1. What are the different parts of UNIX system?
* Programs and services that have made the UNIX system environment.
* Operating system that supports those programs and services.

2. What are the features of UNIX os? (OR)
What are the reasons for popularity of UNIX operating system?
* The system is written in high level language, make it easy to read, understand,
change, and move to other machines.
* It has a simple user interface that has the power to provide the services that
users want.
* It provides primitives that permit complex programs to be built from simpler
programs.
* It uses a hierarchical file system, consistent format for files, byte stream,
* making application programs easier to write.
* It provide simple and consistent interface to peripheral devices.
* It is a multi user multi process system, it hides the machine architecture from
user.

3. What is the purpose of system calls?
It instructs the kernel to do various operations for the calling program and
exchange data between the kernel and the program.

4. Write down the characteristics of UNIX file system?
* ?A hierarchical structure.
* ?Consistent treatment of file data.
* The ability to create and delete files.
* ?Dynamic growth of files.
* The protection of file data.
* The treatment of peripheral devices as files

5. Define path name and component.
A path name is a sequence of component names separated by slash characters; a
component is a sequence of characters that designates a file name that is uniquely
contained in the previous component.
6. What do you mean by access permissions?
Access permissions are set of bits associated with each file. Access permissions
can be set independently to control read, write, and execute permission for three classes of users: the file owner, a file group and every one else.

7. Define program and process.
A program is an executable file, and a process is an instance of the program in
execution.

8. What is the purpose of building block primitives?
Building block primitives enable users to write small, modular programs that
can be used as building blocks to build complex programs. Example for primitives is capability to redirect I/O.

9. What are the services provided by the kernel?
* Controlling the execution of processes by allowing their creation, termination or
* suspension and communication.
* Scheduling processes for execution on the CPU.
* Allocating main memory for an executing process.
* Allocating secondary memory for efficient storage and retrieval of user data.
* Allowing processes controlled access to peripheral devices.

10. What is the purpose of memory management?
Memory management module controls the allocation of memory. If at any time the system does not have enough memory for all processes the kernel moves them between main memory and secondary memory so that all processes get a fair chance to execute.

11. What is the job done by scheduler?
The scheduler module allocates the CPU to process. It schedules them to run in turn until they voluntarily relinquish the CPU while awaiting a resource until the kernel preempts them when their recent run time exceeds a time quantum. The scheduler then chooses a highest priority eligible process to run.

12. What do you mean by swapping?
UNIX system transfer processes from primary memory to swap device and from swap device to primary memory , but did not transfer parts of a process independently, except for a shared text. Such a memory management policy is called swapping.

13. List down the types of file
Regular file, Directory file, FIFO files, Character special file, Block special file,
socket file, symbolic file.

14. What are the different type of commands supported by shell?
* A command can be an executable file that contains object code produced by
* compilation of source code.
* A command can be executable file that contains a sequence of shell command lines.
* A command can be an internal shell command

15. Define shell programming.
It a command interpreter program that users typically execute after logging into system. The shell interprets the first word of command line as a command name. For many commands the shell forks and the child process execs a command associated with the name.

16. What are the execution modes of the user process?
There are two levels of user process ie, user and kernel. When a process executes
a system call the execution mode of the process changes from user mode to kernel mode.

17. What is the difference between user mode and kernel mode?
Processes in user mode can access their own instructions and data but not kernel
instructions and data. Processes in kernel mode can access kernel and user addresses.Some machine instructions are only privileged to kernel mode.

18. Define interrupts.
Interrupts are caused by events that are external to process. Interrupts are considered to happen between the execution of two instructions, and the system continueswith the next instruction after servicing the interrupt.

19. Define exception.
Exception refers to unexpected events caused by a process such as addressing
illegal memory, executing privileged instructions, division by zero and so on. Exception happens in the middle of the execution of an instruction and the system attempts to restart the instruction after handling the exception.

20. Write some sample processor execution levels?
* Machine errors
* Clock
* Disk
* Network devices
* Terminals
* Software interrupts

21. What are the different levels of operation in UNIX kernel?
User Level: The user programs and the libraries are available here.
Kernel level: This is the heart of UNIX operating system. All the operations related to file system, process control sub system,hardware control, and device drivers are available in this level.
Hardware level: The physical components of the system are available here.

22. What is the purpose of file sub system?
The file sub system accesses file data using a buffering mechanism that regulates
data flow between the kernel and secondary storage device.

23. What is the purpose of memory management?
Memory management module controls the allocation of memory. If at
any time the system does not have enough memory for all processes the kernel moves them between main memory and secondary memory so that all processes get a fair chance to execute.

24. Write down the purpose of buffering mechanism?
Buffering mechanism interacts with block I/O device drivers to initiate
data transfer to and from the kernel.

25. What is the job done by device drivers and block I/O devices?
Device drivers are the kernel modules that control the operation of peripheral
devices. Block I/O devices are random access storage devices.

26. What is the purpose of process control subsystem?
The process control subsystem is responsible for process synchronization, inter
process communications, memory management and process scheduling. The file
subsystem and process control subsystem interact when loading a file into memory for execution. The process control subsystem reads executable files into memory before executing them.

27. Write down the system calls useful for control the processes?
* fork: To create a new process
* exec: overlay the image of the program into the running process
* exit: to terminate a process
* wait: to introduce synchronization between the process.
* brk: control the size of the memory allocated to a process.
* signal: control process response to extraordinary events.

28. What are the data structures used for file processing?
File table:
It is a global data structure
User file descriptor table:
It is allocated per process
When a process opens or creats a file the kernel allocates an entry
fromeach table corresponds to the file inode.
Inode:
Internal representation of file is given by an inode, which
contains a description of the disk layout of the file data and other
information related to permission, access time, owner etc.

29. Write about file system layout
Boot block : This occupies the beginning of a file system, typically
first sector,and may contain the bootstrap code
Super block: describes the state of the file system.
Inode list : A list of inodes that follows the super block in the file
system.
Data blocks: Contain file data and administrative data.

30. Define process.
A process is the execution of a program and consists of a pattern of
bytes that the
CPU interprets as machine instructions, data, and stack.

31. What are the contents/ components of executable file?
* Set of headers that describes the attributes of the file.
* A program text.
* A machine language representation of data that has initial values when the program starts execution, and an indication of how much space the kernel should
allocate for uninitialized data.
* Symbol table information

32. What is the content of stack frame?
Stack frame contains the parameters to a function, its local variables, and the data necessary to recover the previous stack frame, including the value of program counter and stack pointer at the time of the program call.

33. What are the contents of user stack and kernel stack?
The user stack contains the arguments, local variables, and other data for functions executing in user mode. The kernel stack contains the stack frames for functions executing in kernel mode.

UNIT – II

34. What are the information available in inode?
A description about the disk addresses the file data and other information such as the file owner, file access permissions and access times.

35. Describe the contents in buffer cache.
The buffer consists of two parts: a memory array that contains data from the disk and a buffer header that identifies the buffer. The data in buffer corresponds to the data in a logical disk block and kernel identifies the buffer contents by examining identifier fields in the buffer header.

36. What are the information available in buffer header?
The buffer header contains a device number field and block number fields that specify the file system and block number of the data on disk and uniquely identify the buffer. It also contains a pointer to a data array, status field.

37. How will you identify the status of the buffer?
* The buffer is currently locked.
* The buffer contains valid data.
* The kernel must write buffer contents to disk before reassigning the buffer
* The kernel is currently reading or writing the contents of the buffer to disk.
* A process is currently waiting for the buffer to become free.

38. What are the different scenarios to allocate a buffer for a disk block.
* The kernel finds the block on its hash queue and its buffer cache.
* The kernel cannot find the block o the hash queue, so it allocates a buffer from the free list.
* The kernel cannot find the block o the hash queue and in attempting to allocate buffer from the free list, find a buffer on a free list that has been marked “delayed write”. The kernel must write the “delayed write” buffer to disk and allocate another buffer.
* The kernel cannot find the block o the hash queue, and the free list of buffers is empty.
* The kernel finds the block on the hash queue but its buffer is currently busy.

39. Write the data structures used for buffer pool.
* Doubly linked list
* Hash queue

40. Write the fields in disk inode.
* File owner Identifier
* File type
* File access functions
* File access times
* Number of links to the file
* Table of contents for the disk addresses
* File Size

41. Write the fields in incore inode.
The following fields in addition to the disk inode fields
* The status of in-core inode
* The logical device number
* Inode number
* Pointers to other in-core inodes
* Reference counts

42. What is the difference between incore inode and and buffer header.
Incode reference count which counts the number of active instances of the file.

43. Write the formula to find block number and byte offset.
The block number =((inode number – 1)/number of inodes per block)+start block of inode list
The byte offset= ((inode number-1)modulo(number of inodes per block))*size of disc inode
44. What is the use of iget and iput?
Iget: To get new inode number
Iput: To release the inode number

45. What are the entries in directory /directory layout.
* Byte offset in directory
* Inode number(2 bytes)
* File names

46. What is the use of namei algorithm.
To convert a path name to an inode

47. What are the fields in superblock.
* The size of the file system
* The number of free blocks in the file system
* A list of free blocks available on the file system
* The index of the next free block in the free block list
* The size of the inode list
* The number of free inodes in the file system
* A list of free inodes in the file system
* The index of the next free inode in the free inode list
* Lock fields for the free block and the free inode lists
* A flag indicating that the super block has been modified

UNIT – III

48. What is the purpose of dup and mount system call?
The dup system call copies a file descriptor into the first free slot of the user file descriptor table, returning new file descriptor to the user.
The mount system call connects the file system in a specified section of a disk to the existing file system hierarchy.

49. What are the different types of pipes?
Unnamed pipe, and named pipe(FIFO)

50. How will you open and close the devices?
The devices are opened or closed by using the ordinary file functions open and close, because the devices are also treated as a file.

51. Write down the difference between named and unnamed pipes
named pipe is called by open system call
unnamed pipe is created by pipe system call

52. Write the system calls for accessing existing files.
Read, write, lseek,open

53. Write the system calls for creating new files.
Creat

54. Write the system calls that manipulate inode.
Namei,iget,iput,ialloc,ifree

55. What are system calls that return file descriptors?
Open,creat

56. Write the syntax for create,open, read , write, close, link.
* Open: fd=open(pathname,flags,modes)
* Read: number=read(fd,buffer,count)
* Write: number=write(fd,buffer,count)
* Close: close(fd)
* Create: fd=creat(pathname,modes)
* Link: link(source filename,target filename)

57. What are the IO parameters saved in uarea.
Mode,count,offset,address,flag

58. Why file and records are locked?
To prevent other processes from reading or writing any part of an entire file or record.

UNIT IV
59. What are the states in the lifetime of a process?
User Running, Kernal running, Ready to run, sleep, Ready to run and swapped, sleep and swapped, preempted, created, zombie

60. What is zombie state?
It is a final state of a process where the process no longer exists,but leaves a record containing an exit code and some timing statistics for its parent process to collect.

61. What is the size of uarea?
4k bytes

62. What are register triples?
There are 3 register triples namely kernel, data and uarea to have the pointers to kernel text,data,uarea of a process respectively.

63. Which command will move from user mode to kernel mode?
Trap system call

64. What are the algorithms used in saving context for abortive return?
Setjmp, longjmp

65. What are the important fields in u area?
* ?Pointer to process table slot of the currently executing process
* Parameters of the current system call return values and error codes
* Internal I/O parameters
* ?Current directory and current root
* ?Process and file size limits

66. What are the factors affect the state of the process?
Text of the process, the values of its global user variables and data structures, the values of machine registers it uses, the values stored in its process table slot and u_area and the contents of its user and kernel stacks.

67. List down the fields in process table.
* State field
* ?Identifiers indicating the user who owns the process
* An event descriptor set when a process is suspended
68. Define context switch.
When the kernel decides to execute another process the kernel saves the necessary information so that it can later switch back to the first process and resume its execution.
69. Define process state and list down the different basic process states.
The lifetime of a process can be divided into a set of states, each with certain characteristics that describe the process. The basic process states are
* The process is currently executing in user mode.
* The process is currently executing in kernel mode .
* The process is not executing, but it is ready to run.
* The process is sleeping.

70. Define state transition diagram.
It is a directed graph whose nodes represent the states a process can enter and whose edges represent the events that cause a process to move from one state to another. State transitions are legal between two states if there exist an edge from the first state to the second.

71. What are the operations are carried out by kernel for fork system call?
* It allocates a slot in the process table for the new process.
* It assigns a unique ID number to the child process.
* It makes a logical copy of the context of the parent process.
* It increments file and inode table counters for files associated with the process.
* It returns the ID number of the child to the parent process, and a 0 value to the child process.

72. What are the different process states in UNIX?
The possible process states are
* ?User running
* ?Kernel running
* ?Ready to run in memory
* ?Asleep in memory
* ?Ready to run, but it is swapped process
* ?Sleep, but it is swapped
* ?Preempted
* ?Created
* ?Zombie state

73. What are the entries in process table?
State field, user identifiers, process identifiers, scheduling parameters, signal field, timer fields

74. What are the I/O parameters in U area?
I/O parameters in U area are
* Mode – indicates read and write
* Count – count of bytes to read and write
* Offset – byte offset in file
* Address – target address to copy data in user or kernel memory
* Flag – indicate a if address is in user or kernel memory

75. Write down the entries in u area.
A pointer to a process table, Real and effective user Ids, array for signals, timer fields, control terminal field, error field, return value field, I/O parameters, current directory, current root, user file descriptor, limit fields, permission modes field.

76. Define region.
System V divides the virtual address space of a process into logical divisions called regions. A region is a continuous area of the virtual address space of a process that can be treated as a distinct object to be shared or protected.

77. What are the components of register context?
The register context consists of the following components
* ?Program counter
* ?Processor status register
* ?Stack pointer
* ?General purpose registers

78. What are the components of system level context?
The system level context consists of the following components
* ?Process table entry of a process
* ?U area of a process
* ?Pregion entries
* Kernel stack
* ?Dynamic part of system level context

79. What are the operations during interrupt handling?
The kernel handles the interrupt with the following sequence of operations
* ?It saves the current register context and creates a new context layer.
* ?It determine the source of interrupt, type of interrupt and unit number
* ?Kernel invoke the interrupt handler
* ?Restore its previous context layer

80. Write down the steps for context switch.
Set of steps used for context switch are
* decide whether to do a context switch and whether a context switch is
permissible now.
* Save the context of old process
* Find the best process to schedule for execution
* Restore its context

81. Write down the contents of region table entry.
The contents in region table entries are
* A pointer to inode of the file
* ?Region type
* ?Size of region
* ?Location of region in physical memory
* ?Status of region – locked, demand, loaded into memory or valid in
* memory
* ?Reference count

82. What are the input needed for loading the region?
* Pointer to per process region table entry
* ?Virtual address to load region
* ?Inode pointer of file for loading region
* ?Byte offset in file for loading region
* ?Byte count for amount of data to load

83. What are the inputs needed for attaching a region?
* Pointer to region being attached
* Process to which region is being attached
* Virtual address in process where region will be attached
* Region type.

84. What are signals? Write the syntax for signal system call.
Signals inform processes of the occurrences of asynchronous events.
Oldfunction=signal(signum,function)

UNIT – V

85. Write down the process-scheduling algorithm.
Algorithm for process scheduling
Input and output are none
{
while(no process picked to execute)
{
for(every process on run queue)
pick highest priority process that is loaded in memory;
if (no process eligible to execute)
idle the machine;
}
remove chosen process from run queue;
switch context to that of chosen process, resume its execution.
}

86. What are the classes of process priorities?
User priorities and kernel priorities

87. What are the different process priority levels in kernel priorities?
Swapper, waiting for disk I/O, waiting for buffer, waiting for inode, waiting for tty input, waiting for tty output, waiting for child exit

88. Write down the principle of fair share scheduler.
This scheduler divides the user community into a set of fair share groups. The member of each group are subjects to the constraints of the regular process scheduler relative to other processes in the group. The system allocates its CPU time proportionally to each group regardless of how many processes are in the groups.

89. Define swap device
The memory management system monitors the amount of available primary memory and may periodically write processes to a secondary memory device called the swap device to provide more space in primary memory.

90. Define demand paging policy
The demand paging policy transferring memory pages instead of processes to and from a secondary device. The entire process does not have to reside in main memory to execute and the kernel loads pages for a process on demand when a process references the pages.

91. What do you mean by map?
The kernel maintains the free space for the swap device in an incore table called a map. Maps used for other resources besides the swap device allows a first fit allocation of contiguous blocks of a resource.

92. What do you mean by protection fault?
The process accessed a valid page but the permission bits associated with the page did not permit access. A process also incurs a protection fault when it attempts to write a page whose copy on write bit was set during the fork system call.

93. What are the different stages are specified as device configuration?
* Administrators can hard_code configuration data into files that are compiled and linked when building the kernel code
* Administrator can supply configuration information after the system is already running; the kernel updates internal configuration tables dynamically.
* Self identifying devices permit the kernel to recognize which devices are installed. The configuration procedure generates or fills in tables.

94. What are the different switch tables available in UNIX?
Block device switch table and character device switch table.

95. What is the purpose of icotl function?
It catch all entry point for device specific commands, allowing a process to setn hardware options associated with a device and software options associated with the driver.

96. What are the functions of line discipline?
* To parse input strings into lines
* To process erase characters
* To process a kill character
* To echo received character to a sequence of blank spaces.
* To generate signals to processes for terminal hang-ups, line breaks.
* To allow a raw mode that does not interpret special characters.

97. What do you mean by stream?
A stream is a full duplex connection between a process and a device driver. It consists of a set of linearly linked queue pairs, one member of each pair for input and other for output.

98. What do you mean by pseudo terminal or pty?
A pty is a software device that operates in pairs: output directed to one
member of the pair is sent to the input of the other member; input is sent to the upstream module.

99. What are the different methods available to identify the address?
The driver gets the address in two ways
* Either the strategy procedure uses a buffer from the buffer pool and the buffer header contains the device and block number or the read and write procedures are passed the minor number as a parameter.
* They convert the byte offset saved in the u area to the appropriate block address.

100. Define page stealer process.
The page stealer process is a kernel process that swaps out memory pages that are no longer part of the working set of a process.

101. What are the different types of page fault?
Validity faults and protection faults

102. When the validity fault occurs?
If a process attempts to access a page whose valid bit is not set it incurs a validity fault and the kernel invokes the validity fault handler.

103. Define page.
The memory management hardware divides physical memory into a set of equal sized blocks called pages.

104. What are the information available in page table entry?
The page table entries contain machine dependent information such as
permission bits to allow reading or writing of the page.

105. Write the states that cause the page fault.
* On a swap device and not in memory
* On the free page list in memory
* In an executable file
* Marked “demand zero”
* Marked “demand fill”

Descriptive questions and key

UNIT I

1. Explain in detail about the high level features of UNIX system
* List down the file system characteristics
* Explain in detail about the file system
* Explain about process control
* Explain about building block primitives.

2. Explain about UNIX system structure in detail.
Fig 1.1 -architecture of UNIX systems

3. Explain the architecture of UNIX operating system.
Draw the block diagram of system kernel
* Explain about process control subsystem
* Explain about file subsystem, buffer cache.
* Explain about IPC, scheduler, memory management
* Explain about drivers and interfaces.

4. Explain in detail about the processes
* Explain about process, process creation, process termination
* Explain basic process states
* Explain process state transition diagram
* Explain about context switch

5. Explain the assumptions about UNIX h/w in UNIX.
* Execution modes: user mode and kernel mode
* Interrupts and exceptions
* Processor execution levels
* Memory management

6. Discuss about system concepts in UNIX
* File system
* Process system
UNIT II

7. Discuss about the structure of buffer pool. Write the advantages and disadvantages of buffer cache.
ADV:
* It allows uniform disk access
* It makes user programs simpler and more portable
* It reduces the amount of disk traffic
* It ensures file system integrity
DISADV:
* It requires an extra data copy when reading and writing to and from user processes
* If too much memory is used for buffers,the system will slow down

8. Explain the scenarios for retrieval of a buffer.
* List down the different scenarios for retrieval a buffer
* Explain each scenario with diagram.
* Explain the algorithms getblk, brelse, bread, bwrite, breada
* Advantages and disadvantages of the buffer cache

9. Write the algorithm of allocating incore inode.

Iget algorithm:

10. Which algorithm is used to convert a path name to an inode? Explain.
namei algorithm

UNIT III

11. What is Pipe? What the different types of pipes? Write difference between regular file and pipe?
Pipe allows transfer of data between processes in a FIFO manner
Types:
Named pipe
Unnamed pipe
Difference:
Regular file uses direct and indirect inode blocks.
Pipe uses only direct inode blocks
12. What is the use of dup system call? Write the algorithm of dup system call.
USE: It copies a file descriptor into the free slot of the user file descriptor table

13. Can two files have the same inode? If so, specify the system call and its algorithm.
* Yes, dup and its algorithm

14. What system call allows another file system to be attached to your file system? What is the attachment point? What table maintains all these information?

* Mount system call
* Mount point
* Mount table

15. What are the entries in mount table? What are the data structures used for mount system call?
* Pointer to a buffer containing the file system super block
* A pointer to the root inode of the mounted file system
* A pointer to the inode of the directory( mount point)
Data Structures
1. Inode table
2. Mount table

16. What are the operations carried out during unmounting the file system?
Unmount algorithm

17. Explain the system calls that return file descriptors as output
Explain the following system calls with the necessary algorithms and example
* Open
* Creat
* Dup
* Pipe
* Close

18. Explain the functions related to file I/O
Explain the following functions in detail with algorithm
* Read
* Write
* Lseek

19. Explain in detail about pipes
* Explain about pipe system call
* Explain about opening a named pipe
* Explain about reading and writing pipes
* Explain about closing pipes with example

UNIT IV

20. What are the process states and explain the process state transition diagram.
* Nine process states
* Process state transition diagram and explanation

21. Write the fields in process table and uarea.
Process table:
State field,User identifiers,Process identifiers,Event descriptor,Scheduling parameters,Signal fields,Various timers
Uarea:
A pointer to the process table,timer fields,an array for signals,error field, IO parameters,curret directory,user file descriptor table,limit field,permission modes

22. Explain the layout of the system memory.
* Regions
* Pages and page tables
* Layout of the kernel

23. Explain the manipulation of the process address space
Explain the following
* Locking and unlocking a region
* Allocating a region with algorithm
* Attaching a region to a process with algorithm
* Changing the size of region with algorithm
* Loading a region freeing a region
* Freeing a region
* Detaching a region from process
* Duplicating a region

24. What is the context of a process? Explain.
Register context
System level context

25. Explain about saving the context of a process?
* Interrupts and exceptions
* System call interface
* Context switch
* Saving context for abortive returns
* Copying data between system and user address space

UNIT V

26. Explain in detail about process scheduling
Explain the following
* System configuration
* System calls and driver interface
* Open and closing device files
* Read and write operations
* Strategy interface
* ioctl function
* Interrupt handlers
27. Explain in detail about swapping
* Explain about allocation of swap space
* Explain in detail about swapping processes out
* Explain fork swap and expansion swap
* Explain swapping processes in with algorithm
28. Explain in detail about demand paging
* Explain in detail about data structures for demand paging
* Explain fork and exec in a paging system
* Explain page stealer process

29. Explain in detail about page fault and fault handlers
* Define page fault
* Explain validity fault handler with algorithm
* Explain protection fault handler with algorithm

30. Explain in detail about the driver interfaces
Explain about
* System configuration
* System calls and driver interface
* xplain about open and close a device
* Explain about read and write a device
* Explain about ioctl function

31. Explain in detail about terminal drivers
* Define terminal drivers
* List down the functions of line discipline
* Explain about clists
* Explain the terminal driver in canonical mode
* Explain the terminal driver in raw mode
* Explain in detail about terminal polling and logging in

1. What are the different parts of UNIX system?
* Programs and services that have made the UNIX system environment.
* Operating system that supports those programs and services.

2. What are the features of UNIX os? (OR)
What are the reasons for popularity of UNIX operating system?
* The system is written in high level language, make it easy to read, understand,
change, and move to other machines.
* It has a simple user interface that has the power to provide the services that
users want.
* It provides primitives that permit complex programs to be built from simpler
programs.
* It uses a hierarchical file system, consistent format for files, byte stream,
* making application programs easier to write.
* It provide simple and consistent interface to peripheral devices.
* It is a multi user multi process system, it hides the machine architecture from
user.

3. What is the purpose of system calls?
It instructs the kernel to do various operations for the calling program and
exchange data between the kernel and the program.

4. Write down the characteristics of UNIX file system?
* ?A hierarchical structure.
* ?Consistent treatment of file data.
* The ability to create and delete files.
* ?Dynamic growth of files.
* The protection of file data.
* The treatment of peripheral devices as files

5. Define path name and component.
A path name is a sequence of component names separated by slash characters; a
component is a sequence of characters that designates a file name that is uniquely
contained in the previous component.
6. What do you mean by access permissions?
Access permissions are set of bits associated with each file. Access permissions
can be set independently to control read, write, and execute permission for three classes of users: the file owner, a file group and every one else.

7. Define program and process.
A program is an executable file, and a process is an instance of the program in
execution.

8. What is the purpose of building block primitives?
Building block primitives enable users to write small, modular programs that
can be used as building blocks to build complex programs. Example for primitives is capability to redirect I/O.

9. What are the services provided by the kernel?
* Controlling the execution of processes by allowing their creation, termination or
* suspension and communication.
* Scheduling processes for execution on the CPU.
* Allocating main memory for an executing process.
* Allocating secondary memory for efficient storage and retrieval of user data.
* Allowing processes controlled access to peripheral devices.

10. What is the purpose of memory management?
Memory management module controls the allocation of memory. If at any time the system does not have enough memory for all processes the kernel moves them between main memory and secondary memory so that all processes get a fair chance to execute.

11. What is the job done by scheduler?
The scheduler module allocates the CPU to process. It schedules them to run in turn until they voluntarily relinquish the CPU while awaiting a resource until the kernel preempts them when their recent run time exceeds a time quantum. The scheduler then chooses a highest priority eligible process to run.

12. What do you mean by swapping?
UNIX system transfer processes from primary memory to swap device and from swap device to primary memory , but did not transfer parts of a process independently, except for a shared text. Such a memory management policy is called swapping.

13. List down the types of file
Regular file, Directory file, FIFO files, Character special file, Block special file,
socket file, symbolic file.

14. What are the different type of commands supported by shell?
* A command can be an executable file that contains object code produced by
* compilation of source code.
* A command can be executable file that contains a sequence of shell command lines.
* A command can be an internal shell command

15. Define shell programming.
It a command interpreter program that users typically execute after logging into system. The shell interprets the first word of command line as a command name. For many commands the shell forks and the child process execs a command associated with the name.

16. What are the execution modes of the user process?
There are two levels of user process ie, user and kernel. When a process executes
a system call the execution mode of the process changes from user mode to kernel mode.

17. What is the difference between user mode and kernel mode?
Processes in user mode can access their own instructions and data but not kernel
instructions and data. Processes in kernel mode can access kernel and user addresses.Some machine instructions are only privileged to kernel mode.

18. Define interrupts.
Interrupts are caused by events that are external to process. Interrupts are considered to happen between the execution of two instructions, and the system continueswith the next instruction after servicing the interrupt.

19. Define exception.
Exception refers to unexpected events caused by a process such as addressing
illegal memory, executing privileged instructions, division by zero and so on. Exception happens in the middle of the execution of an instruction and the system attempts to restart the instruction after handling the exception.

20. Write some sample processor execution levels?
* Machine errors
* Clock
* Disk
* Network devices
* Terminals
* Software interrupts

21. What are the different levels of operation in UNIX kernel?
User Level: The user programs and the libraries are available here.
Kernel level: This is the heart of UNIX operating system. All the operations related to file system, process control sub system,hardware control, and device drivers are available in this level.
Hardware level: The physical components of the system are available here.

22. What is the purpose of file sub system?
The file sub system accesses file data using a buffering mechanism that regulates
data flow between the kernel and secondary storage device.

23. What is the purpose of memory management?
Memory management module controls the allocation of memory. If at
any time the system does not have enough memory for all processes the kernel moves them between main memory and secondary memory so that all processes get a fair chance to execute.

24. Write down the purpose of buffering mechanism?
Buffering mechanism interacts with block I/O device drivers to initiate
data transfer to and from the kernel.

25. What is the job done by device drivers and block I/O devices?
Device drivers are the kernel modules that control the operation of peripheral
devices. Block I/O devices are random access storage devices.

26. What is the purpose of process control subsystem?
The process control subsystem is responsible for process synchronization, inter
process communications, memory management and process scheduling. The file
subsystem and process control subsystem interact when loading a file into memory for execution. The process control subsystem reads executable files into memory before executing them.

27. Write down the system calls useful for control the processes?
* fork: To create a new process
* exec: overlay the image of the program into the running process
* exit: to terminate a process
* wait: to introduce synchronization between the process.
* brk: control the size of the memory allocated to a process.
* signal: control process response to extraordinary events.

28. What are the data structures used for file processing?
File table:
It is a global data structure
User file descriptor table:
It is allocated per process
When a process opens or creats a file the kernel allocates an entry
fromeach table corresponds to the file inode.
Inode:
Internal representation of file is given by an inode, which
contains a description of the disk layout of the file data and other
information related to permission, access time, owner etc.

29. Write about file system layout
Boot block : This occupies the beginning of a file system, typically
first sector,and may contain the bootstrap code
Super block: describes the state of the file system.
Inode list : A list of inodes that follows the super block in the file
system.
Data blocks: Contain file data and administrative data.

30. Define process.
A process is the execution of a program and consists of a pattern of
bytes that the
CPU interprets as machine instructions, data, and stack.

31. What are the contents/ components of executable file?
* Set of headers that describes the attributes of the file.
* A program text.
* A machine language representation of data that has initial values when the program starts execution, and an indication of how much space the kernel should
allocate for uninitialized data.
* Symbol table information

32. What is the content of stack frame?
Stack frame contains the parameters to a function, its local variables, and the data necessary to recover the previous stack frame, including the value of program counter and stack pointer at the time of the program call.

33. What are the contents of user stack and kernel stack?
The user stack contains the arguments, local variables, and other data for functions executing in user mode. The kernel stack contains the stack frames for functions executing in kernel mode.

UNIT – II

34. What are the information available in inode?
A description about the disk addresses the file data and other information such as the file owner, file access permissions and access times.

35. Describe the contents in buffer cache.
The buffer consists of two parts: a memory array that contains data from the disk and a buffer header that identifies the buffer. The data in buffer corresponds to the data in a logical disk block and kernel identifies the buffer contents by examining identifier fields in the buffer header.

36. What are the information available in buffer header?
The buffer header contains a device number field and block number fields that specify the file system and block number of the data on disk and uniquely identify the buffer. It also contains a pointer to a data array, status field.

37. How will you identify the status of the buffer?
* The buffer is currently locked.
* The buffer contains valid data.
* The kernel must write buffer contents to disk before reassigning the buffer
* The kernel is currently reading or writing the contents of the buffer to disk.
* A process is currently waiting for the buffer to become free.

38. What are the different scenarios to allocate a buffer for a disk block.
* The kernel finds the block on its hash queue and its buffer cache.
* The kernel cannot find the block o the hash queue, so it allocates a buffer from the free list.
* The kernel cannot find the block o the hash queue and in attempting to allocate buffer from the free list, find a buffer on a free list that has been marked “delayed write”. The kernel must write the “delayed write” buffer to disk and allocate another buffer.
* The kernel cannot find the block o the hash queue, and the free list of buffers is empty.
* The kernel finds the block on the hash queue but its buffer is currently busy.

39. Write the data structures used for buffer pool.
* Doubly linked list
* Hash queue

40. Write the fields in disk inode.
* File owner Identifier
* File type
* File access functions
* File access times
* Number of links to the file
* Table of contents for the disk addresses
* File Size

41. Write the fields in incore inode.
The following fields in addition to the disk inode fields
* The status of in-core inode
* The logical device number
* Inode number
* Pointers to other in-core inodes
* Reference counts

42. What is the difference between incore inode and and buffer header.
Incode reference count which counts the number of active instances of the file.

43. Write the formula to find block number and byte offset.
The block number =((inode number – 1)/number of inodes per block)+start block of inode list
The byte offset= ((inode number-1)modulo(number of inodes per block))*size of disc inode
44. What is the use of iget and iput?
Iget: To get new inode number
Iput: To release the inode number

45. What are the entries in directory /directory layout.
* Byte offset in directory
* Inode number(2 bytes)
* File names

46. What is the use of namei algorithm.
To convert a path name to an inode

47. What are the fields in superblock.
* The size of the file system
* The number of free blocks in the file system
* A list of free blocks available on the file system
* The index of the next free block in the free block list
* The size of the inode list
* The number of free inodes in the file system
* A list of free inodes in the file system
* The index of the next free inode in the free inode list
* Lock fields for the free block and the free inode lists
* A flag indicating that the super block has been modified

UNIT – III

48. What is the purpose of dup and mount system call?
The dup system call copies a file descriptor into the first free slot of the user file descriptor table, returning new file descriptor to the user.
The mount system call connects the file system in a specified section of a disk to the existing file system hierarchy.

49. What are the different types of pipes?
Unnamed pipe, and named pipe(FIFO)

50. How will you open and close the devices?
The devices are opened or closed by using the ordinary file functions open and close, because the devices are also treated as a file.

51. Write down the difference between named and unnamed pipes
named pipe is called by open system call
unnamed pipe is created by pipe system call

52. Write the system calls for accessing existing files.
Read, write, lseek,open

53. Write the system calls for creating new files.
Creat

54. Write the system calls that manipulate inode.
Namei,iget,iput,ialloc,ifree

55. What are system calls that return file descriptors?
Open,creat

56. Write the syntax for create,open, read , write, close, link.
* Open: fd=open(pathname,flags,modes)
* Read: number=read(fd,buffer,count)
* Write: number=write(fd,buffer,count)
* Close: close(fd)
* Create: fd=creat(pathname,modes)
* Link: link(source filename,target filename)

57. What are the IO parameters saved in uarea.
Mode,count,offset,address,flag

58. Why file and records are locked?
To prevent other processes from reading or writing any part of an entire file or record.

UNIT IV
59. What are the states in the lifetime of a process?
User Running, Kernal running, Ready to run, sleep, Ready to run and swapped, sleep and swapped, preempted, created, zombie

60. What is zombie state?
It is a final state of a process where the process no longer exists,but leaves a record containing an exit code and some timing statistics for its parent process to collect.

61. What is the size of uarea?
4k bytes

62. What are register triples?
There are 3 register triples namely kernel, data and uarea to have the pointers to kernel text,data,uarea of a process respectively.

63. Which command will move from user mode to kernel mode?
Trap system call

64. What are the algorithms used in saving context for abortive return?
Setjmp, longjmp

65. What are the important fields in u area?
* ?Pointer to process table slot of the currently executing process
* Parameters of the current system call return values and error codes
* Internal I/O parameters
* ?Current directory and current root
* ?Process and file size limits

66. What are the factors affect the state of the process?
Text of the process, the values of its global user variables and data structures, the values of machine registers it uses, the values stored in its process table slot and u_area and the contents of its user and kernel stacks.

67. List down the fields in process table.
* State field
* ?Identifiers indicating the user who owns the process
* An event descriptor set when a process is suspended
68. Define context switch.
When the kernel decides to execute another process the kernel saves the necessary information so that it can later switch back to the first process and resume its execution.
69. Define process state and list down the different basic process states.
The lifetime of a process can be divided into a set of states, each with certain characteristics that describe the process. The basic process states are
* The process is currently executing in user mode.
* The process is currently executing in kernel mode .
* The process is not executing, but it is ready to run.
* The process is sleeping.

70. Define state transition diagram.
It is a directed graph whose nodes represent the states a process can enter and whose edges represent the events that cause a process to move from one state to another. State transitions are legal between two states if there exist an edge from the first state to the second.

71. What are the operations are carried out by kernel for fork system call?
* It allocates a slot in the process table for the new process.
* It assigns a unique ID number to the child process.
* It makes a logical copy of the context of the parent process.
* It increments file and inode table counters for files associated with the process.
* It returns the ID number of the child to the parent process, and a 0 value to the child process.

72. What are the different process states in UNIX?
The possible process states are
* ?User running
* ?Kernel running
* ?Ready to run in memory
* ?Asleep in memory
* ?Ready to run, but it is swapped process
* ?Sleep, but it is swapped
* ?Preempted
* ?Created
* ?Zombie state

73. What are the entries in process table?
State field, user identifiers, process identifiers, scheduling parameters, signal field, timer fields

74. What are the I/O parameters in U area?
I/O parameters in U area are
* Mode – indicates read and write
* Count – count of bytes to read and write
* Offset – byte offset in file
* Address – target address to copy data in user or kernel memory
* Flag – indicate a if address is in user or kernel memory

75. Write down the entries in u area.
A pointer to a process table, Real and effective user Ids, array for signals, timer fields, control terminal field, error field, return value field, I/O parameters, current directory, current root, user file descriptor, limit fields, permission modes field.

76. Define region.
System V divides the virtual address space of a process into logical divisions called regions. A region is a continuous area of the virtual address space of a process that can be treated as a distinct object to be shared or protected.

77. What are the components of register context?
The register context consists of the following components
* ?Program counter
* ?Processor status register
* ?Stack pointer
* ?General purpose registers

78. What are the components of system level context?
The system level context consists of the following components
* ?Process table entry of a process
* ?U area of a process
* ?Pregion entries
* Kernel stack
* ?Dynamic part of system level context

79. What are the operations during interrupt handling?
The kernel handles the interrupt with the following sequence of operations
* ?It saves the current register context and creates a new context layer.
* ?It determine the source of interrupt, type of interrupt and unit number
* ?Kernel invoke the interrupt handler
* ?Restore its previous context layer

80. Write down the steps for context switch.
Set of steps used for context switch are
* decide whether to do a context switch and whether a context switch is
permissible now.
* Save the context of old process
* Find the best process to schedule for execution
* Restore its context

81. Write down the contents of region table entry.
The contents in region table entries are
* A pointer to inode of the file
* ?Region type
* ?Size of region
* ?Location of region in physical memory
* ?Status of region – locked, demand, loaded into memory or valid in
* memory
* ?Reference count

82. What are the input needed for loading the region?
* Pointer to per process region table entry
* ?Virtual address to load region
* ?Inode pointer of file for loading region
* ?Byte offset in file for loading region
* ?Byte count for amount of data to load

83. What are the inputs needed for attaching a region?
* Pointer to region being attached
* Process to which region is being attached
* Virtual address in process where region will be attached
* Region type.

84. What are signals? Write the syntax for signal system call.
Signals inform processes of the occurrences of asynchronous events.
Oldfunction=signal(signum,function)

UNIT – V

85. Write down the process-scheduling algorithm.
Algorithm for process scheduling
Input and output are none
{
while(no process picked to execute)
{
for(every process on run queue)
pick highest priority process that is loaded in memory;
if (no process eligible to execute)
idle the machine;
}
remove chosen process from run queue;
switch context to that of chosen process, resume its execution.
}

86. What are the classes of process priorities?
User priorities and kernel priorities

87. What are the different process priority levels in kernel priorities?
Swapper, waiting for disk I/O, waiting for buffer, waiting for inode, waiting for tty input, waiting for tty output, waiting for child exit

88. Write down the principle of fair share scheduler.
This scheduler divides the user community into a set of fair share groups. The member of each group are subjects to the constraints of the regular process scheduler relative to other processes in the group. The system allocates its CPU time proportionally to each group regardless of how many processes are in the groups.

89. Define swap device
The memory management system monitors the amount of available primary memory and may periodically write processes to a secondary memory device called the swap device to provide more space in primary memory.

90. Define demand paging policy
The demand paging policy transferring memory pages instead of processes to and from a secondary device. The entire process does not have to reside in main memory to execute and the kernel loads pages for a process on demand when a process references the pages.

91. What do you mean by map?
The kernel maintains the free space for the swap device in an incore table called a map. Maps used for other resources besides the swap device allows a first fit allocation of contiguous blocks of a resource.

92. What do you mean by protection fault?
The process accessed a valid page but the permission bits associated with the page did not permit access. A process also incurs a protection fault when it attempts to write a page whose copy on write bit was set during the fork system call.

93. What are the different stages are specified as device configuration?
* Administrators can hard_code configuration data into files that are compiled and linked when building the kernel code
* Administrator can supply configuration information after the system is already running; the kernel updates internal configuration tables dynamically.
* Self identifying devices permit the kernel to recognize which devices are installed. The configuration procedure generates or fills in tables.

94. What are the different switch tables available in UNIX?
Block device switch table and character device switch table.

95. What is the purpose of icotl function?
It catch all entry point for device specific commands, allowing a process to setn hardware options associated with a device and software options associated with the driver.

96. What are the functions of line discipline?
* To parse input strings into lines
* To process erase characters
* To process a kill character
* To echo received character to a sequence of blank spaces.
* To generate signals to processes for terminal hang-ups, line breaks.
* To allow a raw mode that does not interpret special characters.

97. What do you mean by stream?
A stream is a full duplex connection between a process and a device driver. It consists of a set of linearly linked queue pairs, one member of each pair for input and other for output.

98. What do you mean by pseudo terminal or pty?
A pty is a software device that operates in pairs: output directed to one
member of the pair is sent to the input of the other member; input is sent to the upstream module.

99. What are the different methods available to identify the address?
The driver gets the address in two ways
* Either the strategy procedure uses a buffer from the buffer pool and the buffer header contains the device and block number or the read and write procedures are passed the minor number as a parameter.
* They convert the byte offset saved in the u area to the appropriate block address.

100. Define page stealer process.
The page stealer process is a kernel process that swaps out memory pages that are no longer part of the working set of a process.

101. What are the different types of page fault?
Validity faults and protection faults

102. When the validity fault occurs?
If a process attempts to access a page whose valid bit is not set it incurs a validity fault and the kernel invokes the validity fault handler.

103. Define page.
The memory management hardware divides physical memory into a set of equal sized blocks called pages.

104. What are the information available in page table entry?
The page table entries contain machine dependent information such as
permission bits to allow reading or writing of the page.

105. Write the states that cause the page fault.
* On a swap device and not in memory
* On the free page list in memory
* In an executable file
* Marked “demand zero”
* Marked “demand fill”

Descriptive questions and key

UNIT I

1. Explain in detail about the high level features of UNIX system
* List down the file system characteristics
* Explain in detail about the file system
* Explain about process control
* Explain about building block primitives.

2. Explain about UNIX system structure in detail.
Fig 1.1 -architecture of UNIX systems

3. Explain the architecture of UNIX operating system.
Draw the block diagram of system kernel
* Explain about process control subsystem
* Explain about file subsystem, buffer cache.
* Explain about IPC, scheduler, memory management
* Explain about drivers and interfaces.

4. Explain in detail about the processes
* Explain about process, process creation, process termination
* Explain basic process states
* Explain process state transition diagram
* Explain about context switch

5. Explain the assumptions about UNIX h/w in UNIX.
* Execution modes: user mode and kernel mode
* Interrupts and exceptions
* Processor execution levels
* Memory management

6. Discuss about system concepts in UNIX
* File system
* Process system
UNIT II

7. Discuss about the structure of buffer pool. Write the advantages and disadvantages of buffer cache.
ADV:
* It allows uniform disk access
* It makes user programs simpler and more portable
* It reduces the amount of disk traffic
* It ensures file system integrity
DISADV:
* It requires an extra data copy when reading and writing to and from user processes
* If too much memory is used for buffers,the system will slow down

8. Explain the scenarios for retrieval of a buffer.
* List down the different scenarios for retrieval a buffer
* Explain each scenario with diagram.
* Explain the algorithms getblk, brelse, bread, bwrite, breada
* Advantages and disadvantages of the buffer cache

9. Write the algorithm of allocating incore inode.

Iget algorithm:

10. Which algorithm is used to convert a path name to an inode? Explain.
namei algorithm

UNIT III

11. What is Pipe? What the different types of pipes? Write difference between regular file and pipe?
Pipe allows transfer of data between processes in a FIFO manner
Types:
Named pipe
Unnamed pipe
Difference:
Regular file uses direct and indirect inode blocks.
Pipe uses only direct inode blocks
12. What is the use of dup system call? Write the algorithm of dup system call.
USE: It copies a file descriptor into the free slot of the user file descriptor table

13. Can two files have the same inode? If so, specify the system call and its algorithm.
* Yes, dup and its algorithm

14. What system call allows another file system to be attached to your file system? What is the attachment point? What table maintains all these information?

* Mount system call
* Mount point
* Mount table

15. What are the entries in mount table? What are the data structures used for mount system call?
* Pointer to a buffer containing the file system super block
* A pointer to the root inode of the mounted file system
* A pointer to the inode of the directory( mount point)
Data Structures
1. Inode table
2. Mount table

16. What are the operations carried out during unmounting the file system?
Unmount algorithm

17. Explain the system calls that return file descriptors as output
Explain the following system calls with the necessary algorithms and example
* Open
* Creat
* Dup
* Pipe
* Close

18. Explain the functions related to file I/O
Explain the following functions in detail with algorithm
* Read
* Write
* Lseek

19. Explain in detail about pipes
* Explain about pipe system call
* Explain about opening a named pipe
* Explain about reading and writing pipes
* Explain about closing pipes with example

UNIT IV

20. What are the process states and explain the process state transition diagram.
* Nine process states
* Process state transition diagram and explanation

21. Write the fields in process table and uarea.
Process table:
State field,User identifiers,Process identifiers,Event descriptor,Scheduling parameters,Signal fields,Various timers
Uarea:
A pointer to the process table,timer fields,an array for signals,error field, IO parameters,curret directory,user file descriptor table,limit field,permission modes

22. Explain the layout of the system memory.
* Regions
* Pages and page tables
* Layout of the kernel

23. Explain the manipulation of the process address space
Explain the following
* Locking and unlocking a region
* Allocating a region with algorithm
* Attaching a region to a process with algorithm
* Changing the size of region with algorithm
* Loading a region freeing a region
* Freeing a region
* Detaching a region from process
* Duplicating a region

24. What is the context of a process? Explain.
Register context
System level context

25. Explain about saving the context of a process?
* Interrupts and exceptions
* System call interface
* Context switch
* Saving context for abortive returns
* Copying data between system and user address space

UNIT V

26. Explain in detail about process scheduling
Explain the following
* System configuration
* System calls and driver interface
* Open and closing device files
* Read and write operations
* Strategy interface
* ioctl function
* Interrupt handlers
27. Explain in detail about swapping
* Explain about allocation of swap space
* Explain in detail about swapping processes out
* Explain fork swap and expansion swap
* Explain swapping processes in with algorithm
28. Explain in detail about demand paging
* Explain in detail about data structures for demand paging
* Explain fork and exec in a paging system
* Explain page stealer process

29. Explain in detail about page fault and fault handlers
* Define page fault
* Explain validity fault handler with algorithm
* Explain protection fault handler with algorithm

30. Explain in detail about the driver interfaces
Explain about
* System configuration
* System calls and driver interface
* xplain about open and close a device
* Explain about read and write a device
* Explain about ioctl function

31. Explain in detail about terminal drivers
* Define terminal drivers
* List down the functions of line discipline
* Explain about clists
* Explain the terminal driver in canonical mode
* Explain the terminal driver in raw mode
* Explain in detail about terminal polling and logging in

I have received Rs 18000/- (in words Rupees Eighteen thousand only) from Mr. Aravinda Aithala against rent for the month of APRIL to JUNE 2008.

Address of the house for which rent is paid
#327,Shanthi Building,
MES Road, Gokula Extn,
Muthyala Nagar,
Bangalore India – 560054
Signature

Dr. RaviShankar

RENT RECEIPT

I have received Rs 18000/- (in words Rupees Eighteen thousand only) from MR. Aravinda Aithala against rent for the month of JULY to SEPTEMBER 2008.

Address of the house for which rent is paid
#327,Shanthi Building,
MES Road, Gokula Extn,
Muthyala Nagar,
Bangalore India – 560054
Signature

Dr. RaviShankar

RENT RECEIPT

I have received Rs 18000/- (in words Rupees Eighteen thousand only) from MR. Aravinda Aithala against rent for the month of OCTOBER to DECEMBER 2008.

Address of the house for which rent is paid
#327,Shanthi Building,
MES Road, Gokula Extn,
Muthyala Nagar,
Bangalore India – 560054
Signature

Dr. RaviShankar

RENT RECEIPT

I have received Rs 18000/- (in words Rupees Eighteen thousand only) from MR. Aravinda Aithala against rent for the month of JAN to MARCH 2009.

Address of the house for which rent is paid
#327,Shanthi Building,
MES Road, Gokula Extn,
Muthyala Nagar,
Bangalore India – 560054
Signature

Dr. RaviShankar

Fig CAN 2.0A Message Frame
- A Control Field containing six bits:
* two dominant bits (r0 and r1) that are reserved for future use, and
* a four bit Data Length Code (DLC). The DLC indicates the number of bytes in the Data Field that follows
- A Data Field, containing from zero to eight bytes.
- The CRC field, containing a fifteen bit cyclic redundancy check code and a recessive delimiter bit
- The ACKnowledge field, consisting of two bits. The first is the Slot bit which is transmitted as a recessive bit, but is subsequently over written by dominant bits transmitted from all other nodes that successfully receive the message. The second bit is a recessive delimiter bit
- The End of Frame field, consisting of seven recessive bits.
Following the end of a frame is the INTermission field consisting of three recessive bits. After the three bit INTermission period the bus is recognised to be free. Bus Idle time may be of any arbitrary length including zero.
2.0B Format
The CAN 2.0B format provides a twenty nine (29) bit identifier as opposed to the 11 bit identifier in 2.0A.
Version 2.0B evolved to provide compatibility with other serial communications protocols used in automotive applications in the USA. To cater for this, and still provide compatibility with the 2.0A format, the Message Frame in Version 2.0B has an extended format.
The differences are:
- In Version 2.0B the Arbitration field contains two identifier bit fields. The first (the base ID) is eleven (11) bits long for compatibility with Version 2.0A. The second field (the ID extension) is eighteen (18) bits long, to give a total length of twenty nine (29) bits.
- The distinction between the two formats is made using an Identifier Extension (IDE) bit.

Fig 4.CAN 2.0B Message Frame
- A Substitute Remote Request (SRR) bit is included in the Arbitration Field. The SRR bit is always transmitted as a recessive bit to ensure that, in the case of arbitration between a Standard Data Frame and an Extended Data Frame, the Standard Data Frame will always have priority if both messages have the same base (11 bit) identifier.
All other fields in a 2.0B Message Frame are identical to those in the Standard format.
2.0A and 2.0B Compatibility
2.0B controllers are completely backward compatible with 2.0A controllers and can transmit and receive messages in either format.
Note, however, that there are two types of 2.0A controllers:
- The first is capable of transmitting and receiving only messages in 2.0A format. With this type of controller, reception of any 2.0B message will flag an error.
- The second type of 2.0A controller (known as 2.0B passive) is also capable of sending and receiving 2.0A messages, but in addition, these devices will acknowledge receipt of 2.0B messages and then ignore them.
Therefore, within the above mentioned constraints it is possible to use both Version 2.0A (with 2.0B passive capabilities) and 2.0B controllers on a single network.
However, because of the lack of full upward compatibility of 2.0A devices with 2.0B devices, only messages in Standard format are meaningful in systems using both types.
The number of unique identifiers available to users, on a single 2.0A network, is 2,032 (2 to the power 11 – 2 to the power 4).
The number of unique identifiers available on a 2.0B network is in excess of 500 million!
Error detection and fault confinement
The error detection, signalling and fault confinement defined in the CAN standard makes the CAN bus very reliable. The built in error detection of the controllers together with the error signalling make sure that the information is correct and consistent. Faulty nodes will go to modes where they do not disturb the traffic on the bus.
The CAN error process
1. The error is detected by the a CAN controller (a transmitter or a receiver).
2. An error frame is immediatly transmitted.
3. The message is cancelled at all nodes).
4. The status of the CAN controllers are updated
5. The message is re-transmitted. If several controllers have messages to send, normal arbitration is used.
Error detection
Error detection is handled automatically by the CAN controller. The detected errors are:
* Bit errors:
1. Bit stuffing error – normally a transmitting node inserts a high after five consecutive low bits(and a low after five consecutive high). This is called bit stuffing. A receiving node that detects violation (more than five consecutive bits will see a bit stuffing violation.
2. Bit error: A transmitting node always reads back the message as it is sending. If it detects a different bit value on the bus than it sent, and the bit is not part of the arbitration field or in the acknowledgement field, and error is detected.
* Message errors:
1. Checksum error – each receiving node checks CAN messages for checksum errors.
2. Frame error – There are certain predefined bit values that must be transmitted at certain points within any CAN Message Frame. If a receiver detects an invalid bit in one of these positions a Form Error (sometimes also known as a Format Error) will be flagged.
3. Acknowledgement Error – If a transmitter determines that a message has not been ACKnowledged then an ACK Error is flagged.
CAN controller error modes
A CAN controller can be in one of three states:
1. Error active – the normal operating mode for a controller. Messages can be received and transmitted. On detecting an error an active error flag is sent
2. Error passive – a mode entered when the controller has frequent problems transmitting or receiving messages. Messages can be received and transmitted. On detecting an error while receiving, a passive error flag is sent.
3. Bus off – entered if the controller has serious problems with transmitting messages. No messages can be received or transmitted until the CAN controller is reset by the host microcontroller or processor.
The mode of the controller is controlled by two error counters – the transmit error counter (tx_count) and the receive error counter (rx_count). The following rules apply:
1. The CAN controller is in error active mode if tx_count <= 127 AND rx_count <= 127.
2. Passive mode is used if (tx_count > 127 OR tx_count > 127) AND tx_count <= 255.
3. Bus off is entered if tx_count > 255.
Once the CAN controller has entered bus off state, it must be reset by the host microcontroller or processor in order to be able to continue operation. In addition, this is only allowed after the reception of 128 occurrences of 11 consecutive recessive bits.
The counters are updated as follows:
1. When a receiver detects an error, the rx_count will be increased by 1, except when the detected error was a bit error during the sending of an active error flag or an overload flag.
2. When a receiver detects a dominant bit as the first bit after sending an error flag, the rx_count will be increased by 8.
3. When a transmitter sends an error flag, the tx_count is increased by 8.
Exception 1:If the transmitter is error passive and detects an ack error because of not detecting a dominant ack and does not detect a dominant bit while sending its passive error flag.
Exception 2: If the transmitter sends an error flag because a stuff error occurred during arbitration whereby the stuff bit is located before the RTR bit, and should have been recessive, and has been sent as recessive but monitored as dominant.
4. If a transmitter detects a bit error while sending an active error flag or an overload flag, the tx_count is increased by 8.
5. If a receiver detects a bit error while sending an active error flag or an overload flag the rx_count is increased by 8.
6. Any node accepts up to 7 consecutive dominant bits after sending an active or passive error flag or an overload flag. After detecting the 14th consecutive dominant bit (in the case of an active error flag or an overload flag), or after detecting the 8th consecutive dominant bit following a passive error flag, and after each sequence of additional 8 consecutive dominant bits every transmitter increases its tx_count by 8 and every receiver increases its rx_count by 8.
7. After the successful transmission of a message (getting ack and no error until end of frame is finished) tx_count is decreased by 1 unless it was already 0.
8. After the successful reception of a message (reception withour error up to the ack slot and the successful sending of the ack bit), rx_count is decreased by 1 if it was between 1 and 127. If rx_count was 0 it stays 0, and if it was greater than 127, it will be set to a value between 119 and 127.
Note: If a node is the only one on the bus (or during startup the only one that has become active), and it transmitts a message, it will get an acknowledgement error, and will retransmit the message. This may lead to that node going to error passive mode, but not to it becoming bus off (due to exception 1 under point 3).
Error signalling
When an error is detected by a node it sends an error flag on the bus. This prevents any other node from accepting the message and ensures consistency of data throughout the network.
The active error flag consists of six low bits, and is used if the node transmitting the error frame is in active error state. As low is dominant all other nodes will detect bit stuffing violation and send their own error flags. After this, nodes that want to transmit (including the one sending the interrupted message) will start to do so. As usual, the node whose message has the highest priority will win arbitration and send its message.
If the CAN controller is in error passive mode the error frame will consist of six passive (high) bits. Since the error flag only consists of passive bits, the bus is not affected. If no other node detected an error, the message will be sent uninterrupted. This ensures that a node having problems with receiving can not block the bus.
All of this advanced error handling is done automatically by the CAN controller, without any need for the host microcontroller to do anything. This is one of the big advantages of CAN.
Bit timing
CAN has advanced features for coping with the time delays found in long bus lengths (in comparison to the bit rate) and coping with differences in clock chrystal frequencies for nodes on the bus.
The choice of bit timing is very important since it decides the bit rate, the sample point and the ability to resynchronise.
Bit segments (as in Bosch standard)
Each bit is divided into four segments – the synchronisation segment, the propagation segment and the phase segments one and two. Each segment consists of one or more time quanta.
A time quantum is a fixed amount of time which is derived from the CAN controller clock with a prescale factor.

Synchronisation segment (Synch_Seg)
The synchronisation segment is used to synchronise the various nodes on the bus. When a bit is sent on the bus, the leading edge is expected to be within this segment.
This segment is always one time quantum long.
Propagation segment (Prop_Seg)
The Propagation Segment is needed to compensate for the delay in the bus lines.
The segment size is programmable between 1 and 8 time quanta.
Phase Segment 1 (Phase_Seg1), Phase Segment 2 (Phase_Seg2)
These segments can be used lengthened or shortened by resynchronisation.
Bit segments (as in implementations like Intel 527 and C167CR)
In most implementations of CAN controllers the segments seem to be implemented in a different way than described in the standard. The synch segment looks as in the standard and consists of one time quantum. The big difference is that the propagation segment and the phase segment 1 in the standard have been combined into one segment, TSEG1. Phase segment 2 is left untouched, but is renamed to TSEG2.

Normally there is only one sample point for each bit. In this case, the sample point is in the edge between TSEG1 and TSEG2. However, some CAN controllers can also sample each bit three times. In this case, the bit will be sampled three quanta in a row, with the last sample being taken in the edge between TSEG1 and TSEG2.
Three samples should only be used for relatively slow baudrates.
Calculation of baudrate and sample point
Baudrate
The baudrate of the bus can be calculated from:
Baudrate = fcrystal / (2*n*(BRP+1))
where n is the number of time quanta for one bit and is defined as :
n = SYNCHSEG+TSEG1+TSEG2
BRP is the value of the BaudRate Prescaler.
Warning: some CAN controllers (like Intel 526) has an other way of calculating the number of time quantas in a bit! Consult your users manual.
Sample point
Quantabeforesample = TSEG1 + 1
Quantaaftersample = TSEG2
Often the sample point is given in percent of the bit time. This is:
(TSEG1+1)/(TSEG1+1+TSEG2)
Warning: some CAN controllers (like the C167CR) use an other way of calculating Quantaaftersample. Consult your manual!
Resynchronisation
Resynchonisation is done to compensate for bus delays and nodes that have different chrystal frequencies. Synchronisation is normally only done on the edge from recessive to dominant bus level.
Hard resynchronisation
When the bus is idle and the controller detects a start bit, it resynchronises itself so that the edge is inside the Synch segment. Hard resynchronisation can only be made for the first bit in a frame.
Resynchronisation within a frame
CAN controllers have the ability to synchronise on bit edges also within a frame. The (re)Synchronisation Jump Width (SJW) decides the maximum number of time quanta that the controller can resynchronise every bit.
* Resynchonisation of a receiver to a slower transmitter is handled as follows:
If a recessive-to-dominant edge appears inside TSEG1 and the edge is less than or equal to SJW quanta inside, TSEG1 is restarted . If the edge was more than SJW quanta inside, TSEG1 is lengthened with SJW quanta.
* Resynchronisation of a receiver to a faster transmitter:
If a recessive-to-dominant edge appears inside TSEG2 , TSEG2 is shortened by the number of quantas necessary to make the edge be outside TSEG2. However, TSEG2 can be shortened no more than SJW quanta.

CAN bus physical layer
The physical layer is not part of the Bosch CAN standard. However, in the ISO standards transceiver characteristics is included.
CAN transmits signals on the CAN bus which consists of two wires, a CAN-High and CAN-Low. These 2 wires are operating in differential mode, that is they are carrying inverted voltages (to decrease noise interferance) The voltage levels, as well as other characteristics of the physical layer, depend on which standard is being used.
ISO 11898
The voltage levels for a CAN network which follows the ISO 11898 (CAN High Speed) standard are described in the picture and table below.
Signal

recessive state

dominant state

unit

min
nominal
max
min
nominal
max

CAN-High
2.0
2.5
3.0
2.75
3.5
4.5
Volt
CAN-Low
2.0
2.5
3.0
0.5
1.5
2.25
Volt
Note that for the recessive state, nominal voltage for the two wires is the same. This decreases the power drawn from the nodes through the termination resistors. These resistors are 120ohm and are located on each end of the wires. Some people have played with using central termination resistors (that is, putting them in one place on the bus). This is not recommended, since that configuration will not prevent reflection problems.
ISO 11519
The voltage levels for a CAN network which follows the ISO 11519 (CAN Low Speed) standard are described in the table below.
Signal

recessive state

dominant state

unit

min
nominal
max
min
nominal
max

CAN-High
1.6
1.75
1.9
3.85
4.0
5.0
Volt
CAN-Low
3.1
3.25
3.4
0
1.0
1.15
Volt
ISO 115519 does not require termination resistors. They are not necessary because the limited bit rates (maximum 125 kB/s) makes the bus insensitive to reflections.
The voltage level on the CAN bus is recessive when the bus is idle.
Bus lengths
The maximum bus length for a CAN network depends on the bit rate used. It is required that the wave front of the bit signal has time to travel to the most remote node and back again before the bit is sampled. This means that if the bus length is near the maximum for the bit rate used, one should choose the sampling point with utmost care – one the other hand, one should always do that!
Below is a table of different bus lengths and the corresponding maximum bit rates.
Bus length (metres)
Maximum bit rate (bit/s)
40
1 Mbit/s
100
500 kbit/s
200
250 kpit/s
500
125 kbit/s
6 km
10 kbit/s

Cable
According to the ISO 11898 standard, the impedance of the cable shall be 120 +- 12 ohms. It should be twisted par, shielded or unshielded. Work is in progress
on the single-wire standard SAE J2411.

CAN implementations
Different implementations – BasicCAN and FullCAN
There is no standard for how CAN controllers shall be implemented or how they shall communicate with their host microcontroller. There are two main implementation stategys for CAN controllers today. They are called BasicCAN and FullCAN.
The main difference between these strategys is how interesting messages are filtered out, that is how it is decided what messages are interesting and which are not. There are also differences in how remote frames are answered, and on how messages are buffered. The differences will effect how much load is put on the host microcontroller.
BasicCAN
BasicCAN is usually used in cheaper standalone CAN controllers or in smaller microcontrollers with integrated CAN controller.
A BasicCAN controller normally has two receive buffers and one transmit buffer. The receive buffers are arranged in a FIFO structure, and a message can be received into the one buffer while the microcontroller is reading the information from the other buffer. If a message is received while both receive buffers are full, the oldest messages are kept. This means that newer messages might be lost if the host microcontroller does not read the messages fast enough.
A message is sent by writing it to the transmit buffer.
Interesting messages are filtered out using two registers, that operate on the message identifier. Each bit in the identifier are checked against the filter. If the message matches the filter it is stored in one of the receive buffers.
Each bit of the identifier filter can be set to ‘1′, ‘0′ or ‘don’t care’. Often the filter only operates on eight of the eleven bits in the identifier (standard CAN). This means the three lower bits in the identifier are always ‘don’t care’.
When BasicCAN is used it is important to choose identifiers with utmost care, so that the window of the filter can be kept as small as possible. All message that is let through the filter must be read and checked by the microcontroller. This means that the final filtering is done in software.
A BasicCAN controller has no support for automatically answering remote frames, which means that the application will have to handle them. This will put extra load on the microcontroller or processor, but will make sure that the value returned is updated.
BasicCAN features
Transmit
The application fills complete Tx register including ID,RTR,datalength,data -> every ID can be transmitted
Receive
Every CAN message can be received
Normally two receive buffers in FIFO structure
Global message filtering. It is normally not possible to set up the filter so that only the interesting messages are let through -> final filtering must be done by the application
Remote frame handling
Remote frame are answered by the application
Overrun philosophy
Keep the oldest message (newer messages will be lost)
FullCAN
FullCAN is used in more expensive, high performance CAN controllers and microcontrollers. The FullCAN controller has a set of buffers called mailboxes. On initialization, each mailbox is assigned an identifier and is set to transmit or receive.
When the CAN controller receives a message it checks the mailboxes in order to see is there is a receive mailbox with the same identifier as the message. If such a mailbox is found, the message is stored in it and the host controller is notified. Otherwise the message is discarded.
When transmitting a message the message length and data is written to the transmit mailbox with the correct identifier.
If a remote message is received the controller checks the remote identifier against the transmit mailboxes. If a match is found, the controller automatically sends a message with the identifier and data contained in that mailbox. This means that the microcontroller gets a lower load, and that the software does not have to handle remote messages. However, if the mailbox has not been updated in a long time, the information sent to the network will be old. This have to be considered when writing the software.
With a FullCAN controller it is possible to filter out only the exact message types that are interesting. This type of controller will therefore give a lower load on the host microcontroller. However, the number of mailboxes are limited. The largest number of mailboxes present in a CAN controller today is ???.
With some controllers it is possible to reconfigure the mailboxes dynamically. However, that does not completely solve the problem. Therefore some controllers are mixed CAN controllers, that is they have mailboxes, but also BasicCAN buffers.
FullCAN controllers have support for automatically answering remote frames. This will decrease the load on the host microcontroller or processor, but may also mean that old information is sent. It is very important to take this into consideration when writing your application.
FullCAN features
Transmit
Transmit mailboxes initialised once
Only data bytes written before transmission
Receive
Only messages with the IDs defined in receive mailboxes can be received
No double buffering for mailboxes
Full acceptance filtering (only the exact message IDs are let through)
Remote frame handling
Remote frames are answered automatically by the controller
Overrun philosophy
Keep the newest message (older messages with the same ID will be lost)

FAQ – Frequently Asked Questions
I will update this section as I receive questions from the readers of this page.
Question: The transmitter considers the data / remote frame transfer valid if no error occurs until the end of the corresponding EOF delimiter.
Why does the receiver consider the transfer valid when there is no error until the last but one bit of the EOF delimiter ( why doesn’t the receiver check until the end of the corresponding EOF delimiter, like what the transmitter do. ) ?
Answer: If a receiver detecs a corrupt frame, it sends an error frame (in the normal case, an active error flag). This will cause all other receivers AND the transmitter to detect a corrupt frame, they all send their error frames and the transmitter resends the frame (normal arbitration is used).

Now assume that the protocol was implemented the way that you suggest, what would happen if the receiver detects an error on the last bit of the EOF? It will start to send its error frame, but the transmitter (and other receivers) may already have have accepted the frame as valid (not having detected any errors up to and including the whole eof). This means that one (or more) receivers have not received a valid frame, but the transmitter thinks that it is sent ok and will not repeat it!!!

The problem is solved in CAN – if a receiver detects an error in the last bit that it cares about (the last but one bit of the eof) it will send an error frame. This will corrupt the last bit that the transmitter cares about (the last bit of the eof) and he will retransmit the message.
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