UNIX O/S Tutorial

1. How are devices represented in UNIX?

All devices are represented by files called special files that are located in/dev directory. Thus, device files and other files are named and accessed in the same way. A 'regular file' is just an ordinary data file in the disk. A 'block special file' represents a device with characteristics similar to a disk (data transfer in terms of blocks). A 'character special file' represents a device with characteristics similar to a keyboard (data transfer is by stream of bits in sequential order).

2. What is 'inode'?

All UNIX files have its description stored in a structure called 'inode'. The inode contains info about the file-size, its location, time of last access, time of last modification, permission and so on. Directories are also represented as files and have an associated inode. In addition to descriptions about the file, the inode contains pointers to the data blocks of the file. If the file is large, inode has indirect pointer to a block of pointers to additional data blocks (this further aggregates for larger files). A block is typically 8k.

Inode consists of the following fields:

 File owner identifierØ

 File typeØ

 File access permissionsØ

 File access timesØ

 Number of linksØ

 File sizeØ

 Location of the file dataØ

3. Brief about the directory representation in UNIX

A Unix directory is a file containing a correspondence between filenames and inodes. A directory is a special file that the kernel maintains. Only kernel modifies directories, but processes can read directories. The contents of a directory are a list of filename and inode number pairs. When new directories are created, kernel makes two entries named '.' (refers to the directory itself) and '..' (refers to parent directory).

System call for creating directory is mkdir (pathname, mode).

4. What are the Unix system calls for I/O?

 open(pathname,flag,mode) - open fileØ

 creat(pathname,mode) - create fileØ

 close(filedes) - close an open fileØ

 read(filedes,buffer,bytes) - read data from an open fileØ

 write(filedes,buffer,bytes) - write data to an open fileØ

 lseek(filedes,offset,from) - position an open fileØ

 dup(filedes) - duplicate an existing file descriptorØ

 dup2(oldfd,newfd) - duplicate to a desired file descriptorØ

 fcntl(filedes,cmd,arg) - change properties of an open fileØ

 ioctl(filedes,request,arg) - change the behaviour of an open fileØ

The difference between fcntl anf ioctl is that the former is intended for any open file, while the latter is for device-specific operations.

5. How do you change File Access Permissions?

Every file has following attributes:

 owner's user ID ( 16 bit integer )Ø

 owner's group ID ( 16 bit integer )Ø

 File access mode wordØ

'r w x -r w x- r w x'

(user permission-group permission-others permission)

r-read, w-write, x-execute

To change the access mode, we use chmod(filename,mode).

Example 1:

To change mode of myfile to 'rw-rw-r--' (ie. read, write permission for user - read,write permission for group - only read permission for others) we give the args as:

chmod(myfile,0664) .

Each operation is represented by discrete values

'r' is 4

'w' is 2

'x' is 1

Therefore, for 'rw' the value is 6(4+2).

Example 2:

To change mode of myfile to 'rwxr--r--' we give the args as:

chmod(myfile,0744).

6. What are links and symbolic links in UNIX file system?

A link is a second name (not a file) for a file. Links can be used to assign more than one name to a file, but cannot be used to assign a directory more than one name or link filenames on different computers.

Symbolic link 'is' a file that only contains the name of another file.Operation on the symbolic link is directed to the file pointed by the it.Both the limitations of links are eliminated in symbolic links.

Commands for linking files are:

Link ln filename1 filename2

Symbolic link ln -s filename1 filename2

7. What is a FIFO?

FIFO are otherwise called as 'named pipes'. FIFO (first-in-first-out) is a special file which is said to be data transient. Once data is read from named pipe, it cannot be read again. Also, data can be read only in the order written. It is used in interprocess communication where a process writes to one end of the pipe (producer) and the other reads from the other end (consumer).

8. How do you create special files like named pipes and device files?

The system call mknod creates special files in the following sequence.

1. kernel assigns new inode,

2. sets the file type to indicate that the file is a pipe, directory or special file,

3. If it is a device file, it makes the other entries like major, minor device numbers.

For example:

If the device is a disk, major device number refers to the disk controller and minor device number is the disk.

9. Discuss the mount and unmount system calls

The privileged mount system call is used to attach a file system to a directory of another file system; the unmount system call detaches a file system. When you mount another file system on to your directory, you are essentially splicing one directory tree onto a branch in another directory tree. The first argument to mount call is the mount point, that is , a directory in the current file naming system. The second argument is the file system to mount to that point. When you insert a cdrom to your unix system's drive, the file system in the cdrom automatically mounts to /dev/cdrom in your system.

10. How does the inode map to data block of a file?

Inode has 13 block addresses. The first 10 are direct block addresses of the first 10 data blocks in the file. The 11th address points to a one-level index block. The 12th address points to a two-level (double in-direction) index block. The 13th address points to a three-level(triple in-direction)index block. This provides a very large maximum file size with efficient access to large files, but also small files are accessed directly in one disk read.

11. What is a shell?

A shell is an interactive user interface to an operating system services that allows an user to enter commands as character strings or through a graphical user interface. The shell converts them to system calls to the OS or forks off a process to execute the command. System call results and other information from the OS are presented to the user through an interactive interface. Commonly used shells are sh,csh,ks etc.

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Microprocessor - II

1. Access time is faster for
a) ROM b) SRAM c) DRAM

2. In 8279 Strobed input mode, the control line goes low. The data on return lines is strobed in the ____.
a) FIFO byte by byte b) FILO byte by byte c) LIFO byte by byte
d) LILO byte by byte.

3. ___ bit in ICW1 indicates whether the 8259A is cascade mode or not?
a) LTIM=0 b) LTIM=1 c) SNGL=0 d) SNGL=1

4. In 8255, under the I/O mode of operation we have __ modes. Under which mode
will have the following features
i) A 5 bit control port is available.
ii) Three I/O lines are available at Port C.
a) 3, Mode2 b) 2, Mode 2 c) 4, Mode 3 d) 3, Mode 2

5. In ADC 0808 if _______ pin high enables output.
a) EOC b) I/P0-I/P7 c) SOC d) OE

6. In 8279, a scanned sensor matrix mode, if a sensor changes its state, the ___ line goes ____ to interrupt the CPU.
a) CS, high b) A0, high c) IRQ, high d) STB, high

7. In 8279 Status Word, data is read when ________ pins are low, and write to the
display RAM with ____________ are low.

a) A0, CS, RD & A0, WR, CS. b) CS, WR, A0 & A0, CS, RD
c) A0, RD & WR, CS d) CS, RD & A0, CS.

8. In 8279, the keyboard entries are debounced and stored in an _________, that is
further accessed by the CPU to read the key codes.
a) 8-bit FIFO b) 8-byte FIFO c) 16 byte FIFO d) 16 bit FIFO

9. The 8279 normally provides a maximum of _____ seven segment display
interface with CPU.
a) 8 b) 16 c) 32 d) 18

10. For the most Static RAM the write pulse width should be at least
a) 10ns b) 60ns c) 300ns d) 1µs
11. BURST refresh in DRAM is also called as
a) Concentrated refresh b) distributed refresh c) Hidden refresh d) none

12. For the most Static RAM the maximum access time is about
a) 1ns b) 10ns c) 100ns d) 1µs

13. Which of the following statements on DRAM are correct?
i) Page mode read operation is faster than RAS read.
ii) RAS input remains active during column address strobe.
iii) The row and column addresses are strobed into the internal buffers using RAS
and CAS inputs respectively.
a) i & iii b) i & ii c) all d) iii

14. 8086 microprocessor is interfaced to 8253 a programmable interval timer. The
maximum number by which the clock frequency on one of the timers is divided by
a) 216 b) 28 c) 210 d) 220

15. 8086 is interfaced to two 8259s (Programmable interrupt controllers). If 8259s
are in master slave configuration the number of interrupts available to the 8086
microprocessor is
a) 8 b) 16 c) 15 d) 64