1. UNIX
Unit 1- Architecture of Unix
- By
Pratima

2. Unix Architecture Typical computer system consists of:
Hardware
Operating system
Applications and utilities
Typical OS includes a user interface or command interpreter 2

3. Unix Architecture 3

4. UNIX is no longer a small system but it certainly is a powerful one.
Unix Architecture
UNIX is no longer a small system but it certainly is a powerful one.
Layered approach is basis for Unix security
Division of Labor: Kernel n Shell
The File Process
The System Calls 4

6. Levels of Unix Software
Unix uses a layer approach of defining software
Layered approach is basis for Unix security
At the lowest level, the level closest to the hardware, are the basic hardware interface modules
Process scheduling
Memory management
Interrupt handling
Low level device control
Disk system management and data buffering
Process synchronization and inter-process communication (IPC) 6

7. Unix Functional Layer Model7

8. 1. Division of Labor: Kernel & Shell
“Filter ideas” is division of labor between two agencies- The Kernel and Shell
The Kernel interacts with the machine's hardware & the Shell with the user (Mapping between user requests and device driver actions)
The Kernel is the core of the operating system
1. Its a collections of routines mostly written in c
2. Its loaded into memory when its booted n communicates with hardware directly
3. Programs access the Kernel through set of System calls
It actually also performs the requested service
Process creation and termination (Schedules the process)
I/O services
File/disk access services (manages sys memory)
Terminal handling services
Decides the Priorities n perform other tasks. 8

9. Shell The uppermost layer consists of user processes running:
Shells
Unix commands
Utility programs
User application programs
User processes:
Protected from from other users
Have no access to Unix kernel routines except through the system call interface
Cannot directly access kernel memory space 9

10. Shell
Command Interpreter is a job that is handled by the outer part of the OS called as Shell OR
Unix command interface is a separate program referred to as the shell
The Shell actually acts as an interface between user & the Kernel
There could be several Shells in action-one for each user who logged in.
After using commands, shell communicates with Kernel to see that command is executed.
It is represented by sh(Bourne shell), csh(C shell),
ksh(Korn shell) or bash(Bash shell) 10

11. Kernel Space
Kernel space is the region of memory where all kernel services are provided via kernel processes
Any process executing in kernel space is said to be executing in kernel mode
Kernel space is a privileged area; the user has access to it only through the system call interface
A user does not have direct access to either all machine instructions or devices
A kernel process does have direct access to both 11

12. 2. The File and Process “File have places and Process have Life”
A File is a an array of bytes n also a part of single hierarchical structure.
Can be moved from one place to another
It considers even directories and devices as a single file system
The Process is an entity, name given to a file wen its executed as a program
IT also belongs to a separate hierarchical tree structure 12

13. 3. The System Calls
System calls provide a programming interface that allows user programs to access kernel functions
There are a large number of system calls that perform functions to manage system resources such as memory, disk storage, and peripherals
System calls are defined in a run-time library that provides a mapping of the system call interface to the kernel routine(s) that perform the system functions
The shell also provides functionality equivalent to many of the system calls
System call arguments are passed in the u area (user area) of the calling process 13

14. System Usage Accounting Services
Unix provides some general accounting services
As each process terminates, an accounting record is made available describing the resources used by the process
Name of the process
Amount of user and system CPU time used
Elapsed (wall clock) time
Average amount of memory used
Number of disk I/O operations done
UID and GID of the process
Terminal from which the process was started 14

15. Features A Multiuser System A Multitasking System
The Building-Block Approach
The Unix Toolkit
Pattern Matching
Programming Facility
Documentation 15