UNIX Unit 1- Architecture of Unix - By Pratima

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

Unix Architecture 3

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

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

Unix Functional Layer Model 7

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

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

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

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

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

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

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

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