CS4315A. Berrached:CMS:UHD1 Operating System Structures Chapter 3.

CS4315 A. Berrached:CMS:UHD 1

Operating System Structures

Chapter 3


Operating System Structures

• OS Design Constraints• OS Basic Functions• OS Structures


Design Constraints

• Performance• Protection & Security• Correctness• Maintainability• Commercial factors


Performance• People use computers for the potential of rapid

information processing• There are several measures of performance

– throughput– response time

• The OS is an overhead function => should not use too much of machine resources

• Provide an environment in which programmers can produce solutions in a cost-effective manner

==> trade off


Correctness & Maintainability

• Correctness - refers to whether OS functions meet their requirements.

• Correctness is the most basic requirement on which all other requirement are based- e.g. security depends on correct operation of OS => trusted vs un-trusted software

• Maintainability - refers to the ease with which software can be changed/extended, bugs can be fixed, etc.


OS Basic Functions

• Device management• Process & resource management• Memory Management• File Management


Device Management

• OS Manages the allocation, sharing and isolation of I/O devices (disks, tapes, terminals, etc.)

• Most Operating Systems treat all devices in the same general manner– UNIX treats them all like files

• Chapters 4 & 5 discuss Device Management


Process & Resource Management

• A process is the basic unit of computation• Resources are the elements needed by a process so

that it can execute– CPU, Memory, I/O devices, data etc.

• OS provides a set of process management mechanisms: for process creation , blocking, resumption, termination ,etc


Process & Resource Management

• OS manages computer resources so that multiple processes can execute simultaneously– CPU scheduling– resource allocation, sharing & process

synchronization– resource allocation

• Chapters 6 - 10


Memory Management

• Allocation and use of the primary memory resource– memory allocation among competing processes– enforce memory isolation and sharing

• Most modern OS support virtual memory.– Virtual memory allows processes to access

data in secondary storage as if it were in main memory.

• Chapter s11&12


File Management

• Information that need to be saved "permanently" must be stored in a secondary storage device e.g. a disk, tape, etc.

• Files are an abstraction of secondary storage devices• File manager is responsible for

– managing the file system: file & directory creation and manipulation

– mapping files into physical storage devices


Basic OS Functions


OS Structures-Simple Approach

• MS-DOS - written to provide the most functionality in the least space– not divided into modules– Although MS-DOS has some structures, its

interfaces and levels of functionality are not well separated.

• application programs are able to access BIOS routines directly (bypassing DOS).


OS Structures--UNIX

• UNIX -- modular• UNIX consists of two separate parts:

– System programs (Shells and commands, compilers and interpreters, system libraries)

– The kernel: part of OS that is most critical to its correct operation (trusted)

• provides CPU scheduling, memory management, file management, and other operating system functions.

CS4315A. Berrached:CMS:UHD1 Operating System Structures Chapter 3.

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