CMSC 412 Lecture 3: System Calls and Processes - Prof. Jeffrey K. Hollingsworth, Study notes of Operating Systems

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• S02 (lect 3)

Announcements

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Program #1^ – Is on the web

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Reading^ – Chapter 3– Chapter 4 (for Thursday)

• S02 (lect 3)

System Calls

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Provide the interface between application programsand the kernel

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Are like procedure calls^ – take parameters– calling routine waits for response

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Permit application programs to access protectedresources

load r0, xsystem call 10User Program

Operating System

Code forsys call 10 (kernel)

register r

• S02 (lect 3)

Types of System Calls

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File Related^ –

open, create

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read, write

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close, delete

-^

get or set file attributes

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Information^ –

get time

-^

set system data (OS parameters)

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get process information (id, time used)

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Communication^ –

establish a connection

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send, receive messages

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terminate a connection

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Process control^ –

create/terminate a process (including self)

• S02 (lect 3)

System Structure

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Simple Structure (or no structure)^ – any part of the system may use the functionality of the rest of

the system

• MS-DOS (user programs can call low level I/O routines)

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Layered Structure^ – layer n can only see the functionality that layer n-1 exports– provides good abstraction from the lower level details

• new hardware can be added if it provides the interface

required of a particular layer

• system call interface is an example of layering– can be slow if there are too many layers

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Hybrid Approach^ – most real systems fall somewhere in the middle

• S02 (lect 3)

Processes

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What is a process?^ – a program in execution– “An execution stream in the context of a particular state”– a piece of code along with all the things the code can affect

or be affected by.

• this is a bit too general. It includes all files and

transitively all other processes

• only one thing happens at a time within a process

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What’s not a process?– program on a disk - a process is an active object, but a

program is just a file

• S02 (lect 3)

Multi-programming

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Systems that permit more than one process at once^ – virtually all computers today

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Permits more efficient use of resources^ – while one process is waiting another can run

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Provides natural abstraction of different activities^ – windowing system– editor– mail daemon

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Preemptive vs. non-preemptive muti-programming^ – preemptive means that a process can be forced off the

processor by the OS

• provides processor protection

• S02 (lect 3)

Process State Transitions

new

readyready

runningrunning

waitingwaiting

terminatedterminated

admitted

interrupt dispatch

I/O request or event wait

I/O request orevent wait done

Kill

exit

• S02 (lect 3)

Components of a Process

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Memory Segments^ – Program - often called the text segment– Data - global variables– Stack - contains activation records

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Processor Registers^ – program counter - next instruction to execute– general purpose CPU registers– processor status word

• results of compare operations
  • floating point registers