Topic 6 : Introduction to Operating Systems L & E: Pages 1-19, 17-29 Tanenbaum: Pages 1-5, 15-16, 50-53

C.Sc. 110: Computer SystemsC.Sc. 110: Computer Systems

Topic 6 : Introduction to Operating Systems

L & E: Pages 1-19, 17-29

Tanenbaum: Pages 1-5, 15-16, 50-53


Book Details Reminder:

• We will use two different books for this course.• “Fundamentals of Operating Systems”,

A. M. Lister and R. D. Eager, Fifth Edition, Macmillan Computer Science Series, ISBN. 0-333-59848-2, £13.99.

• “Operating Systems Design and Implementation”, A. S. Tanenbaum, Prentice-Hall International, ISBN. 0-13-630195-9, about £29.99.

• www.comp.lancs.ac.uk/computing/staff/kc/keiths_teaching.html


Operating SystemsProvide a Virtual Machine

Hardware

Operating System Specific Interface

O/S

An OS is responsible for:

Sharing resources

Ensuring optimisation of

resources


Wrote a minimal dispatcher.

• Basic idea is to load the program from disk and start it executing.

• When its finished start executing another one.

• During its period of execution the program is called a process.


Approach for Simple Dispatcher

• Sit in a loop waiting for a program to be ready to run.

• Load the specified program from disk into memory.

• Use base and limit registers to make sure program addresses work.

• When the program has finished go back to the loop and wait for another to be ready.


This Lecture….

• Doing many things at the same (or apparently the same) time…


Some Simple Definitions

• Sequential = doing things one at a time.

• Concurrent = doing things (apparently) at the same time.

• Parallel = doing things really at the same time.


Example Programs

• Two programs to be run.Begin

putline (“1”);putline (“2”);putline (“3”);

End.

Beginputline (“A”);putline (“B”);putline (“C”);

End.


Sequential Execution

Job One Job Two Output




123




123ABC


Concurrent Execution





1




1A




1A2




1A2B




1A2B3




1A2B3C


Parallel Execution



Parallel Execution


1 A


Parallel Execution


12

AB


Parallel Execution


123

ABC


Summary of Execution Patterns

Sequential

Concurrent

Parallel


Extending Our Dispatcher to Deal With Concurrency• Load programs into different parts of

memory.• Make each program save its data and where

it is when it wants to temporarily stop running (c.f. interrupts).

• Modify the dispatcher so it can jump to these saved points.


Why Multiple Programs?

• Support multiple tasks by users.

• Allow tasks to proceed in background.

• Allow programs to be structured as

independent tasks.

• Support multiple users.


The Old Application Program

Main Memory Address Machine Language Instruction

2

345

6

LOAD 6

MULT 6STORE 6

JUMP

0

1

NULL

LDBASE #0

1

10


The New Application Program

Main Memory Address

4 LDA #8

0

1

STB

LDBASE #0

3

2

3

JUMP

4 # PC Stored Here

@3

12 STPC+2 B

13 JUMP 0


The New Dispatcher

• Sit in a loop waiting for a program to be ready to run.

• If it’s a new program load the specified program from disk into memory and jump to the start.

• If it’s an old program ... restore the registers and jump back via stored PC.


The New Dispatcher

JUMP IDLE STARTIT: LDBASE BASETABLE+

JUMPSUB SAVE_IT

IDLE: LDA PTEJNZ DISPATCHJUMP IDLE

0

12

345

PTE = 0Acc = 0

Base = 0Limit = 0

Index = 0


The New Dispatcher

DISPATCH: STILDA BASE_TABLE+JNZ RESTORE_ITJUMPSUB LOADITLDLIMIT LIMITTABLE+JUMP STARTIT

PTE = 0Acc = 0

Base = 0Limit = 0

Index = 0

67891011


The New Dispatcher

SAVE_IT: STA ACC_TABLE+RETURN

RESTORE_IT LDA ACC_TABLE+LDLIMIT LIMIT_TABLE+JUMP START_IT

1213

141516

PTE = 0Acc = 0

Base = 0Limit = 0

Index = 0


Supporting Data Structures

BASETABLE: 000

LIMITTABLE: 000

ACCTABLE: 000


Demo


JUMPSUB SAVE_IT


0

12

345

PTE = 0Acc = 0

Base = 0Limit = 0

Index = 0


Demo


JUMPSUB SAVE_IT


PTE =1Acc = 1

Base = 0Limit = 0

Index = 0

0

12

345


Demo


JUMPSUB SAVE_IT


PTE = 1Acc = 1

Base = 0Limit = 0

Index = 0

0

12

345


Demo


PTE = 1Acc = 1

Base = 0Limit = 0

Index = 1

67891011


Demo


PTE = 1Acc = 0

Base = 0Limit = 0

Index = 1

67891011


Demo


PTE = 1Acc = 0

Base = 0Limit = 0

Index = 1

67891011


Demo


PTE = 1Acc = 0

Base = 0Limit = 0

Index = 1

67891011

Assume LOADIT finds some space in memory and fixes thebase and limit registers accordingly.



BASETABLE: 01000

LIMITTABLE: 01990

ACCTABLE: 000


Demo


PTE = 1Acc = 0

Base = 0Limit = 199

Index = 1

67891011


Demo


PTE = 1Acc = 0

Base = 0Limit =199

Index =1

67891011


Demo


JUMPSUB SAVE_IT


PTE = 1Acc = 0

Base = 100Limit = 199

Index = 1

0

12

345


Demo

Main Memory Address

104 LDA #8

100

101

STB

LDBASE #0

3

102

103

JUMP

4 # PC Stored Here

@3

112 STPC+2 B

113 JUMP 0

PTE =0Acc = 0


Index = 1


Demo

Main Memory Address

104 LDA #8

100

101

STB

LDBASE #0

3

102

103

JUMP

4 # PC Stored Here

@3

112 STPC+2 B

113 JUMP 0

PTE = 0Acc = 8

Base = 100Limit =199

Index =1


Demo

Main Memory Address

104 LOAD #8

100

101

STB

LDBASE #0

3

102

103

JUMP

4 # PC Stored Here

@3

112 STPC+2 B

113 JUMP 0

PTE = 0Acc =9


Index = 1

program instructions leading to it wanting to halt temporarily


Demo

Main Memory Address

104 LOAD 8

100

101

STB

LDBASE #0

3

102

103

JUMP

4 # PC Stored Here

@3

112 STPC+2 B

113 JUMP 0

PTE = 0Acc = 8


Index = 1

B = 14


Demo

Main Memory Address

104 LOAD 8

100

101

STB

LDBASE #0

3

102

103

JUMP

4 # PC Stored Here

@3

112 STPC+2 B

113 JUMP 0

PTE = 0Acc = 8


Index = 1

B = 14


Demo

Main Memory Address

104 LOAD 8

100

101

STB

LDBASE #0

3

102

103

JUMP

14 # PC Stored Here

@3

112 STPC+2 B

113 JUMP 0

PTE = 0Acc = 8


Index = 1

B = 14


Demo

Main Memory Address

104 LOAD 8

100

101

STB

LDBASE #0

3

102

103

JUMP

14 # PC Stored Here

@3

112 STPC+2 B

113 JUMP 0

PTE = 0 Acc = 8

Base = 0Limit = 199

Index = 1

B = 14


Demo


JUMPSUB SAVE_IT


0

12

345

PTE = 0Acc = 8

Base = 0Limit = 199

Index = 1


Demo



1213

141516

PTE = 0Acc = 8

Base = 0Limit = 199

Index = 1



BASETABLE: 01000

LIMITTABLE: 01990

ACCTABLE: 080


Demo



PTE =0Acc = 8

Base = 0Limit = 199

Index = 1

1213

141516


Demo


JUMPSUB SAVE_IT


PTE =0Acc = 0

Base = 0Limit = 199

Index = 1

0

12

345


Demo


JUMPSUB SAVE_IT


PTE = 0Acc = 0

Base = 0Limit = 0

Index = 1

0

12

345


Demo


JUMPSUB SAVE_IT


PTE = 0Acc = 0

Base = 0Limit = 0

Index = 1

0

12

345


Demo


JUMPSUB SAVE_IT


PTE = 1Acc = 1

Base = 0Limit = 0

Index = 1

0

12

345


Demo


JUMPSUB SAVE_IT


PTE =1Acc = 1

Base = 0Limit = 0

Index = 1

0

12

345


Demo


PTE =1Acc = 1

Base = 0Limit =0

Index = 1

67891011


Demo


PTE = 0Acc = 100

Base = 0Limit =0

Index = 0

67891011


Demo


PTE =1Acc =100

Base = 0Limit = 0

Index =1

67891011


Demo



1213

141516

PTE =1Acc =8

Base = 0Limit = 0

Index =1


Demo



PTE =1Acc =8

Base = 0Limit = 199

Index =1

1213

141516


Demo



1213

141516

PTE =1Acc =8

Base = 0Limit = 199

Index =1


Demo


JUMPSUB SAVE_IT


0

12

345

PTE =1Acc =8


Index =1


Demo

Main Memory Address

104 LOAD #8

100

101

STB

LDBASE #0

3

102

103

JUMP

14 # PC Stored Here

@3

112 STPC+2 B

113 JUMP 0

PTE =0Acc =8


Index =1


Demo

Main Memory Address

104 LOAD 8

100

101

STB

LDBASE #0

3

102

103

JUMP

14 # PC Stored Here

@3

112 STPC+2 B

113 JUMP 0

114

PTE =0Acc =8


Index =1


Whew ! Glad That’s Over


Process Context

• We save two pieces of information: accumulator and PC

• In practice processes have much more context that needs to be saved.

• Term process context is used to describe all of the state a process must save to be able to resume execution from the same point.


Which Process to Run

• Question: Why stop a process and change to another during its execution ?

• Answer: Lots of reasons .....• Want to give other processes a chance to run

(concurrent execution).• The process can’t continue until another process has

finished its task.• The process is waiting for something to happen (e.g.

I/O).• An exceptional event has occurred (i.e. an interrupt).


The Process State Model

• The decision on which process to run therefore has to be based on knowing whether the process can be run.

• During their lifetime processes typically cycle through 3 states• Ready• Running• Blocked


The Process State Model .. contd

Ready



Ready

Running

Process is selected to run


Either



Ready

Running


Process finishes time slice


OR



Ready

Running


BlockedProcess blocks (e.g. I/O)



Ready

Running


BlockedProcess blocks (e.g. I/O)

Blocking operation finishes (e.g. I/O)


Summary

• Described different execution patterns: sequential, concurrent and parallel.

• Developed a simple dispatcher which can handle concurrent processes.

• Started to look at why you might want to change between concurrent processes - the process state model.


Coming next week

• Problems with concurrent processing.

Documents

Topic 6 : Introduction to Operating Systems L & E: Pages 1-19, 17-29 Tanenbaum: Pages 1-5, 15-16, 50-53