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INVITATION TO

Computer Science 1 1

Chapter 6

An Introduction to System Software and Virtual Machines

Objectives

In this chapter, you will learn about:

• System software

• Assemblers and assembly language

• Operating systems

Invitation to Computer Science, 6th Edition 2

Objectives

After studying this chapter, students will be able to:

• Describe the main categories of system tasks

• Compare the virtual machine created for the user by

system software with the naked machine

• Explain the benefits of writing system software in

assembly language, rather than machine language

• Read and write short assembly language programs

• Describe how an assembler translates assembly

language programs into machine instructions


Objectives (continued)

After studying this chapter, students will be able to:

• List five key tasks of an operating system, and explain

what each is and why it is critical to modern systems

• Describe the different generations of operating

systems, what the features of each generation were,

and how each generation solved a drawback of the

previous generation


Introduction

• A bare or naked machine has no tools or programs

to help the user

– Write instructions in binary

– Write data in binary

– Load instructions into memory one cell at a time

– Initiate program run

• Too difficult for humans to do

• We must build an interface to hide the details and

make the computer easier to build


Interface

• Von Neumann computer

– “Naked machine”

– Hardware without any helpful user-oriented features

– Extremely difficult for a human to work with

• An interface between the user and the hardware is

needed to make a Von Neumann computer usable

and people oriented ( like “dashboard” on a car )


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Interface

• Tasks of the interface – Hide details of the underlying hardware from the

user

– Present information in a way that does not require

in-depth knowledge of the internal structure of the

system

– Allow easy user access to the available resources

– Prevent accidental or intentional damage to

hardware, programs, and data


Software

• Application software

– programs primarily for users ( word processors,

spreadsheets, databases, presentation graphics, etc.)

• System software

– primarily used to run the computer (operating

system, device drivers, assemblers, compilers )


System Software

• System software is the intermediary between the

users and hardware

– Hides complex internal structure of the Von Neumann

architecture

• Virtual Machine/Virtual Environment

– The set of services and resources created by the

software and seen by the user


System Software

• System software is a collection of programs to

– manage resources of the computer

– serve as intermediary between user and hardware

• System software creates a virtual machine (or

virtual environment) that user sees


System Software (continued)

• Operating system: central system software

– Communicates with users

– Starts up other system software and applications as

needed

• Graphical user interface (GUI), often pronounced

“gooey”

– visual interface to operating system (or other system

software/applications)

• I/O systems: communicate with various devices


Types of System Software

• System software is a collection of many different

programs

• Operating system

– Controls the overall operation of the computer

– Communicates with the user

– Determines what the user wants

– Activates system programs, applications packages, or

user programs to carry out user requests


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• User interface

– Graphical user interface (GUI) provides graphical

control of the capabilities and services of the

computer

• Language services

– Assemblers, compilers, and interpreters

– Allow you to write programs in a high-level, user-

oriented language, and then execute them



• Memory managers

– Allocate and retrieve memory space

• Information managers

– Handle the organization, storage, and retrieval of

information on mass storage devices

• I/O systems

– Allow the use of different types of input and output

devices



• Scheduler

– Keeps a list of programs ready to run and selects the

one that will execute next

• Utilities – Collections of library routines that provide services

either to user or other system routines



• Language services: support high level languages

• Memory managers: allocate memory to programs

• Information managers: organize mass storage

• Scheduler: manage programs waiting to run

• Utilities: tools, including program libraries



Naked machine:

1. Write program in binary

2. Load instructions one-by-

one into memory

3. Insert start into memory

address 0 and push “go”

button

4. Read results from

memory one-by-one, in

binary

Virtual machine:

1. Write program using text

editor in high-level

language

2. Save program to folder

3. Use translator to convert

to binary

4. Use scheduler to load

and run

5. Use I/O system to print

results


What’s the Problem with Machine

Language?

• Written from machine’s view

• Complicated, difficult to understand

• Program is written, loaded, debugged in binary - no

English-like words

• Allows only numeric memory addresses

• Difficult to read and change

• Difficult to create data


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Assembly Language

• Assembly language

– Designed to overcome shortcomings of machine

languages

– Create a more productive, user-oriented

environment

– Earlier termed second-generation languages

– Now viewed as low-level programming languages


Assembly Language

• Second-generation language

• Uses mnemonics to represent machine language

opcodes

• Uses symbolic memory addresses

• Provides data generation operations

• Format:

label: opcode address --comment


Assemblers and Assembly Language

• Assembly language

– Instructions map one-to-one to machine language

– Symbolic op codes (not binary)

– Symbolic addresses for instructions and data

– Pseudo-ops for data generation and more (data in

human-friendly terms)

• Assembly = a low-level programming language

• Java, C, C++, VB, Python = high-level

programming languages

Invitation to Computer Science, 6th Edition 21 Invitation to Computer Science, 6th Edition 22


• Source program – An assembly language program

• Object program – A machine language program

• Assembler – Translates source program into an object program

Assembly language process:

• Source program, in assembly language

• Translated by the assembler to

• Object program, in machine language

• Loader places in memory

• Hardware runs


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(continued)

Example Assembly language:

NEXTSTEP: LOAD X -- Put X into reg. R

label: opcode address field -- comment

• Label is optional name for this instruction’s location

• Op code mnemonic and address field translate to

machine language

• Comments are ignored by assembler, just for

human use



(continued)

Advantages over machine code:

• Clarity, readability, and maintainability

• Can be placed at different locations in memory

• Pseudo-op: commands in the program directed to

the assembler, not converted to machine

instructions

– .BEGIN and .END to mark where instructions are

– .DATA to mark memory location as holding data:

COUNTER: .DATA 0

X: .DATA 12


Examples of Assembly Language

Instructions

• Assembly language translation

LOAD ONE --Put a 1 into register R.

STORE X --Store the constant 1 into X

:

INCREMENT X --Add 1 to Value in memory

location X

:

I: .DATA 0 --The index value. Initially it is 0

ONE: .DATA 1 --The constant 1.



Instructions

• Assigns name to address where data is stored

• Converts data value to proper binary representation

• Example:

FIVE: .DATA +5

NEGSEVEN: .DATA -7

LOAD FIVE

ADD NEGSEVEN

• Arithmetic expression

A = B + C – 7

(Assume that B and C have already been assigned values)



Instructions • Assembly language translation

.BEGIN

LOAD B --Put the value B into register R.

ADD C --R now holds the sum (B + C).

SUBTRACT SEVEN --R now holds the expression (B + C - 7).

STORE A --Store the result into A.

: -- Data should be placed after the HALT

A: .DATA 0

B: .DATA 0

C: .DATA 0

SEVEN: .DATA 7 --The constant 7

.END


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Simple examples:

x = y + 3

input a and b

while a > b do

print a

a = a – 2

LOAD Y

ADD THREE

STORE X

… -- Data comes after .END

X: .DATA 0 -- X is initially 0

Y: .DATA 5 – Y is initially 5

THREE: .DATA 3 –The constant 3

IN A

IN B

LOOP1: LOAD A A: .DATA 0

COMPARE B B: .DATA 0

JUMPLT LOOP1END TWO: .DATA 2

JUMPEQ LOOP1END

OUT A

LOAD A

SUBTRACT TWO

STORE A

JUMP LOOP1

LOOP1END: …


Example – Summing Numbers

• Problem

– Read in a sequence of non-negative numbers, one

number at a time, and compute a running sum

– When you encounter a negative number, print out

the sum of the non-negative values and stop


35


Translation and loading:

• Assembler translates to machine language

– Convert symbolic op codes to binary equivalents

– Convert symbolic labels to memory addresses

– Perform pseudo-op actions

– Write object file containing machine instructions

• Loader gets program ready to run

– Places instructions in memory

– Triggers the hardware to run the program


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Converting symbolic op codes to binary ones

• Assembler maintains a table

• Assembler looks up symbolic op codes in the

table and substitutes the binary analogue

• Use binary search to optimize table lookups

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Converting symbolic labels to memory addresses

• Assembler needs two passes

– looks over assembly code two times

• First pass:

– Keep a count of how many instructions from the

start

– Collect symbolic labels and add to symbol table

along with location counter


Invitation to Computer Science, 6th Edition 39 40


(continued)

• Second pass:

– Look up and replace op codes

– Substitute label references with location from symbol

table

– Set up .DATA pseudo-ops with location and binary value

– Write instructions to object file

Invitation to Computer Science, 6th Edition 41 42

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Translation and Loading

• The object program becomes the input to another piece of

software – the loader

• The loader reads instructions from the object file and stores

them in memory for execution

• To do this it reads and address and stores the machine

instruction into the specified memory address.

• After the instructions are loaded into memory, the loader

places the address of the first instruction (0 in our example)

into the program counter (PC) .

• The computer fetches, decodes and executes the program

starting with the instruction, whose address is in the PC.


Operating Systems

• What is an operating system?

– A library of programs that make a computer

• Easier to user for the user

• Use resources more efficiently

– It can also be viewed as:

• A Government – making policy and rules

• An Environment – in which users write and run programs

• A Resource allocator

• A Security guard

• A Traffic cop

• A Wizard – providing “illusions”


Purpose of an Operating System

• The primary purposes of an Operating

System are:

• Ease of use for the user

• Efficient use of the system resources

• Operating system

– Waits for requests and activates other system

programs to service these requests

• System commands

– Used to translate, load, and run programs


Operating Systems

• System commands:

- user instructions about what the computer should do


Operating Systems (continued)

• User interface: user communicates with operating

system

• Operating system as receptionist and dispatcher

• User command system software scheduled and

runrepeat

• Text-based:

– System commands typed at a prompt in a terminal

– Command language must be learned

• GUI-based:

– System commands by visual/mouse interface


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GUIs


Example of a Graphical User Interface


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Operating Systems

• System security and protection: permit only

authorized access to resources

• Operating system as security guard

• Access protected by usernames and passwords

– Superusers have more privileges

– Encrypt data to increase security

• Folders and files have authorization lists:

– read a file, add new information to a file, change

existing information, delete a file




• Efficient allocation of resources: keep CPU busy

and all programs making progress

• Operating system as traffic cop

• Multiple programs active at one time

• Every program is in one of three states:

– Running: program currently using the CPU

– Waiting: programs waiting for I/O operations to

complete

– Ready: programs ready for a turn on CPU

• Keep a queue of ready programs and share


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Example of sharing CPU: Waiting Ready Running

B A

C

A makes I/O request Waiting Ready Running

A C B

A finishes with I/O Waiting Ready Running

C B

A


Operating Systems

• Safe use of resources: ensure that computer

doesn’t get stuck in deadlock

• Multiple programs requesting access to resources

• Deadlock occurs when all programs have some

resources, and are waiting for resources held by

others

• Deadlock prevention: if you can’t get all resources,

release all you have and try again later

• Deadlock recovery: if no acknowledgement, send

message again


Summary of OS Responsibilities

• Major responsibilities of operating systems

– User interface management (a receptionist)

– Control of access to system and files (a security

guard)

– Program scheduling and activation (a dispatcher)

– Efficient resource allocation (an efficiency

expert)

– Deadlock detection and error detection (a traffic

officer)


Operating Systems

Historical development of operating systems

• First generation: (1945–1955)

• Nearly “naked computer”

– No operating systems

– Assemblers and loaders were almost the only system

software provided

– Programmer hand-loaded programs


Operating Systems

• Second generation: (1955–1965)

• Batch operating system

– Programmers gave programs to operators

– Operators collects a “batch” of programs

• Used I/O computer to translate programs to tape

• Ran programs as a group

• Used I/O computer to translate output to text/paper

– Job control language: instructions to OS

– Ran collections of input programs one after the other

– Command language -specifying the operation


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Operating Systems

• Third-generation operating systems(1965–1985)

• Multiprogrammed operating systems

– Time sharing systems

– Permitted multiple user programs to run at once

– User operation codes: could be included in any user

program

– Privileged operation codes: use restricted to the

operating system or other system software (eg. HALT

instruction) ( Why can’t users execute this on a multiuser

system?)



• Third generation:

• Multiprogrammed operating systems

– Switch between programs when I/O happens

– Computer security now required!

• Privileged operations only available to administrator

• Bound memory a program can legitimately access



• Time-sharing system (also third generation)

– Multiprogrammed, but users are on system

interactively

– Users need illusion of sole access

– Allocate run time in time slices. Each program runs

until I/O OR time runs out




• Fourth generation: ( 1985- present)

• Network operating system

– Operating system supports all the same local services

– Also supports services that access resources that are

available over a network • Shared printer

• Servers: e-mail, data, web

• Connections to Internet

– Real-time operating system or embedded system are

special-purpose computers in other equipment

– Distributed environment - computing done remotely in

the office, laboratory, classroom, and factory


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Operating Systems

• Fourth Generation (1985-Present)

– Real-time operating system

• Manages resources of embedded computers that are

controlling ongoing physical processes

• Guarantees that it can service important requests

within a fixed amount of time

– Virtual environment

• treats resources physically residing on the computer in

the same way as resources available through the

computer’s network




• Fifth generation, the near future

– Multimedia interfaces (integrate images, speech,

video seamlessly)

– Parallel processing system, to perform multimedia

and to permit larger scale tasks

– Distributed computing environment, users don’t

know where resources are stored

• Cloud computing



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Summary

• System software creates a virtual machine that is

easy for users to use

• Assemblers and loaders are system software:

translate human-friendly programs to machine

language

• Assembly language uses symbolic names,

symbolic op codes, and pseudo-ops to describe

algorithms

• Assembler translates source programs to object

files; loader places object instructions in memory


Summary (continued)

• Operating systems communicate with users

through text or GUI actions

• The operating system includes key tasks such as:

the user interface, system security, scheduling of

programs, allocating resources and system safety

• Operating systems developed through four

generations of improvements, each adding new

features and improving efficiency

• Future operating systems: parallel, distributed, and

multimedia, with resources stored remotely


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