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ihs/CSCS323/Spring 2012 1

CSCS 323

Course Title Computer Organization with Assembly Language

Course Code COMP 323

Credits 3

Instructor Dr. Iftikhar Hussain Shah

Recommended Books / Material

Assembly Language Programming and Organization of the IBM PC

by Ytha Yu and Charles Marut (McGraw-Hill)

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A Computer System

What is a computer?

A computer is an electronic machine which can read, store,

retrieve, process and display information. What is a Computer System?

A computer system consists of two major components:

1. HardwareIt consists of circuits which can read, store, retrieve,process and display information.

2. SoftwareIt consists of programs which:

a. drive the computer (Operating System)

b. facilitate users to perform specific jobs such as word processing(Application Software).

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Number Systems

A number system consists of digits.

The number of digits in a number system is called its base.

The following four number systems are commonly used.

Number System Digits Base

Decimal 0-9 10

Binary 0,1 (bits) 2Octal 0-7 8

Hexadecimal 0-9, A-F (A=10, F=15) 16

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Conversion from Decimal to Binary

First divide the decimal number by 2 and then repeatedly divide

the quotient by 2 until the quotient becomes o. The remainders taken in the reverse order form the binaryequivalent of that decimal number.

Example: The binary equivalent of 23 is calculated below.

2 23

2 11 1

2 5 1

2 2 1

2 1 0

2 0 1

(23)10 = (10111)2

Base

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Conversion from Binary to Decimal

Multiply rightmost bit by 20, next on left by 21, next by 22

and so on. Then add all the products. This gives the decimalequivalent of the given binary number.

Example:

(10111)2 = 1*20

+ 1*21+ 1*22

+ 0*23

+ 1*24

= (23)10

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Representation of Numbers

(10111)2is binary number; it can also be written as 10111b or

10111B where b or B indicate that the preceding number is

binary.

Similarly, H or h are used to represent a hexadecimal number,

D or d are used to represent a decimal number & O or o are

used to represent an octal number.

The default is a decimal number; e.g. 101B is binary number

(equivalent to 5 in decimal, while 101 is a decimal number

representing one hundred and one.

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Conversion from Decimal to Hexadecimal

First divide the decimal number by 16 and then repeatedly

divide the quotient by 16 until the quotient becomes o. The remainders taken in the reverse order form thehexadecimal equivalent of that decimal number.

Example1: The hex. equivalent of 37 is calculated below.

(37)10 = (25)16

Ex.1. Show that (140)10 = (8C)16

Ex.2. Compute hex. Equivalents of the following decimal numbers:

16, 32, 255, 256, 2047

16 37

16 2 5

0 2

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Conversion from Hex. to Decimal

Multiply rightmost digit of the hex number by 160, next on left by

161

, next by 162

and so on. Then add all the products. This givesthe decimal equivalent of the given binary number.

Example:

(25)2 = 5*160

+ 2*161

= (37)10

Ex. Write down a general rules for convertinga decimal number to an equivalent number

in any other system and vice versa.

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Conversion from Hexadecimal to Binary

Two methods:

1. Convert hex to decimal and then decimal to binary.

2. Replace each digit of the hexadecimal number by its four-bitbinary representation.( This is commonly used)

Examples:

(25)16 = (00100101)2 or 25h = 00100101b

(8C)16 = (10001100)2 or 8Ch = 10001100b

EX. Convert the following hex. numbers to their binary equivalents andcheck the result: 0A7Dh, 48GDh

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Conversion from Binary to Hexadecimal

Two methods:

1. Convert binary to decimal and then decimal to hex.

2. Starting from right form groups of bits each consisting of four bitsof the given binary number. If the last group does not contain 4bits place required 0s to its left. Then replace each group of four

bits by it hexadecimal equivalent.

Examples:

100101b = 00100101b = 25h

10001100b = 8Ch

EX. Convert the following hex. numbers to their binary equivalents and

check the result: 0A7Dh, 48GDh

25

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Octal Number System

Assignment:

What is an Octal number system?How would you convert a number from:

1. Decimal to octal and vice versa

2. Octal to binary and vice versa

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Language of Computer

A computer understands machine language only. In this language, everything

is expressed in terms of 0s and 1s (Bit Patterns).

The bit

The bit stands for a binary digit, and it can be either 0 or 1. A bit cannot store

much information. To store meaningful information, we use a collection of 8,

16, 32, bits. The Byte

A collection of 8 bits is called a Byte. It can store a character in some way

and is also called a cell.

The Word

A word can be 1, 2, 4, 8, bytes is called a word.

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Representation of Information The information consists of letters, digits, punctuation symbols and other

symbols; these are called characters.

A unique integer code has been assigned to each character; inside

computer a character is represented by the eight bit binary equivalent of

its code so that it can fit into a byte of memory.

A few coding systems have been developed for this purpose; the most

common coding system is American Standard Code for Information

interchange (ASCII).

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Exercise

How is information represented inside thecomputer. Write a detailed note ondifferent coding Systems.

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Example (ASCII CODES)

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Logic instructions

o Logic instructions can be used to change individual bits in the computer

o The truth tables for the logic operators AND, OR, XOR & NOT shown in fig.Here a, b are logic / bit variables whose values cab be 0 (false),1(true)

a b a AND b a OR b a XOR b

0 0 0 0 0

0 1 0 1 1

1 0 0 1 1

1 1 1 1 0

a NOT a

0 1

1 0

L i i i

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Logic instructions (continued)

o Logic instructions can be used to change individual bits in the computero The truth tables for the logic operators AND, OR, XOR, NOT are shown in fig.

Here a, b are logic / bit variables whose values cab be FALSE (F), TRUE (T).

a b a AND b a OR b a XOR b

F F F F F

F T F T T

T F F T T

T T T T F

a NOT a

F T

T F

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Signals and Buses

A signal consists of a bit pattern.

Processor uses signals to communicate with memory and I/O circuits.

Signals are of three types: address, data and control signals.

A bus is a collection of wires that can carry signals

Buses are of three types: address bus, data bus and control bus.

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Input / Output Ports

I/O devices are connected to the computer through circuits. Each of these

circuits contain several registers called I/O ports.

These registers are identified by their addresses. These addresses are used in

I/O operations only.

I/O ports act as transfer points between the CPU and I/O devices.

An input device places data into a port from where CPU reads it.

To output, the CPU places data in the port from where the output

device reads it.

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Input / Output Ports

Serial and Parallel Ports

Serial ports transfer data one bit at a time

Parallel ports transfer 8, 16, bits at a time

Slow devices like keyboard are connected to serial ports

Fast devices like disk drives are connected to parallel ports.

Printer can be connected to any port.

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Execution of an Instruction (Fetch-Execute Cycle)

The CPU performs the following steps to execute a Machine Language instruction;

these steps constitute Fetch-Execute Cycle. A machine instruction has two

components:

Opcode : specifies type of operation

Operands : are usually registers or memory addresses

Fetch-Execute Cycle

a. Fetch

Bring instructions from the main memory

Decode the instruction into an operation

Fetch data from the main memory

b. Execute

Perform the operation on data

Store result back into the main memory if required

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The Clock To ensure that all the steps required to execute a machine language instruction

are performed in an order, a clock circuit controls the processor by generating a

train of pulses.

Period interval between two pulses

Clock speed No. of pulses generated per second

(Unit: Hz, MHz, GHz)

o The 8086 processor had a clock speed of 4.77 MHz

o Now speed are in GHz.

The processing speed depends upon the clock speed.

Computer circuits are activated by clock pulses. Each step in Fetch-Execute cycle

requires one or more clock pulses; e.g. 8086 processor takes 4 pulses to perform a

read operation and 70 pulses to perform a multiplication operation.

Programming Languages

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Programming Languages

Machine LanguageThe computer can execute a program written in machine language only.

In Machine Language, operations and operands are expressed in terms of 0s and 1s (bit

patterns).Example:

Instruction

00000101 00000100 00000000

Meaning

Add 4 to AX and store the result in AX

Disadvantage of machine language?

It is very difficult to write programs in machine language It is even more difficult to maintain (correct / modify) them.

What is solution then? Assembly Languages was invented in early 1950s

Programming Languages (contd )

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Programming Languages (contd.)

Assembly Language

The binary codes of operations were replaced by code words called mnemonics

Memory addresses were replaced by variables

Codes for registers were replaced by their names.

The above instruction written in assembly language becomes:

ADD AX, 4 Add 4 to AX and store the result in AX

Disadvantaes of Assembly language?

Instructions are very primitive; not suitable for writing large programs.

What is solution then?

High Level Languages (Fortran, Pascal, C, C++, Java, have been invented.

High Level languages

Ex. Point out distinguishing feature of High Level languages.

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Organization of the IBM PC

Intel introduced first 16 bit processor 8086 in 1978

The 8 bit 8088 processor was introduced in 1979.

The 8086 processor had faster clock and better performance

IBM selected 8088 to build PC because it was more economical

Both the processors had the same instruction set which forms the basicinstruction set for the subsequent processors.

Then Intel gradually introduced 80186, 80286, 80386, 80486, processors.

The subsequent processors usually had more facilities including faster

clock speed, bigger bus, bigger instruction set, etc.

EX.Highlight distinguishing features of 80186, 80286, 80386, 80486 processors.

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Organization of the 8086 Processor

The information inside the processor is stored in registers

The 8086 processor has 14 registers each of size 16 bit.

Type of registers:

Register FunctionData registers Contain data for operationsAddress registers Contain address of an instructionFlags register Contains status of the processor

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Data Registers AX, BX, CX, DX

These registers are available to the programmers for general use.

The processing of the instructions stored in the registers is faster than

those stored in the RAM.

Upper and lower halves of these registers can be used separately; e.g.

the upper half of AX is called AH and lower half is called AL and same

is true of the other registers.

In addition, these registers also perform the following special functions.

Data Registers AX BX CX DX

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Data Registers AX, BX, CX, DX

Register Special FunctionAX(Accumulator register) It is preferred to be used in arithmetic, logic and data transferinstructions because its use generates shortest machine code.

Also used in I/O and multiplication & division operations

BX(Base register)

Used as address register; e.g. in XLATE instruction.

CX(Counter register) Used as a counter in LOOP and REP (repeat) instructions andstring operations.

DX(Data register) used in Output, multiplication and division operations.

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Segment Registers CS, DS, SS, ES

The segment registers are used to store RAM addresses of instructions

and of data

The processor uses these addresses to access the memory locations.

The 8086 processor used 20 bit address to each byte in its 1 MB RAM;

these are called physical addresses.

The size of each segment register is 16 bit and cannot accommodate

20 bit address. What is solution?

o Divide the 20 bit physical address into two components and load

each component in a separate register.

o For this purpose main memory is divided into segments.

Segments

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Segments

What is a segment?

o A segment is a contiguous block of216 bytes (64 KB) of RAM so that the address

size of each of its bytes is 16 bit. The byte number within the segment is called

offset.

o Each segment is identified by a 16-bit segment number.

o Address of each byte has two parts:

1. segment number2. offset.

o This is called logical address and is written as:

Segment# : Offset

Example: 02AB : 0255H is a logical Address, where,

02AB is segment number,

0255 is offset

H indicates that these numbers are Hexadecimal.

Conversion of Logical Address into Physical Addresses

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At the time of use, 2-dimensional logical address must be converted into

20-bit physical address because RAM has a 20-bit address associated

with each byte.

To do this:

1. Shift Segment # (expressed in Hex) by one position towards left,

2. Add offset (expressed in Hex) to it.

The physicaladdress corresponding to 02AB : 0255H is:

02AB0

+ 02 5 5

02D05


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Assignment # 1

Marks = 4 Last Date of Submission: 14 March 20121. Show that segments are overlapping.

2. What is minimum size of a segment and why?

3. How can the size of segment be increased?

4. What is maximum size of a segment and why?

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Segment Registers (Contd.)

The logical address consists of 16-bit segment number and a 16-bit offset.

The segment number is stored in a segment register (CS, DS, SS or ES).

The offset is stored in the companion of that segment register.

The following are companion registers.

Segment Register Companion register

CS IP ( Instruction pointer)

DS SI (Source Index)

SS SP (Stack Pointer)

BP ( Base Pointer)

ES DI ( destination Index)

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Flags Register

9 out of its 16 bits are used as flags.

Six bit (0, 2, 4, 6, 7 and 11) represent status flags

Three bits (8,9,10) represent control flags

Six status flags reflect the result of the preceding instruction; if result of

the preceding instruction is zero then the flag ZF is made 1 (true).

Three control flags enable or disable certain operations of the processor;

e.g. if the interrupt flag IF is made zero then the input from the

keyboard is ignored by the processor.

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