The CPURevision Typical machine code instructions Using op-codes and operands Symbolic addressing. Conditional and unconditional branches

The CPUThe Central Processing Unit is

one of the main parts of a computer

It consists of;1. Control Unit (CU)2. The Arithmetic Logic Unit (ALU)3. Registers

Structure of the CPU

Arithmetic Logic Unit

Accumulator

Control Unit

Program Counter

Instruction Register

Parts of the CPU

1. Control Unit: The manager of the CPU, decodes instructions and controls the operations done by the ALU

2. Arithmetic Logic Unit: Arithmetic and logic operations are carried out.

3. Instruction register: Stores a copy of the current instruction being executed

4. program counter: Stores the address of the next instruction in the program to be executed

5. Bus: Transfer of data

Assembly Language The CPU uses what is known as

Assembly Language

Assembly Language is made up of op-codes and operands

Instructions in assembly language are rather simple

Assembly Language Operations

A few examples

Moving Data Set a register (a temporary

location in the CPU) to a fixed

value

STO 10This will store a result (found in

the accumulator) to a memory

location, which in this case is 10.

Move data from a memory

location to a register, or vice

versa. This is done to obtain the

data to perform a computation

on it later, or to store the result

of a computation.

mov ah, 09This will move the constant

number 9 to the register ah.

Read and write data from

hardware devicesINP ah, 145Receives input from the input

device with the code 145 and

stores it into register ah.

Computing Data Add, subtract, multiply, or

divide the values of two

registers, placing the result in a

register

ADD AX, BXAdd the contents found in the

register AX with the contents in

register BX and store the result

in register AX.

Perform bitwise operations,

taking the conjunction /

disjunction (and/or) of

corresponding bits in a pair of

registers, or the negation (not)

of each bit in a register

OR AX, BXExecutes a bit-wise OR between

the two registers and stores the

result in BX.

Compare two values in registers

(for example, to see if one is

less, or if they are equal)

CMP AX, BXCompares the values by

subtracting the BX from AX, the

result is not stored.

Program Flow

Jump to another location in the

program and execute

instructions there

JMP destThis will force the flow of the

program to jump to the

destination specified as an

argument.

Jump to another location if a

certain condition holdsJNZ destThis will jump to the

destination if the accumulator

is not Zero.

For more info:http://en.wikibooks.org/wiki/X86_Assembly/X86_Instructions

Using Op-Codes (mnemonics) Op-Codes are used instead of binary

Op - Codes are short words normally made up of 3 characters

When used they perform a function

Op-Codes are easier to remember by programmers, since their name refers to what they will perform

Op – Code Examples Op.code

(binary) Mnemonic Function

0000 LDA

LOAD Accumulator with contents of specified

address.

0001 STA STore Accumulator contents in specified address.

0010 ADD

ADD contents of the specified address to the

accumulator.

0010 SUB

SUBtract contents of specified address from the

accumulator.

0110 JPU JumP Unconditionally to the specified address

0111 JAZ

Jumps to the specified address if Accumulator is

Zero.

1001 JAL Jumps to specified address if Accumulator < zero.

1010 JAG

Jumps to the specified address if the Accumulator

> zero.

1111 HLT Stop the program

Addressing With Assembly Language, we

have three ways in which the computer can find locations from memory;

1. Absolute Addressing2. Relative Addressing3. Symbolic Addressing

Absolute Addressing When we use Assembly Language different

memory locations are used

In some cases we can actually specify which memory location we wish to use (example, LDA 10 is specifying that the accumulator should be filled with the contents found in memory location 10)

910 11 12 13

LDA 10

Relative AddressingThis is indicated by specifying the distance

from another address

In this case it is called the base address, hence the exact memory location is not given (example, MOV CX, [BX+4] is specifying that the contents of Register C must be moved in the location which is found 4 locations off register B)

BxBx+1

Bx+2

Bx+3 Bx+4

MOV CX,[BX+4]Note:BX is the base address

Symbolic Addressing A label can be given to a memory location in

assembly language

Instead of referring to the memory location by the location a label is used.

The memory location 106 can be assigned a label Num1, and from then on, whenever location 106 is required, instead of calling memory location 106, Num1 is called, LDA Num1.

103 104 105 106 107

LDA NUM1

NUM1

Conditional & Unconditional branches

The program counter (PC) found in the CPU points to the next instruction to be fetched

If we change the PC, instructions can be executed in a different order not in a sequence

In order to use jump instructions, labels must be used. Labels can be created by simply specifying a label name and adding a colon at the end of the label. For example:

label1: mov ax,5 mov bx,3

Unconditional JumpThis type of branching instructs

the processor to jump to a label without any conditions.

The command is JMP

When the JMP command is encountered, the flow of the program will resume from the specified label; in this case lab1.

mov ax, 5jmp lab1add ax, 4lda 10lab1: sub ax, 3

…

In the program,;1. As soon as the command jmp lab1 is

encountered, the program will jump to the label

2. The program will continue from the label3. The commands add ax,4 and lda 10 will

be skipped4. sub ax,3 will be worked next.

As already mentioned, an unconditional branch does not have any conditions, so when the jump command is encountered it will simply skip to the label.

Conditional Branching Conditional branching allows the

program to skip to another location (through the use of labels) only if it satisfies a condition

In order to use this type of branching, a comparison must be done first

First we carry out the comparison, and then we jump to a label if the comparison satisfies the jump’s criteria

Assembly

cmp ax, bx

jg isgreater

: ; block 1 (else part)

jmp after

isgreater:

: ; block 2 (then part)

after:

: ; after if

1. The registers ax and bx are compared with each other using the command CMP

2. Then the command jg (jump if greater) is used to jump to the label isgreater, if ax is greater than bx, otherwise it will jump to label after

3. The actual comparison is carried out using the CMP command, and then different jump statements can be used.

Different Jumps

Instruction Meaning

jg Jump if greater

jge Jump if greater or equal

jl Jump if less

jle Jump if less or equal

je Jump if equal

jne Jump if not equal

jc Jump if carry flag is set