[PDF] 8086/8088 Machine language Instruction format Addressing Modes

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8086/8088 Machine language

Instruction format, Addressing

Modes, Data addressing,

Program memory & stack

addressing mode

Software

yThe sequence of commands used to tell a microcomputer what to do is called a program, yEach command in a program is called an instruction y8088 understands and performs operations for 117 basic instructions yThe native language of the IBM PC is the machine language of the 8088 yA program written in machine language is referred to as machine code

yIn 8088 assembly language, each of the operations is described by alphanumeric symbols instead of 0-1s.

ADDAX, BX

(Opcode) (Destination operand) (Source operand)

Instructions

LABEL: INSTRUCTION ; COMMENT

Address identifierDoes not generate any machine

code yEx. START:MOV AX, BX; copy BX into AX yThere is a one-to-one relationship between assembly and machine language instructions yA compiled machine code implementation of a program written in a high-level language results in inefficient code

²More machine language instructions than an

assembled version of an equivalent handwritten assembly language program yTwo key benefits of assembly language programming

²It takes up less memory

²It executes much faster

Applications

yOne of the most beneficial uses of assembly language programming is real-time applications. Real time means the task required by the application must be completed before any other input to the program that will alter its operation can occur For example the device service routine which controls the operation of the floppy disk drive is a good example that is usually written in assembly language yAssembly language not only good for controlling hardware devices but also performing pure software operations ²Searching through a large table of data for a special string of characters

²Code translation from ASCII to EBCDIC

²Table sort routines

²Mathematical routines

Assembly language: perform real-time operations

High-level languages: used to write those parts

that are not time critical

Converting Assembly Language Instructions to

Machine Code

yAn instruction can be coded with 1 to 6 bytes yByte 1 contains three kinds of information ²Opcode field (6 bits) specifies the operation (add, subtract, move) ²Register Direction Bit (D bit) Tells the register operand in REG field in byte 2 is source or destination operand

1: destination 0: source

-Data Size Bit (W bit) Specifies whether the operation will be performed on

8-bit or 16-bit data

0: 8 bits1: 16 bits

15 -------10987 ----65 ----32 ----0

yByte 2 has three fields

²Mode field (MOD)

²Register field (REG) used to identify the register for the first operand

²Register/memory field (R/M field)

2 bit MOD field and 3 bit

Mode Field encoding

R/M field together specify the

second operand

Register/memory (R/M) Field Encoding

Examples

MOV BL,AL(88C316)

Opcode for MOV = 100010

D = 0 (AL source operand)

W bit = 0 (8-bits)

Therefore byte 1 is 100010002=8816

‡ 02G 11 UHJLVPHU PRGH

‡ 5(* 000 ŃRGH IRU $I

‡ 5C0 011 GHVPLQMPLRQ LV %I

Therefore Byte 2 is 110000112=C316

Examples:

MOV BL, AL = 10001000 11000011 = 88 C3h

ADD AX, [SI] = 00000011 00000100 = 03 04 h

ADD [BX] [DI] + 1234h, AX = 00000001 10000001 __ __ h =

01 81 34 12 h

Addressing Modes and

Formats

Addressing Modes

yImmediate yDirect yIndirect yRegister yRegister Indirect yDisplacement (Indexed) yStack

Immediate Addressing

yOperand is part of instruction yOperand = address field ye.g. ADD 5

ŃAdd 5 to contents of accumulator

Ń5 is operand

yNo memory reference to fetch data yFast yLimited range

Immediate Addressing Diagram

OperandOpcode

Instruction

Direct Addressing

yAddress field contains address of operand yEffective address (EA) = address field (A) ye.g. ADD A

ŃAdd contents of cell A to accumulator

ŃLook in memory at address A for operand

ySingle memory reference to access data yNo additional calculations to work out effective address yLimited address space

Direct Addressing Diagram

Address AOpcode

Instruction

Memory

Operand

Indirect Addressing (1)

yMemory cell pointed to by address field contains the address of (pointer to) the operand yEA = (A) ŃLook in A, find address (A) and look there for operand ye.g. ADD (A)

ŃAdd contents of cell pointed to by contents

of A to accumulator

Indirect Addressing (2)

yLarge address space y2nwhere n = word length yMay be nested, multilevel, cascaded

Ńe.g. EA = (((A)))

xDraw the diagram yourself yMultiple memory accesses to find operand yHence slower

Indirect Addressing Diagram

Address AOpcode

Instruction

Memory

Operand

Pointer to operand

Register Addressing (1)

yOperand is held in register named in address filed yEA = R yLimited number of registers yVery small address field needed

ŃShorter instructions

ŃFaster instruction fetch

Register Addressing (2)

yNo memory access yVery fast execution yVery limited address space yMultiple registers helps performance

ŃRequires good assembly programming or

compiler writing

ŃN.B. C programming

xregister int a; yc.f. Direct addressing

Register Addressing Diagram

Register Address ROpcode

Instruction

Registers

Operand

Register Indirect Addressing

yC.f. indirect addressing yEA = (R) yOperand is in memory cell pointed to by contents of register R yLarge address space (2n) yOne fewer memory access than indirect addressing

Register Indirect Addressing Diagram

Register Address ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

Displacement Addressing

yEA = A + (R) yAddress field hold two values

ŃA = base value

ŃR = register that holds displacement

Ńor vice versa

Displacement Addressing Diagram

Register ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

Address A

Relative Addressing

yA version of displacement addressing yR = Program counter, PCquotesdbs_dbs8.pdfusesText_14

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