Programming The 8086 In Assembly Language … CMP source, dest is used to compare two numbers CMP...

Lecture 05

Programming The 8086 In

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Programming The 8086 In

Assembly Language…Continued

Daniel D.Daniel D.Daniel D.Daniel D. DECEECEgECEgECEgECEg –––– 4501 Microcomputers & Interfacing4501 Microcomputers & Interfacing4501 Microcomputers & Interfacing4501 Microcomputers & Interfacing

� Basic Instructions

In This LectureAAiT

� Details of the basic instructions

1ECEg ECEg ECEg ECEg ---- 4501450145014501

Basic Instructions

1. Data transfer instructions

MOV, LEA, IN/OUT, PUSH/POP, PUSHF/POPF

2. Arithmetic instructions

ADD, INC, SUB, DEC, NEG, COMP, MUL/IMUL, DIV/IDIV

3. Bit manipulation instructions

logical (AND, OR, TEST, XOR), SHR/SHL, RCL/RCR

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logical (AND, OR, TEST, XOR), SHR/SHL, RCL/RCR

4. String instructions

REP/REPE/REPNE, LOADS, STOS

5. Program execution transfer instructions

JMP/JE/JG/JL/JGE/JLE/JC, CALL/RET, LOOP, INT/IRET

6. Processor control instructions

flags(set/clear), HLT, WAIT, NOP


1. Data Transfer Instructions

MOV destination, source

Destination: can be a register or a memory location.

Source: can be a register, a memory location, or an

immediate number.

The source and destination must both be of type byte, or they

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The source and destination must both be of type byte, or they

must both be of type word.

e.g.

MOV CX, 037AH Put the immediate number 037AH in CX

MOV BL, [437AH] Copy byte in DS at offset 437AH to BL

MOV AX, BX Copy contents of register BX to AX

MOV DL, [BX] Copy byte from memory at [BX] to DL.


PUSH source, POP destination

Destination: can be a register or a memory location.

Source: can be a register or a memory location

The stack segment register and the stack pointer must be initialized

before these instructions can be used.

Data Transfer Instructions…cntdAAiT

before these instructions can be used.

e.g.

PUSH BX Decrement SP by 2, copy BX to stack

PUSH DS Decrement SP by 2, copy DS to stack

POP DX Copy a word from top of stack to DX, Increment SP by 2

PUSH AL Illegal, must push a word

POP CS Just Illegal

PUSHF/POPF can be used with flags


IN accumulator, port OUT port, accumulator

Port: 8 or 16-bit I/O port address. Can be fixed or variable

�For the fixed-port type, the 8-bit address of a port is specified

directly in the instruction.

e.g.

Data Transfer Instructions…cntd AAiT

e.g.

IN AX,34H Input a word from port 34H to AX

OUT 3BH,AL Copy the contents of AL to port 3BH

�For the variable port, the port address is loaded into the DX

register before the IN/OUT instruction.

e.g.

MOV DX, 0FF78H Initialize DX to point to port

IN AL, DX Input a byte from 8-bit port 0FF78H to AL


LEA register, source (load effective address)

This instruction determines the offset of the variable or memory

location named as the source and puts this offset in the indicated

16-bit register.

Data Transfer Instructions…cntdAAiT

e.g.

LEA BX, PRICES Load BX with offset of PRICES in DS

LEA BP, SS: STACK_TOP Load BP with offset of STACK_TOP in SS

Assume PRICES is an array of bytes in data segment. The instruction

LEA BX, PRICES will load the displacement of the first price in the list

directly into BX.


ADD destination, source /also ADC (add with carry)

e.g.

ADD AL, 74H Add immediate number 74H to contents of AL.

Result In AL.

ADD DX,[SI] Add word from memory at offset [SI] in DS to

2. Arithmetic InstructionsAAiT

ADD DX,[SI] Add word from memory at offset [SI] in DS to

contents of DX. Result in DX.

NB: Arithmetic instructions affect the flags.

INC destination can be used to add 1 to a number

e.g

INC BX, INC CX, …

INC doesn’t affect the carry flag.


SUB destination, source

e.g.

SUB CX, BX CX - BX. Result in CX

SUB AX,3427H Subtract immediate number 3427H from AX

�The decrement instruction DEC destination can be used to

subtract 1 from a number

2. Arithmetic Instructions…cntdAAiT

subtract 1 from a number

DEC CX subtract 1 from the content of the CX register

�The NEG destination instruction can be used to form 2’s

complement of the number in destination

NEG AL Replace number in AL with its 2’s complement

�The CMP source, dest is used to compare two numbers

CMP BH, CL compare a byte in BH with a byte in CL


MUL source IMUL source (signed)

•Source: can be memory or registers.

•one of the operands is in AL if source contains a byte, else in AX if

source holds a word.

AL x source = AH: AL (AX)……………byte x byte = word


AL x source = AH: AL (AX)……………byte x byte = word

AX x source = DX: AX ………………..word x word = double word

e.g.

MUL BH AL times BH, result in AX

MUL CX AX times CX, result high word in DX, low word in AX

•IMUL can be used the same way for signed numbers taking care of

the sign bit.


DIV source IDIV source (signed)

• Source: contains the divisor. It can be in memory or registers.

• The dividend is in AX if word/byte or in DX:AX if double/word

• Word/byte: AX/source…quotient in AL, remainder in AH

• Double/word: DX:AX/source…quotient in AX, remainder in DX

e.g.


e.g.

DIV BL Divide word in AX by byte in BL. Quotient in AL,

remainder in AH.

DIV CX Divide double word in DX and AX by word in CX.

Quotient in AX, remainder in DX.

•IMUL can be used the same way for signed numbers taking care of

the sign bit.

•CPU generates interrupts on divide by zero and quotient overflow


3. Bit-oriented Instructions

Logical (AND/TEST, OR, XOR, NOT)

AND/OR/XOR dest, source NOT dest

Do the indicated logic operation on the given source and

destination operands and keep the result in destination

e.g.

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e.g.

AND BH,CL AND byte in BH with byte in CL Result in BH

TEST AL, BH AND BH with AL, no result stored. Update PF, ZF

OR BL,80H BL ORed with immediate 80H. Set MSB of BL to a 1

XOR CL,BH Byte in BH Exclusive-ORed with byte in CL. Result in

CL. BH not chanced.

NOT BX Complement contents of BX register

Logical operators affect the zero flag and the parity flag


Bit-oriented Instructions…cntd

SHL dest, count SHR dest, count

Shift the binary number in dest. count times

CF MSB LSB 0 SHL ( *2)

0 MSB LSB CF SHR (/2)

e.g.

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e.g.

SHL BX, 1 Shift word in BX 1 bit position left, 0 in LSB

MOV CL, 02H Load desired number of shifts in CL

SHL BP, CL Shift word in BP left (CL) bit positions, 0's in 2 LSBs

SHR AL, 4 Shift AL four bit positions right and put 0's in

upper 4 bits.

• shift operators affect CF, ZF, OF, SF, and PF


4. String Instructions

MOVS {dest string, source string} LODS source / STOS dest

�MOVS: copies string in data segment to a string in extra segment

�LODS: copies a string in memory pointed by SI in to accumulator

�STOS: Copies a string in accumulator to memory at [DI]

Use the REP/REPE/REPNE prefixes for multiple operations

e.g.

CLD Clear direction flag, i.e. SI will be auto incremented.

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CLD Clear direction flag, i.e. SI will be auto incremented.

MOV SI, OFFSET SOURCE_STRING Point SI at start of string

LODS SOURCE_STRING Copy byte or word from string to AL or AX

MOV CX, 04H Load length of string into CX as counter

REP

MOVSB Decrement CX and copy string bytes until CX = 0

MOV DI, OFFGET TARGET_STRING Point DI at destination

STOS TARGET_STRING Store string starting @ [DI]


String Instructions…cntd

CMPS {dest string, source string}

The comparison is done by subtracting the byte or word pointed to

by DI from the byte or word pointed to by SI.

The AF, CF, OF, PF, SF, and ZF flags are affected by the comparison,

but neither of the operands is affected.

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but neither of the operands is affected.

e.g.

MOV SI, OFFSET FIRST-STRING Point SI at source string

MOV DI, OFFSET SECOND-STRING Point DI at destination string

CLD DF cleared, so SI and DI will auto-increment after compare

MOV CX,100 Put number of string elements in CX

REPE Repeat the comparison of string bytes until end

CMPSB of string or until compared bytes are not equal


5. Program Execution Instructions

CALL subroutine

There are two basic types of calls, near and far.

Near: is a call to a procedure, which is in the same segment

as the CALL instruction.

Far: a call to a procedure, which is in a different segment from the

one that contains the CALL instruction.

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CPU saves the return address in stack before branching to the

subroutine

e.g.

CALL BX near call to a subroutine whose address is in BX

CALL SAMPLE far call to a subroutine labeled SAMPLE

There should be the RET instruction at the end of a subroutine


... Program Execution Instructions

JUMP instructions

JMP/JE/JZ/JC/JO/JA/JB/JG/JGE/JL/JLE…address

Address: is the destination to jump to

Short jump: within the same segment; long jump: to a different seg.

e.g.

…

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…NEXT: CMP BX,DX ;Compare (BX-DX)

JGE DONE ;Jump to DONE if BX ≥ DX

SUB BX,AX ;Else subtract AX from BX

INC CX ;Increment counter

JMP NEXT ;Check again

DONE: MOV AX,CX ;Copy count to AX

IN AL, 8FH ;Read data from port 8FH

SUB AL,30H ;Subtract 30h from AX

JZ START_MACHINE ;Jump to label if result of subtraction was 0



LOOP instructions

LOOP/LOOPE/LOOPNE label

LOOP: loop while CX ≠ 0 aBer auto decrement

LOOPE/LOOPZ: loop while CX ≠ 0 or ZF = 1

LOOPNE/LOOPNZ: loop while CX ≠ 0 or ZF = 0

e.g.

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e.g.

MOV BX, OFFSET ARRAY ;Point BX to start of array

DEC BX

MOV CX, 100 ;Put number of array elements in CX

NEXT: INC BX ;Point to next element in array

CMP [BX], FFH ;Compare array element with FFH

LOOPE NEXT ;Loop until an array element is equal to

;FFH or all elements of the array are checked.



Interrupt instruction

INT type

Type: a number between 0 and 255 which identifies the interrupt.

• CPU pushes current program address and status in to stack before

executing the ISR.

• In DOS, INT 21 is used for basic I/O functions, like display, print…

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• In DOS, INT 21 is used for basic I/O functions, like display, print…

Example of DOS INT 21 ISRs are

o INT 21h / AH=1 read character from standard input, result is

stored in AL. If there is no character in the keyboard

buffer, the function waits until any key is pressed.

o INT 21h / AH=2 write character to standard output. DL = character to write

o INT 21h / AH=9 printout a string from DS:DX. String must be terminated by '$’



…Interrupt instruction

e.g. (prints a number in binary on the console window)…..

mov al, 5 ; 05h, or 00000101b

mov bl, 10 ; 0Ah, or 00001010b

add bl, al ; 5 + 10 = 15 (decimal), 0Fh, or 00001111b

; print result in binary

mov cx, 8 ;for the 8 bits

print: mov ah, 2 ; print function.

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print: mov ah, 2 ; print function.

mov dl, '0‘

test bl, 10000000b ; test first bit.

jz zero

mov dl, '1‘

zero: int 21h ;print to console

shl bl, 1

loop print

; print binary suffix:

mov dl, ‘b’

int 21h

;Print the string “hello world”

msg db "hello world $“

mov dx, offset msg

mov ah, 9

int 21h

;wait for any key press

mov ah, 1

int 21h

ret ;return to the operating system


6. Processor control Instructions

� flag set/clear instructionSTC/CLC: set/clear the carry flag

STD/CLD: set/clear the direction flag, etc.

� HLT Stops the processor

To get the processor out of the halt state, apply an interrupt signal

on the INTR pin, an interrupt signal on the NMI pin, or a reset

signal on the RESET input.

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signal on the RESET input.

� WAIT puts the processor in idle state (no processing)

CPU stays in this idle state until the TEST input pin is made low or

an interrupt signal is received on the INTR or the NMI input pins.

� NOP fetch- decode- no execution

refer: ESC, LOCK instruction for multi-processor system


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Programming The 8086 In Assembly Language … CMP source, dest is used to compare two numbers CMP...

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