microprocessors & interfacing (a1423 unit ii 8086 … · department of electronics and...

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Department of Electronics and Communication Engineering

MICROPROCESSORS & INTERFACING (A1423)UNIT – II 8086 ASSEMBLY LANGUAGE PROGRAMMING

VARDHAMAN COLLEGE OF ENGINEERING(AUTONOMOUS)

Shamshabad, Hyderabad - 501218

Microprocessors & Interfacing (A1423)

UNIT-II

2

8086 Family Assemble Language Programming

8086 Instruction Set

Simple Programs

Assembly language programs involving

1. logical, branch and call instructions

2. Sorting, evaluation of arithmetic expressions

3. String manipulations

Assembler directives

Procedures

Macros

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Machine Language Programming

3

One way to write a program is to assign a fixed binary

pattern for each instruction and represent the program

by sequencing these binary patterns.

Such a program is called a Machine language

program or an object program

Ex:

0011 1110 ; LOAD AL Register with

0000 0101 ; Value 5


Assembly Language Programming

4

Use of symbols in programming to improve the

readability.

Giving symbolic names for each instruction.

These names are called mnemonics and a program

written using these symbols are called as Assembly

Language Program.

Ex:

MOV AL, 05H ; load AL register with 5

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Features of ALP

C.LOKANATH REDDY5

For using Assembly language, the programmer needs

to know the internal Architecture of the Microprocessor.

The Assembly language written for one processor will

not usually run on other processors.

An assembly language program cannot be executed by

a machine directly, as it not in a binary form.


Features of ALP cont..

6

An Assembler is needed in order to translate an

assembly language (source program) into the object

code executable by the machine.

Unlike the other programming languages, assembly

language is not a single language, but rather a group

of languages.

Each processor family (and sometimes individual

processors within a processor family) has its own

assembly language.

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Format of Assembly Program

7

LABEL OPCODE MNEMONICS OPERAND COMMENT

Start B8,0010 MOV AX,0010H Move 0010H into AX

Labels :Begins from column1. Should start with a

letter. After the letter a combination of letters and

numbers may be used. Restricted by most

assemblers from 6 to 8 characters. Use is to identify

opcode and operands. Are needed on executable

statements, so that the statement can be branched to.


Format of Assembly Program cont..

8

Opcode: It indicates the type of operation to be performed

by the CPU.

Mnemonics: Symbolic abbreviations for operation codes or

comment to the Assembler. Specifies how data are to be

manipulated, which is residing within a microprocessor

register or in the main memory. Ex: MOV, LD, ADD

Operand: use to specify the addresses or registers used

by the operands by the Machine instructions. Operands

may be Registers, memory location, constants etc.

Comments: prefixed by ; for documentation

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Simple Program

9

Write a program to add a data byte located at offset 0500H

in 2000H segment to another data byte located at 0600H

in the same segment and store the result at 0700H in the

same segment. START

Initialize Seg. Reg

Get content of 0500H in a G.P. register

Perform addition

Store result in 0700H

STOP


Simple Program cont..

10

MOV AX, 2000H; Initializing DS with value 2000H

MOV DS, AX

MOV AX, [500H]; Get first byte from 0500H offset

ADD AX, [600H]; Add this to the second byte from

0600H

MOV [700H], AX; Store AX in 0700H (result)

HLT ; Stop

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Instruction Set of 8086

11

An instruction is a binary pattern designed inside a

microprocessor to perform a specific function.

The entire group of instructions that a microprocessor

supports is called Instruction Set.

8086 has more than 20,000 instructions.

There are 117 basic instructions in the instruction set

of 8086


Classification of Instruction Set

12

Data Transfer Instructions

Arithmetic Instructions

Bit Manipulation Instructions

Program Execution Transfer Instructions

String Instructions

Processor Control Instructions

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Data Transfer Instructions

13

General purpose byte or word transfer instructions

1. MOV: copy byte or word from specified source to specified destination.

Syntax: MOV Destination, Source

Source operand can be register, memory location or immediate operand.

Destination can be register or memory location.

Both Source and Destination cannot be memory location at the same time.


Data Transfer Instructions cont..

14


1. MOV: copy byte or word from specified source to

specified destination.

EX:

MOV CX, 037AH

MOV BL, [437AH]

MOV AX,BX

MOV DL, [BX]

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15


2. PUSH: copy word to top of stack.

The PUSH instruction decrements the stack pointer by 2 and copies a word from a specified source to the location in the stack segment where the stack pointer then points

EX:

PUSH BX

PUSH DS

PUSH AL; illegal, must push a word



C.LOKANATH REDDY16


3. POP: copy word from top of stack to specified

location.

EX:

POP BX

POP DS

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4. XCHG: Exchange bytes or exchange words.

* The segment registers cannot be used in this

instruction.

EX:

XCHG AX, DX

XCHG BL, CH



18

Simple input and output port transfer instructions

5. IN: Copy a byte or word from specified port to

accumulator .

EX:

IN AL, 0C8H

IN AX, 34H

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19

Simple input and output port transfer instructions

6. OUT: Copy a byte or word from accumulator to

specified port .

EX:

OUT 0C8H, AL

OUT 34H, AX



20

Special address transfer instructions

7. LEA: Load effective address of operand into

specified register.

It loads a 16-bit register with the offset address of the

data specified by the Source.

EX:

LEA BX, [DI]

This instruction loads the contents of DI (offset) into the BX register.

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C.LOKANATH REDDY21


8. LDS: Load DS register and another specified register from

memory.

It loads 32-bit pointer from memory source to destination

register and DS

EX: LDS BX, [4326]

Copy contents of memory at displacement 4326H in DS to BL,

contents of 4327H to BH. Copy contents at displacement of 4328H

and 4329H in DS to DS register.



22


9. LES: Load ES register and another specified register

from memory.

It loads 32-bit pointer from memory source to destination

register and ES

EX: LES BX, [4326]

Copy contents of memory at displacement 4326H in DS to BL,

contents of 4327H to BH. Copy contents at displacement of

4328H and 4329H in DS to ES register.

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23

Flag transfer instructions

10. LAHF: It copies the lower byte of flag

register to AH.

11. SAHF: It copies the contents of AH to lower

byte of flag register.

12. PUSHF: Pushes flag register to top of stack.

13. POPF: Pops the stack top to flag register.


ARITHMETIC INSTRUCTIONS

24

Addition instructions

Subtraction instructions

Multiplication instructions

Division instructions

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Addition instructions

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ADD : Add specified byte to byte or specified word to word.

This instruction add a number from source to number from

destination and put the result in the specified destination.

The source may be an immediate number, a register or a

memory location specified by any of the addressing modes.

The destination may be a register or a memory location

specified by any of the addressing modes.

The source and destination in an instruction cannot both be

memory locations.

Ex: ADD AL, 74H

Flags affected: AF, CF, OF, PF, SF, ZF.


Addition instructions Cont..

26

ADC : Add byte + byte + carry flag

Add word + word + carry flag

This instruction add a number from source to number

from destination and put the result in the specified

destination. The add with carry also adds the status of

carry flag into the result

Ex: ADC CL, BL; Add contents of BL plus carry status

to content of CL


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INC : Increment specified byte or specified word by 1

The INC instruction adds 1 to a specified register or to

a memory location

Ex: INC CL

Flags affected: AF, OF, PF, SF, ZF.

Carry flag is not affected i.e. if an 8-bit destination

containing FFH or a 16-bit destination containing

FFFFH is incremented, the result will be all 0‟s with no

carry



28

AAA : ASCII adjust after addition

Numerical data coming into a computer from a

terminal is usually in ASCII code. In this code the

numbers 0 to 9 are represented by the ASCII codes

30H to 39H.

The 8086 allows to add the ASCII codes for two

decimal digits without masking off the “3” in the upper

nibble of each.

After addition, the AAA instruction is used to make

sure the result is the correct unpacked BCD

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AAA : ASCII adjust after addition

Ex: assume AL=0011 0101, ASCII 5

BL=0011 1001, ASCII 9

ADD AL,BL; Result: AL=0110 1110= 6EH, in

correct BCD

AAA ; Now AL=00000100, unpacked BCD

4.

;CF=1 indicates answer is 14 decimal

Flags affected: AF, CF.

The AAA instruction works only on the AL register.



30

DAA : Decimal (BCD) adjust after addition

This instruction is used to make sure the result of

adding two packed BCD numbers is adjusted to

be a legal BCD number.

The result of the addition must be in AL for DAA

to work correctly.

If the lower nibble in AL after an addition is

greater than 9 or AF was set by addition, then the

DAA instruction will add 6 to lower nibble in AL.

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DAA : Decimal (BCD) adjust after addition

If the result upper nibble of AL is greater than 9 or CF

was set by addition, then the DAA instruction will add

60H to AL.

Ex: assume AL=0101 1001, BCD 59

BL=0011 0101, BCD 35

ADD AL,BL; Result: AL=1000 1110= 8EH

DAA ;add 0110 because 1110>9

;AL=1001 0100= 94 BCD

Flags affected: AF, PF, ZF, CF.


Subtraction instructions

C.LOKANATH REDDY32

SUB : Subtract byte from byte or word from word.

For subtraction the carry flag (CF) functions as borrow

flag.

The source and the destination must both be of type

byte or both be of type word.

Ex: SUB CX, BX; CX-BX. Result in CX


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Subtraction instructions Cont..

33

SBB : Subtract byte and carry flag from byte or word

and carry flag from word.

Ex: SBB CH, AL; subtract contents of AL and

;contents of CF from

;contents of CH. Result in CH




34

DEC : Decrement specified byte or specified word by 1

Ex: DEC CL

Flags affected: AF, OF, PF, SF, ZF.

Carry flag is not affected i.e. if an 8-bit destination

containing 00H or a 16-bit destination containing

0000H is decremented, the result will be all FFH or

FFFFH with no carry(borrow).

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35

NEG : Negate– invert each bit of a specified byte or

word and add 1 (forms 2‟s complement).

Ex: NEG AL; Replace number in AL with its 2‟s

complement




36

CMP : Compare two specified bytes or two specified

words.

The comparison is actually done by subtracting the

source byte or word from destination byte or word.

The source and the destination are not changed but

the flags are set to indicate the result of comparison.

Ex: CMP CX, BX


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37

CMP : Compare two specified bytes or two specified

Words.

For the instruction CMP CX, BX; CF, ZF, SF are left as

follows

Comparison CF ZF SF comment

CX = BX 0 1 0 Result of subtraction is zero

CX > BX 0 0 0 No borrow is required, so CF = 0

CX < BX 1 0 1 Subtraction required borrow, so CF = 1



38

AAS : ASCII adjust after subtraction

After subtraction, the AAS instruction is used to make

sure the result is the correct unpacked BCD

Flags affected: AF, CF

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DAS : Decimal (BCD) adjust after subtraction

This instruction is used to make sure the result of

subtracting two packed BCD numbers is adjusted to

be a legal BCD number.

The result of the subtraction must be in AL for DAS to

work correctly.

If the lower nibble in AL after subtraction is greater

than 9 or AF was set by subtraction, then the DAS

instruction will subtract 6 from lower nibble in AL.


Multiplication instructions

40

MUL : Multiply unsigned byte by byte or unsigned

word by word.

This instruction multiplies an unsigned byte from some

source times an unsigned byte in AL register or an

unsigned word from some source times an unsigned word

in AX

When a byte is multiplied by the contents of AL, the result

(product) is put in AX. A 16-bit destination is required

because the result of multiplying an 8-bit number by an 8-

bit number can be as large as 16 bit.

The MSB of result is put in AH, LSB of result is put in AL

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Multiplication instructions Cont..

41

When a word is multiplied by the contents of AX, the

result (product) can be as large as 32 bit.

The MSB of result is put in DX, LSB of result is put in

AX

Ex: MUL CX ; AX times CX, result high word in DX

;low word in AX

Flags affected: CF, OF.



42

IMUL : Multiply signed byte by byte or signed word by

word.

This instruction multiplies an signed byte from some

source times an signed byte in AL register or an

unsigned word from some source times an unsigned

word in AX.

Ex: IMUL AX ; AX times AX, result high word in DX

;low word in AX


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C.LOKANATH REDDY43

AAM : ASCII adjust after multiplication.

Before multiply of two ASCII digits, first the upper 4

bits of each digit must be masked. This leaves

unpacked BCD (one BCD digit per byte) in each byte

After the two unpacked BCD digits are multiplied, the

AAM instruction is used to adjust the product to two

unpacked BCD digits in AX



44

AAM : ASCII adjust after multiplication.

Ex: assume AL=0011 0101, ASCII 5

BL=0011 1001, ASCII 9

MUL BL; AL× BL; Result in AX

AX=00000000 00101101= 002DH,

AAM ; Now AX=00000100 00000101= 0405H,

Which is unpacked BCD for 45.

If ASCII codes for the result are desired, put 3 in upper

nibble of each byte. i.e. 3435H (ASCII code for 45)

Flags affected: PF, ZF, SF.

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Division instructions

45

DIV : Divide unsigned word by byte or unsigned double

word

by word.

When a word is divided by a byte, the word must be in

the AX register. The divisor can be in register or a

memory location.

After division, AL will contain an 8 bit result (quotient)

and AH will contain an 8-bit remainder

For a DIV, the dividend (numerator) must always be in

AX or DX and AX

Ex: DIV CX


Division instructions Cont..

46

IDIV : Divide signed word by byte or signed double

word by word.

Ex: IDIV BL

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47

AAD : ASCII adjust before division

AAD converts two unpacked BCD digits in AH and

AL to equivalent binary number in AL. This

adjustment must be made before dividing two

unpacked BCD digits in AX by an unpacked BCD

byte.

Ex:



48

AAD : ASCII adjust before division

Ex: assume AX=0607H unpacked BCD for 67

CH=09H, now adjust to binary

AAD; Result: AX=0043=43H=67 decimal

DIV CH ; Quotient: AL= 07H,

; Remainder: AH=04

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49

CBW : Convert Signed Byte to Signed Word.

Fill upper byte of word with copies of sign bit of lower

byte.

Ex: assume AX= 00000000 10011011=-155 decimal

CBW; Result: AX= 11111111 10011011=-155



50

CWD : Convert Signed Word to Signed Double word.

Fill upper word of double word with copies of sign bit

of lower word.

Ex: assume DX=00000000 00000000

AX= 11110000 11000111=-3897 decimal

CWD; Result: DX=11111111 11111111

AX= 11110000 11000111=-3897 decimal

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BIT MANIPULATION INSTRUCTIONS

51

Logical Instructions

Shift Instructions

Rotate Instructions


Logical Instructions

52

NOT : Invert each bit of a byte or word.

Ex: NOT BX

Flags affected: NO.

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Logical Instructions Cont..

53

AND : AND each bit in a byte or word with the

corresponding bit in another byte or word.

The result for each bit position will follow the truth table for

a two-input AND gate.

Ex: AND BH,CL

Flags affected: PF, SF, ZF.

CF and OF are both 0‟s after operation

PF has meaning only for the lower 8 bits of the destination.

AF will be undefined.



54

OR : OR each bit in a byte or word with the



a two-input OR gate.

Ex: OR BH,CL





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55

XOR : XOR each bit in a byte or word with the



a two-input Exclusive OR gate.

Ex: XOR BH,CL







56

TEST: AND operands to update flags, but don‟t

change operands.

The TEST instruction is often used to set flags before a

conditional jump instruction.

Ex: TEST AH,CL





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Shift Instructions

57

SAL/SHL: Shift bits of word or byte left, put zeros in

LSB.

The MSB will be shifted into CF

Ex: MOV CL, 02H

SAL BP,CL


For multiple shifts OF is undefined.

PF has meaning only for the lower 8 bits of the

destination.



Shift Instructions Cont..

58

SHR: Shift bits of word or byte right, put zeros in

MSB.

The bit shifted out of LSB will be shifted into CF

Ex: MOV CL, 02H

SHR BP,CL



PF has meaning only for the lower 8 bits of the

destination.


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Shift Instructions Cont..

59

SAR: Shift bits of word or byte right, copy old MSB into

new MSB.

The sign bit is copied into MSB.

The LSB will be shifted into CF

Ex: MOV CL, 02H

SAR BP,CL






Rotate Instructions

60

ROL: Rotate bits of word or byte left, MSB to LSB and

to

CF

The operation can be thought of as circular , because

the data bit is rotated out of MSB is circled back into

the LSB

Ex: MOV CL, 02H

ROL BL,CL


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Rotate Instructions Cont..

61

ROR: Rotate bits of word or byte right, LSB to MSB

and to CF

Ex: MOV CL, 02H

ROR BL,CL




62

RCL: Rotate bits of word or byte left, MSB to CF and

CF to LSB

Ex: MOV CL, 02H

RCL BL,CL


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63

RCR: Rotate bits of word or byte right, LSB to CF and

CF to MSB

Ex: MOV CL, 02H

RCR BL,CL



STRING INSTRUCTIONS

64

String is a series of bytes or a series of words in

sequential memory locations.

REP INS/INSB/INSW

REPE/REPZ OUTS/OUTSB/OUTSW

REPNE/REPNZ SCAS/SCASB/SCASW

MOVS/MOVSB/MOVSW LODS/LODSB/LODSW

COMPS/COMPSB/COMPSW STOS/STOSB/STOSW

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STRING INSTRUCTIONS Cont..

65

REP/REPE/REPZ/REPNE/REPNZ: Repeat String

Instruction

REP is a prefix which is written before one string

instructions. It will cause the CX register to be

decremented and the string instruction to be repeated

until CX=0.

The instruction REP MOVSB, for example will

continue to copy string bytes until the number of bytes

loaded into CX has been copied.



66

REPE and REPZ are two mnemonics for the same prefix.

They stand for Repeat if Equal and Repeat if Zero

respectively.

REPE or REPZ will cause the string instruction to be

repeated as long as the compare bytes or words are equal

(ZF=1) and CX is not yet counted down to zero.

There are two conditions to stop the repetition

1.CX=0

2.Sting bytes or words not equal.

EX: REPE CMPSB

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67

REPNE and REPNZ are two mnemonics for the same

prefix.

They stand for Repeat if Not Equal and Repeat if Not Zero

respectively.

REPNE or REPNZ will cause the string instruction to be

repeated until the compare bytes or words are equal

(ZF=1) and CX=0.

EX: REPNE SCASW



68

MOVS/MOVSB/MOVSW- Move String Byte or String

Word

This instruction copies a byte or a word from a location in

the data segment to a location in extra segment.

The offset of the source byte or word in the data segment

must be in the SI register.

The offset of the destination byte or word in the extra

segment must be contained in the DI register.

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69

SIMPLE PROGRAM

MOV SI, OFFSET SOURCE_STRING; Load offset of start of

source in DS into SI

MOV DI, OFFSET DESTINATION_STRING; Load offset of

start of destination in ES into DI

CLD; Clear direction flag to auto increment SI & DI after move

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

REP MOVSB; Decrement CX and copy string bytes until CX=0



70

CMPS/CMPSB/CMPSW- Compare String Bytes or

String Words

The CMPS instruction can be used to compare a byte or

word in one string to a byte or word in another string.

SI is used to hold the offset of a byte or word in the source

string .

DI is used to hold the offset of a byte or word in the other

string .

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71

CMPS/CMPSB/CMPSW- Compare String Bytes or

String Words

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 operand is affected.

The CMPS instruction can be used with REPE or REPNE

as prefix to compare all the elements of a string.

EX: REPE CMPSB



C.LOKANATH REDDY72

SCAS/SCASB/SCASW- Scan a String Byte or String

Word

SCAS compares a byte in AL or a word in AX with a byte or

word pointed to by DI in ES.

The string to be scanned must be in the extra segment and

DI must contain offset of the byte or word to be compared.

SCAS affects AF, CF, OF, PF, SF and ZF flags, but it does

not change either the operand in AL (AX) or the operand in

the string.

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73

LODS/LODSB/LODSW- Load a String Byte into AL or

Load a String Word into AX

This instruction copies a byte from a string location pointed

to by SI to AL, or a word from a string location pointed to

by SI to AX.

LODS affects no flags.

EX: LODS SOURCE_STRING



74

STOS/STOSB/STOSW- Store Byte from AL or Word

from AX into String.

This instruction copies a byte from AL or a word from AX to

a memory location in the extra segment pointed to by DI.

STOS affects no flags.

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PROGRAM EXECUTION TRANSFER INSTRUCTIONS

75

These instruction are used to tell the 8086 to start

fetching instructions from some new address, rather

than continuing in sequence.

1. Unconditional transfer instructions

2. Conditional transfer instructions

3. Iteration control instructions

4. Interrupt instructions


Unconditional transfer instructions

76

MNEMONIC Description Example

CALL Call a procedure (sub program), save return

address on stack

CALL BX

RET Return from a procedure to calling program. RET

JMP Go to specified address to get next instruction JMP BX

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Conditional transfer instructions

77

These instructions are often used after a compare

instruction.

The terms above and below refers to unsigned binary

numbers. Above means larger in magnitude.

The terms greater than or less than refer to signed

binary numbers. Greater means more positive.



Cont..

78

MNEMONIC Description Flags

JA/JNBE Jump if above/ Jump if not below or

Equal

CF=0

ZF=0

JAE/ JNB/

JNC

Jump if above or Equal/ Jump if Not

Below/ Jump if No Carry

CF=0

JB/ JC/

JNAE

Jump if below/ Jump if Carry/ Jump if

Not Above or Equal

CF=1

JBE/ JNA Jump if Below or Equal/ Jump if Not

Above

Either ZF=1 or

CF=1

JCXZ Jump if the CX register is Zero

JE/ JZ Jump if Equal/ Jump if Zero ZF=1

JG/ JNLE Jump if Greater/ Jump if Not Less

than or Equal

ZF=0 &

CF=OF

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Cont..

79


JGE/ JNL Jump if Greater than or Equal/ Jump

if Not Less than

SF=OF

JL/ JNGE Jump if Less than/ Jump if Not

Greater than or Equal

SF≠ OF

JLE/ JNG Jump if Less than or Equal/ Jump if

Not Greater than

ZF=1 or

SF≠ OF

JNE/ JNZ Jump if Not Equal/ Jump if Not Zero ZF=0

JNO Jump if Not Overflow OF=0

JNP/ JPO Jump if No parity/ Jump if Parity Odd PF=0

JNS Jump if not Signed SF=0

JO Jump if Overflow OF=1



Cont..

80


JP/ JPE Jump if Parity/ Jump if Parity Even PF=1

JS Jump if Signed SF=1

EX: CMP AX, 4371H

JA LABEL

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Iteration Control instructions

C.LOKANATH REDDY81

These instructions can be used to execute a series of

instructions some number of times.

MNEMONIC DESCRIPTION

LOOP Loop through a sequence of instructions until

CX=0

LOOPE/ LOOPZ Loop through a sequence of instructions while

ZF=1 and CX≠0

LOOPNE/ LOOPNZ Loop through a sequence of instructions while

ZF=0 and CX≠0


Iteration Control instructions Cont..

82

Sample Program

MOV BX, OFFSET PRICES

MOV CX, 40

NEXT: MOV AL, [BX]

ADD AL, 07H

DAA

MOV [BX], AL

INC BX

LOOP NEXT

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Interrupt instructions

83

MNEMONIC DESCRIPTION

INT Interrupt program execution, call service

procedure

INTO Interrupt program execution if OF=1

IRET Return from Interrupt service procedure to

main program


PROCESSOR CONTROL INSTRUCTIONS

84

Flag set/ clear instructions

External hardware synchronization instructions

No operation instruction

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Flag set/ clear instructions

85

MNEMONIC Description

STC Set carry flag CF=1

CLC Clear carry flag CF=0

CMC Complement the state of the carry flag CF

STD Set direction flag DF to 1(decrement string pointers)

CLD Clear direction flag DF to 0(increment string pointers)

STI Set interrupt enable flag to 1(enable INTR input)

CLI Clear interrupt enable flag to 0(disable INTR input)


External hardware synchronization instructions

86


HLT Halt(do nothing) until interrupt or reset

WAIT Wait(do nothing) until signal on the TEST pin is low

ESC Escape to external coprocessor such as 8087 or 8089

LOCK An instruction prefix. Prevents another processor from

taking the bus while adjacent instruction executes

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No operation instructions

87


NOP No action except fetch and decode


Assembler Directives

88

An assembler is a program used to convert an ALP into

equivalent machine code modules which may further be

converted to executable codes.

Assembler Directives are directions to the assembler.

Assembler directives are the commands to the assembler that

direct the assembly process.

They indicate how an operand is treated by the assembler and

how assembler handles the program.

They also direct the assembler how program and data should be

arranged in the memory.

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List of Assembler Directives

89

ASSUME DB DW DD DQ

DT END ENDP ENDS EQU

EVEN EXTRN GLOBAL GROUP INCLUDE

LABEL LENGTH NAME OFFSET ORG

PROC PTR SEGMENT SHORT TYPE


Data Definition and Storage Allocation Directives

90

Data definition directives are used to define the program

variables and allocate a specified amount of memory to them.

DB: Define Byte:

The DB directive is used to reserve byte or bytes of memory

locations in the available memory.

Syntax: Name of the variable DB Initialization Value

Ex : RANKS DB 01H, 02H, 03H, 04H

This statement direct the assembler to reserve four memory

locations for a list named RANKS and initializes them with the

above specified four values.

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Microprocessors & Interfacing (A1423)Data Definition and Storage Allocation Directives

Cont..

91

DW: Define Word

The DW directive serves the same purposes as the DB directive,

but it now makes the assembler the number of memory words (16-

bit) instead of bytes.

Syntax: Name of the variable DW Initialization Value

Ex: WORDS DW 1234H, 4567H, 78AH, 045CH

This makes the assembler to reserve four words in memory (8

bytes), and initialize the words with the specified values in the

statements.

Another option of the DW directive is explained with the DUP

operator.

Ex: WDATA DW 5 DUP (6666H)

This statement reserves five words, i.e., 10-bytes of memory for a

word label WDATA and initializes all the word locations with 6666H.


Cont..

92

DQ: Define Quad word

This directive is used to direct the assembler to reserve 4 words (8 bytes) of memory for the specified variable and may initialize it with the specified values.

Syntax: Name of the variable DQ Initialization Value

Ex: TOTAL DQ 0

Reserves 8 bytes of memory for the variable TOTAL and initializes with 0.

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Cont..

93

DD: Define Double

The directive DD is used to define a double word (4

bytes) variable.

Syntax: Name of the variable DD Initialization value

Ex: TOTAL DD 0

Reserves 4 bytes of memory for the variable TOTAL

and initializes with 0.


Cont..

94

DT: Define Ten bytes

The DT directive directs the assembler to define the

specified variable requiring 10 bytes for its storage

and initialize the 10 bytes with the specified values.

Syntax: Name of the variable DT Initialization

Value

Ex: ACODE DT 0

The above statement informs the assembler to

reserve 10 bytes of memory for a variable named

ACODE and initialize with 0.

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PROGRAM ORGANIZATION DIRECTIVES

95

The 8086 programs are organized as a collection of logical

segments. The directives used to organize the program segments

are: SEGMENT, ASSUME, ENDS etc.

SEGMENT:

The directive „SEGMENT‟ is used to indicate the beginning

of a logical segment. The directive segment follows the name of

the segment. To end the segment, SEGMENT must be ended

with ENDS statement.

Syntax: DATA SEGMENT

;

;

; program data definition

DATA ENDS


PROGRAM ORGANIZATION DIRECTIVES Cont..

96

ENDS:

The directive ENDS informs the assembler the

end of the segment. The directives ENDS and

SEGMENT must enclose the segment data or

code of the program.

Syntax: Segment name ENDS.

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97

ASSUME:

The directive assume informs the assembler the

name of the logical segment that should be used for a

specified segment when program is loaded and the

processor segment registers should point to the

respective logical segments.

Syntax: ASSUME SEGREG : SEGNAME

Ex: ASSUME CS : _code

ASSUME CS : _code, DS : _Data, SS : _stack



98

END: End of program

Program termination directive and it is used to

inform the assembler the physical end of the

program.

The statement after the directive END will be

ignored by the assembler.

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ASSEMBLER DIRECTIVES Cont..

99

OFFSET: The directive OFFSET informs the assembler to

determine the displacement of the specified variable with respect to

the base of the segment. It is usually used to load the offset of a

variable into the register using this OFFSET value a variable can be

referenced using indexed addressing modes.

Syntax: OFFSET Variable Name

Ex: DATA Segment

MSG DB „Hello World‟

A DW 50 DUP (?)

DATA ENDS

MOV DX, OFFSET MSG; DX= offset of the variable MSG

MOV SI, OFFSET A; SI= offset of the array.

MOV AX, [SI]; AX= element of array pointed to by SI i.e.[SI] refers to A[1]

[SI+2] refers to A[2]



100

PROC: Procedure

The directive PROC indicates the beginning of the

procedure. The directive PROC follows the name of

the procedure and the term NEAR or FAR follows the

PROC indicating the type of procedure. The directive

PROC is used with the directive ENDP to enclose the

procedure code.

Syntax: Procedure Name PROC NEAR/FAR

Ex: ONE PROC NEAR

TWO PROC FAR

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101

ENDP: End of Procedure

The directive ENDP informs the assembler the end of

procedure.

Syntax: Procedure Name ENDP

Ex: ONE1 ENDP.



102

EVEN: Align on Even Memory address

The directive EVEN updates the location counter to the

next even address, if the current location counter

contents are not even.

EX: EVEN

Proc ROOT

ENDP

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103

EQU: Equate

The directive EQU is used to assign a label with a value

or symbol. The use of this directive is just to reduce

recurrence of the numerical values or constants in the

program.

EX: LABEL EQU 0500H

LABEL EQU ADD



104

LABEL:

The directive LABEL is used to assign a name to the

current content of the location counter.

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105

EXTRN: External and PUBLIC:

The directive EXTRN informs the assembler that the names, procedures and labels declared after this directive have already been defined in some other assembly language modules.

while in the other module, where the names, procedures and labels actually appear, they must be declared public, using PUBLIC directive.

EX: MOD1 SEGMENT

PUBLIC FACTORIAL

MOD1 ENDS

MOD2 SEGMENT

EXTRN FACTORIAL

MOD2 ENDS



106

GROUP: Group the related Segments

The directive GROUP informs the assembler to

form logical groups of segments with similar purpose or

type. This directive inform the assembler to form a logical

group of the following segment names.

EX: PROGRAM GROUP CODE, DATA, STACK

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107

ORG: Origin

The directive ORG directs the assembler to start

the memory allocation for the particular segment, block

or code from the declared address in the ORG

statement.

EX: ORG 0200H



108

NAME: Logical Name of a Module

LOCAL: The labels, variables, constants or procedures

declared LOCAL in a module are to be used only by

that particular module.

LOCAL a, b, DATA, ARRAY, ROUTINE

LENGTH: This directive is not available in MASM. This

is used to refer to the length of a data array or a

string.

MOV CX, LENGTH ARRAY

GLOBAL: The labels, variables, constants or

procedures declared GLOBAL in a module are to be

used by other modules of the program.

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Procedures

109

A procedure is a collection of instructions to which we can direct

the flow of our program, and once the execution of these

instructions is over control is given back to the next line to

process of the code which called on the procedure.

At the time of invoking a procedure the address of the next

instruction of the program is kept on the stack so that, once the

flow of the program has been transferred and the procedure is

done, one can return to the next line

of the original program, the one which called the procedure.

Intrasegment procedure(IP in stack)

Intersegment procedure(CS:IP in stack)


Procedures Cont..

110

Syntax:

Procedure name PROC

near

instruction 1

instruction 2

RET

Procedure name ENDP

Example:

ADD2 PROC near

ADD AX,BX

RET

ADD2 ENDP

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Macros

111

A macro is a group of repetitive instructions in

a program which are codified only once and

can be used as many times as necessary.

Macro with in a macro is a nested MACRO

A macro can be defined anywhere in program

using the directives MACRO and ENDM


Defining a MACRO

112

Display MACRO

MOV AX, SEG MSG

MOV DS,AX

MOV DX, OFFSET MSG

MOV AH, 09H

INT 21H

ENDM

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Passing parameters to a MACRO

113

Display MACRO MSG

MOV AX, SEG MSG

MOV DS,AX

MOV DX, OFFSET MSG

MOV AH, 09H

INT 21H

ENDM

Parameter MSG can be replaced

by MSG1 or MSG2 while calling

the MACRO

.

.

DISPLAY MSG1

.

.

.

DISPLAY MSG2


PROCEDURE Vs MACRO

114

Procedures Macros

Accessed by CALL and RET

mechanism during program execution

Accessed by name given to macro

when defined during assembly

Machine code for instructions only put

in memory once

Machine code generated for

instructions each time called

Parameters are passed in registers,

memory locations or stack

Parameters passed as part of

statement which calls macro

Procedures uses stack Macro does not utilize stack

A procedure can be defined anywhere

in program using the directives PROC

and ENDP

A macro can be defined anywhere in

program using the directives MACRO

and ENDM

Procedures takes huge memory for

CALL(3 bytes each time CALL is used)

instruction

Length of code is very huge if macro‟s

are called for more number of times

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