Review of Assembly language

Review of Assembly language

Recalling main concepts


Segment: special areas defined to contain CODE, DATA and STACK

Paragraph boundary: location evenly divisible by 16 or 10H


Stack Segment

Data Segment

Code Segment

SSDSCS

Segment Registers

Example program; Add two numbers and store the results into the third variablepage 60,132TITLE A04ASM1 (EXE) Move and add operations; ---------------------------------------------STACK SEGMENT PARA STACK 'Stack'

DW 32 DUP(0)STACK ENDS; ----------------------------------------------DATASEG SEGMENT PARA 'Data'

FLDD DW 215FLDE DW 125FLDF DW ?

DATASEG ENDS; -----------------------------------------------CODESEG SEGMENT PARA 'Code'MAIN PROC FAR

ASSUME SS:STACK,DS:DATASEG,CS:CODESEGMOV AX,DATASEG ;Set address of data MOV DS,AX ; segment in DS MOV AX,FLDD ;Move 0215 to AXADD AX,FLDE ;Add 0125 to AXMOV FLDF,AX ;Store sum in FLDFMOV AX,4C00H ;End processingINT 21H

MAIN ENDP ;End of procedureCODESEG ENDS ;End of segment

END MAIN ;End of program

; Add two numbers and store the results into the third variablepage 60,132TITLE A04ASM1 (EXE) Move and add operations

; ---------------------------------------------STACK SEGMENT PARA STACK 'Stack'

DW 32 DUP(0)STACK ENDS

; ----------------------------------------------DATASEG SEGMENT PARA 'Data'


DATASEG ENDS

; -----------------------------------------------CODESEG SEGMENT PARA 'Code'MAIN PROC FAR

ASSUME SS:STACK,DS:DATASEG,CS:CODESEGMOV AX,DATASEG ;Set address of data MOV DS,AX ;Segment in DS MOV AX,FLDD ;Move 0215 to AXADD AX,FLDE ;Add 0125 to AXMOV FLDF,AX ;Store sum in FLDFMOV AX,4C00H ;End processingINT 21H

MAIN ENDP ;End of procedureCODESEG ENDS ;End of segmentEND MAIN ;End of program

Comments

; <your comments>

COMMENTS

; Add two numbers and store the results into the third variablepage 60,132TITLE A04ASM1 (EXE) Move and add operations

; ---------------------------------------------STACK SEGMENT PARA STACK 'Stack'

DW 32 DUP(0)STACK ENDS

; ----------------------------------------------DATASEG SEGMENT PARA 'Data'


DATASEG ENDS

; -----------------------------------------------CODESEG SEGMENT PARA 'Code'

MAIN PROC FARASSUME SS:STACK,DS:DATASEG,CS:CODESEGMOV AX,DATASEG ;Set address of data MOV DS,AX ;Segment in DS MOV AX,FLDD ;Move 0215 to AXADD AX,FLDE ;Add 0125 to AXMOV FLDF,AX ;Store sum in FLDFMOV AX,4C00H ;End processingINT 21H


IDENTIFIERS

Identifiers

Identifier is a name applied to an item in a program to reference

• Name (e.g: FLDD DW 215)

• Label (e.g: MAIN PROC FAR) Identifiers must not a reserved word and only contain:

• Alphabetic letters (A-Z,a-z)

• Digits (0-9)

• ?,_,$,@,dot (.) (but not for the first character) Maximum length is 247

IDENTIFIERS

Instructions: ADD, MOV Directives: .TITLE, .MODEL Operators: FAR, SIZE Pre-defined symbols: @Data, @Model Register: AX,BX

RESERVED WORDS

STATEMENT

Instructions: are translated to object code

MOV, ADD, LEA.. Directives: tell the assembler to perform

a specific action.

[identifier] operation [operand(s)] [;comments]

; Add two numbers and store the results into the third variablepage 60,132TITLE A04ASM1 (EXE) Move and add operations; ---------------------------------------------STACK SEGMENT PARA STACK 'Stack'




ASSUME SS:STACK,DS:DATASEG,CS:CODESEGMOV AX,DATASEG ;Set address of data MOV DS,AX ;Segment in DS MOV AX,FLDD ;Move 0215 to AXADD AX,FLDE ;Add 0125 to AXMOV FLDF,AX ;Store sum in FLDFMOV AX,4C00H ;End processingINT 21H


STATEMENTS

Directives

Control the way a source program assembles and lists

Generate no machine code (unlike instructions which generate object code)

; Add two numbers and store the results into the third variablepage 60,132TITLE A04ASM1 (EXE) Move and add operations; ---------------------------------------------

STACK SEGMENT PARA STACK 'Stack'DW 32 DUP(0)

STACK ENDS; ----------------------------------------------DATASEG SEGMENT PARA 'Data'



ASSUME SS:STACK,DS:DATASEG,CS:CODESEGMOV AX,DATASEG ;Set address of data MOV FLDF,AX ;Store sum in FLDFMOV AX,4C00H ;End processingINT 21H


Segment directive

Segment directive

Name Operation OperandSegment-name SEGMENT [align][combine] [`class’]

Segment-name ENDS

Example:

STACK SEGMENT PARA STACK 'Stack‘

STACK ENDS

; Add two numbers and store the results into the third variable

page 60,132TITLE A04ASM1 (EXE) Move and add operations; ---------------------------------------------STACK SEGMENT PARA STACK 'Stack'



DATASEG ENDS; -----------------------------------------------CODESEG SEGMENT PARA 'Code'

MAIN PROC FARASSUME SS:STACK,DS:DATASEG,CS:CODESEGMOV AX,DATASEG ;Set address of data MOV DS,AX ;Segment in DS MOV AX,FLDD ;Move 0215 to AXMOV FLDF,AX ;Store sum in FLDFMOV AX,4C00H ;End processingINT 21H


PROC directive

PROC directive

Format:Procedure-name PROC Operand

Comment

Procedure-name ENDP

Operand: relates to program execution (FAR)

; Add two numbers and store the results into the third variablepage 60,132TITLE A04ASM1 (EXE) Move and add operations; ---------------------------------------------STACK SEGMENT PARA STACK 'Stack'




ASSUME SS:STACK,DS:DATASEG,CS:CODESEGMOV AX,DATASEG ;Set address of data MOV DS,AX ;Segment in DS MOV AX,FLDD ;Move 0215 to AXMOV FLDF,AX ;Store sum in FLDFMOV AX,4C00H ;End processingINT 21H


ASSUME directive

ASSUME directive

Tells the assembler the purpose of each segment in the program

Example:

ASSUME SS:STACK,DS:DATASEG,CS:CODESEG

Simplified Segment Directives

Model memory-model# Code segment #Data segment

Small: 1, <=64K 1,<=64KMedium: any number,size 1, <=64KCompact: 1, <=64K any

number,size

Large: any number,size any number,size

Huge: any number,size any number,size

Simplified Segment Directives

STACK [size] (default: 1K) DATA (default size is 1K) CODE (default size is 1K) .EXIT directive

EQUATE directives

Equal-Sign directiveCOEFFICIENT= 100

EQU directive

COEFFICIENT EQU 100

Data type Format for data definition

[name] Dn expression

Name: identifierDn: Directives and can be:

DB: byte DF:farwordDW: word DQ:quadwordDD: doubleword DT:tenbytes

Expression:can be unnitialized: ?can be assigned a constant: such as 25, 21.

Example:• DATAZ DB 21,22..• DW 10 DUP(?)

Data type

Constant:• String: is defined within ‘ ‘ or “ “

MESSAGE DB “I am learning assembly language”

• Numeric:

• Is stored in reverse sequence

• Binary: 01B

• Decimal: 9D( D is optional)

• Hexadecimal: 1FH

• Real: 12R

Directives for defining Data

Byte: DBWord: DW

Doubleword: DD

Farword: DF

Quadword: DQ

Tenbytes: DT

Some instructions on arithmetic calculation

ADD:ADD register register/memory/immediateExample: ADD AX,FLDE

SubtractSBB register register/memory/immediateExample: SUB AX, 100

MultiplicationIMUL registerExample: IMUL CX

DivisionDIV registerExample DIV CX

Data transfer instructions MOV instruction

• Transfers data referenced by the address of the second operand to the address of the first operand

• Destination has to have the same length as source[label:] MOV register/memory register/memory/immediate

Example:

MOV F, AX ; // Move content of AX to the variable F

MOV CX, D ;// Move value of D to CX

MOV ES, AX

MOV AX, 215

Note Note

• MOV instruction can’t:

• set the value of the CS and IP registers.

• copy value of one segment register to another segment register (should copy to general register first).

MOV ES, DS

•copy immediate value to segment register (should copy to general register first).

MOV DS, 100

• MOV instruction can’t:

• set the value of the CS and IP registers.

• copy value of one segment register to another segment register (should copy to general register first).

MOV ES, DS

•copy immediate value to segment register (should copy to general register first).

MOV DS, 100

MOVSB:

Copy byte at DS:[SI] to ES:[DI]. Update SI and DI.

Algorithm:

ES:[DI] = DS:[SI]

if DF = 0 then SI = SI + 1 DI = DI + 1

else SI = SI - 1 DI = DI - 1

DF: direction flag from the flag register

MOVSB and MOVSW

MOVSW:

Copy word at DS:[SI] to ES:[DI]. Update SI and DI.

ES:[DI] = DS:[SI]

if DF = 0 then SI = SI + 2 DI = DI + 2

else SI = SI - 2 DI = DI - 2

DF: direction flag from the flag register

MOVSB and MOVSW

XCHG swap the two data items

[label:] XCHG register/memory, register/memory

Example:

MOV AL, 5

MOV AH, 2

XCHG AL, AH ; AL = 2, AH = 5

XCHG AL, AH ; AL = 5, AH = 2

XCHG instruction

Load Effective Address. REG = address of memory (offset)

[label:] LEA register/memory

Example:

LEA AX, m ;load offset address of m to AX

LEA instruction

Arithmetic instructions INC and DEC instruction

• Increasing or decreasing the contents of register or memory location by 1

[label:] INC/DEC register/memory

Flag: OF, SF and ZFOF:is set when an instruction resulted in a carry into the

sign bit of the result. SF: is set if the sign bit of a result is set ZF: is set if the result is equal to 0.

Arithmetic instructions

ADD[label:] ADD/SUB operand1, operand 2

operand1 =operand 1 + operand 2

Operand 1: register/memory

Operand 2: register/memory/immediate


SUB[label:] SUB operand1, operand 2

operand1 =operand 1 - operand 2

operand 1: register/memory

operand 2: register/memory/immediate


MUL operandUnsigned multiply.

Operand: register/memory


IMUL operandSigned multiply.


Example:

MOV AX, -2

MOV CX, -3

IMUL CX ; AX = +6

CF = 0


DIV operandUnsigned multiply.


when operand is a byte:AL = AX / operandAH = remainder (modulus)

when operand is a word:DX = remainder (modulus)


IDIV operandSigned multiply.


when operand is a byte:AL = AX / operandAH = remainder (modulus)

when operand is a word:DX = remainder (modulus)

Repetitive move instructionsTITLE A04ASM1 (EXE) Move and add operations; ---------------------------------------------STACK SEGMENT PARA STACK 'Stack'

DW 32 DUP(0)STACK ENDS; ----------------------------------------------DATASEG SEGMENT PARA 'Data' STRING1 DW "12345678","$" STRING2 DW ? DATASEG ENDS

Repetitive move instructionsMAIN PROC FAR MOV AX, dataseg MOV DS, AX MOV ES, AX MOV CX, 09 ; Initialize to move 9 characters LEA SI, STRING1 ; Initialize source index register to offset of string 1 LEA DI, STRING2 ; Initialize destination index register to offset of string 2

BEGINLOOP: MOV AL,[SI] ; Get a current character from string 1 to AL MOV [DI], AL ; Move it to the current character in string 2 INC SI ; Move to the next character in string 1 INC DI ; Move to the next character in string 2 DEC CX ; Decrease the count for loop JNZ BEGINLOOP ; Continue to loop if count is not 0 MOV AH, 09H LEA DX, STRING2 int 21H ; Display String 2 MAIN ENDP ;End of procedureEND MAIN ;End of programCODESEG ENDS

Result

Repetitive move instructions

DEC CXZF = 1 if CX = 0

JNZ LABEL

if ZF = 0 then jump to the label

Addressing mode

Register addressing: E.g ADD AX, BX

fastest type of operations Immediate addressing

Immediate contains a constant value or an expressionE.g: MOV AX, 0245H

Direct memory addressingOne of operand references a memory location and the

other operand references a registerE.G MOV FLDF, AX

Addressing mode

Direct-Offset addressing

use arithmetic instruction to modify an address

e.g MOV CX, DATAZ+2 Indirect memory addressing

Use BX and BP, DI and SI within [ ]

e.g. MOV [BX], CL

Addressing mode

Base Displacement AddressingUses BX, BP and DI, SI and combine with a displacement to form an effective addressE.g MOV AL,[SI+2]

Base-Index AddressingCombine BX,BP with DI,SI to form effective address

E.G MOV AL,[BX+SI]

Addressing mode

Base-Index Displacement Addressing

Combine BX, BP and DI, SI and a displacement to form an effective address

E.g MOV AL,[BX+SI+2]

NEAR and FAR address

NEAR address

consists of 16 bit offset portion of an address

used in real mode

FAR address

consists of both the segment and offset portions in the form of 32 bit segment:offset

CMP Instruction

[label:] CMP register/memory, register/memory/immediate

Compares the first to the second operand Affects: AF, CF, OF, PF, SF and ZF flag

CMP AX, DX

JE Startloop

Conditional Jump instructions

Jump based on unsigned data

[label:] JE/JZ short-address

Jump if equal or Jump if zero

[label:] JNE/JNZ short-address

Jump if not equal or Jump if not zero

Flag: ZF

Example

MOV AL, 5

CMP AL, 5

JE label1

JMP exit

label1: MOV CX, BX

exit: …..

Conditional Jump instructions

JG: Jump if first operand is Greater then second operand (as set by CMP instruction). Signed.

if (ZF = 0) and (SF = OF) then jump

Syntax: [label:] JG short-address

Example

MOV AL, 5

CMP AL, -5

JG label1

JMP exit

label1: MOV CX, -5 ; in this case AL > -5

exit:

Conditional Jump Instruction

JL: Jump if first operand is Less then second operand (as set by CMP instruction). Signed.

if SF <> OF then jump

Syntax: [label:] JL short-address

Example

MOV AL, -2

CMP AL, 5

JL label1

JMP exit

label1: MOV CX, 5 ; in this case AL < 5

exit: …

Review of Assembly language

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