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Fundamentals of Assembly Language

Lesson plan• Review of existing concepts

• Data transfer instructions– Practice

• Basic arithmetic instructions and

Repetitive move operations– Practice

Review of existing concepts• Comments

• Directives (page, segment, title)

• Data type (Byte (DB), Word(DW), Doubleword(DD), String

• Some arithmetic operations: ADD,SUB,MUL,DIV

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

Arithmetic instructions

• SUB[label:] SUB operand1, operand 2

operand1 =operand 1 - operand 2

operand 1: register/memory

operand 2: register/memory/immediate

Arithmetic instructions

• MULoperandUnsigned multiply.

Operand: register/memory

Arithmetic instructions

• IMUL operandSigned multiply.

Operand: register/memory

Example:

MOV AX, -2

MOV CX, -3

IMUL CX ; AX = +6

CF = 0

Arithmetic instructions

• DIV operandUnsigned multiply.

Operand: register/memory

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

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

Arithmetic instructions

• IDIV operandSigned multiply.

Operand: register/memory

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

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

Write a program to convert from Celsius to Fahrenheit and vice versa:

Tc = (5/9)*(Tf-32)

Tc: censius

Tf: fahrenheit

(The result may not be accurate due to the integer division but that is fine)

Practice

Repetitive move instructions

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

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

Repetitive move instructionsCODESEG SEGMENT PARA 'Code'MAIN 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 procedureCODESEG ENDSEND MAIN ;End of program

Result

Repetitive move instructions• DEC CX

ZF = 1 if CX = 0

• JNZ LABEL

if ZF = 0 then jump to the label

Practice• Develop an assembly program to:

– Define byte items: BYTE1 and BYTE2 (Assign any values for these two variables)

– Define a word item: WORD3 and WORD3=0– Move content of Byte1 to AL– Add content of Byte2 to AL– Set DL= 42H– Exchange the content of AL and DL– Multiply the contents of AL by DL– Transfer product from AX to WORD3

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

SEGMENT OVERRIDE PREFIX

Example:

MOV ES:[SI+2], CL ; move CL to ES:[SI+2]