Assembly LanguageFundamentals of Assembly
language
Motaz K. SaadSpring 2007
1Motaz K. Saad, Dept. of CS
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
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Operand Types
• Three basic types of operands:– Immediate – a constant integer (8, 16, or 32 bits)
• value is encoded within the instruction
– Register – the name of a register• register name is converted to a number and encoded
within the instruction
– Memory – reference to a location in memory• memory address is encoded within the instruction, or a
register holds the address of a memory location
Motaz K. Saad, Dept. of CS 3
Data transfer instructions• MOV instruction
• Move from source to destination. Syntax:• MOV destination,source
– 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 FMOV CX, D ;// Move value of D to CXMOV ES, AXMOV AX, 215
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MOV Instruction
.datacount DB 100wVal DW 2.code
mov bl,countmov ax,wValmov count,al
mov al,wVal ; errormov ax,count ; errormov eax,count ; error
• Move from source to destination. Syntax:
MOV destination,source• No more than one memory operand permitted
MOV VAR1,VAR2 • CS, IP, and IP cannot be the destination
MOV IP, 100• No immediate to segment moves
MOV DS, @DATA
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• Load Effective Address. REG = address of memory (offset)
LEA register/memory
Example:LEA AX, m ;load offset address of m to AX
LEA instruction
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Arithmetic instructions• INC and DEC instruction
– Increasing or decreasing the contents of register or memory location by 1
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.
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Arithmetic instructions
• ADDADD/SUB operand1, operand 2
operand1 =operand 1 + operand 2Operand 1: register/memoryOperand 2: register/memory/immediate
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Arithmetic instructions
• SUBSUB operand1, operand 2
operand1 =operand 1 - operand 2
operand 1: register/memoryoperand 2: register/memory/immediate
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Arithmetic instructions
• MUL operandUnsigned multiply.
Operand: register/memory
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Arithmetic instructions
• IMULoperandSigned multiply.
Operand: register/memoryExample:
MOV AX, -2MOV CX, -3IMUL CX ; AX = +6
CF = 0
11Motaz K. Saad, Dept. of CS
Arithmetic instructions
• DIV operandUnsigned multiply.
Operand: register/memorywhen operand is a byte:
AL = AX / operandAH = remainder (modulus)
when operand is a word:DX = remainder (modulus)
12Motaz K. Saad, Dept. of CS
Arithmetic instructions
• IDIV operandSigned multiply.
Operand: register/memorywhen operand is a byte:
AL = AX / operandAH = remainder (modulus)
when operand is a word:DX = remainder (modulus)
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Write a program to convert from Celsius to Fahrenheit and vice versa:
Tc = (5/9)*(Tf-32)
Tc: censiusTf: fahrenheit
(The result may not be accurate due to the integer division but that is fine)
Practice
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Repetitive move instructionscopying a string to another
TITLE A04ASM1 (EXE) Move and add operations; ---------------------------------------------.STACK; ----------------------------------------------.DATA STRING1 DB "12345678","$" STRING2 DB ?
15Motaz K. Saad, Dept. of CS
Repetitive move instructions.CODEMAIN PROC FAR MOV AX, @DATA 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 .EXITMAIN ENDP ;End of procedureEND MAIN ;End of program 16Motaz K. Saad, Dept. of CS
Repetitive move instructions• DEC CX
ZF = 1 if CX = 0
• JNZ LABEL
if ZF = 0 then jump to the label
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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
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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
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Addressing mode
• Direct-Offset addressinguse arithmetic instruction to modify an address
e.g MOV CX, DATAZ+2• Indirect memory addressingUse BX and BP, DI and SI within [ ]e.g. MOV [BX], CL
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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]
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Addressing mode
Base-Index Displacement AddressingCombine BX, BP and DI, SI and a displacement to form an effective addressE.g MOV AL,[BX+SI+2]
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