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Computer Organization & Assembly Language Assembly Language Fundamentals Outline Assembly Language – Basic Elements Sample Hello Program 2 Basic Elements of Assembly Language Constants and Expressions Statements Numeric Literals Character or String constant Name Field Operation Field Operands(s) Field Comment Field Names Labels for Variables Code Labels Irvine: Assembly Language for Intel-Based Computers (1999) Numeric Literals A numeric literal is any combination of digits plus: optional decimal point, exponent, sign Examples of decimal literals: 5234 5.5 -5.5 26.0E+05 +35d Irvine: Assembly Language for Intel-Based Computers (1999) Contd.. Use a radix symbol (suffix) to select binary, octal, decimal, or hexadecimal 6A15h ; hexadecimal 0BAF1h ; leading zero required 32q ; octal 1011b ; binary 35d ; decimal (default) Irvine: Assembly Language for Intel-Based Computers (1999) Symbolic Constant (Integer) Defined using the = operator Must evaluate to a 16-bit integer or 32-bit integer when .386 directive used COUNT = 25 ROWS = 10 tablePos = ROWS * 5 Irvine: Assembly Language for Intel-Based Computers (1999) Constant Expression Combination of numeric literals, operators, and defined symbolic constants must be evaluated at assembly time Examples: 4 * 20 -3 * 4 / 6 ROWS - 3 ; (ROWS is a constant) COUNT MOD 5 Irvine: Assembly Language for Intel-Based Computers (1999) Statements Syntax: name operation operand(s) comments name and comment are optional Number of operands depend on the instruction One statement per line Each statement is either: Instruction (translated into machine code) Assembler Directive (instructs the assembler to perform some specific task such as allocating memory space for a variable or creating a procedure) At least one blank or tab character must separate the field. Statement Example mnemonic Operands Comment Label ; Code Here: mov ax,count ; store count into ax ; Data, using DW directive myArray dw 1000h,2000h Data name dw 3000h,4000h Main PROC Label Operation Irvine: Assembly Language for Intel-Based Computers (1999) Contd.. Name Field 10 The assembler translates names into memory addresses. Names can be 1 to 31 character long and may consist of letter, digit or special characters. If period is used, it must be first character. Embedded blanks are not allowed. May not begin with a digit. Not case sensitive Contd.. Operation Field: Symbolic operation (Op code) 11 Symbolic opcode translated into ML opcode In an assembler directive, the operation field represents Pseudo-opcode Pseudo-op is not translated into ML opcode, it only tells assembler to do something. Example: PROC psuedo-op is used to create a procedure Contd.. Operand Field An instruction may have zero, one or more operands. In two-operand instruction, first operand is destination, second operand is source. For an assembler directive, operand field represents more information about the directive Comments Field 12 Optional Marked by semicolon in the beginning Ignored by assembler Good practice The Hello World Program title Hello World Program (hello.asm) ; This program displays “Hello, world!” .model small .stack 100h .data message db “Hello, world!”,0dh,0ah,’$’ .code main proc mov ax,@data mov ds,ax mov ah,9 mov dx,offset message int 21h mov ax,4C00h int 21h main endp end main Irvine: Assembly Language for Intel-Based Computers (1999) Sample Hello Program program title (comment) title Hello World Program (hello.asm) ; This program displays “Hello, world!” comment line .model small .stack 100h memory model set the stack size Irvine: Assembly Language for Intel-Based Computers (1999) Sample Hello Program starts the data segment .data message db “Hello, world!”,0dh,0ah,’$’ starts the code segment .code main proc mov ax,@data sets DS to the offset of the mov ds,ax data segment mov ah,9 mov dx,offset message int 21h calls DOS display function 9 mov ax,4C00h int 21h main endp end main halts program Irvine: Assembly Language for Intel-Based Computers (1999) Legal Combinations of Operands for XCHG Destination Operand Source Operand Legal General Register General Register YES General Register Memory Location YES Memory Location General Register YES Memory Location Memory Location NO Legal Combinations of Operands for MOV Destination Operand Source Operand Legal General Register General Register YES General Register Memory Location YES General Register Segment Register YES General Register Constant YES Memory Location General Register YES Memory Location Memory Location NO Memory Location Segment Register YES Memory Location Constant YES Legal Combinations of Operands for MOV Destination Operand Source Operand Legal Segment Register General Register YES Segment Register Memory Location YES Segment Register Segment Register NO Segment Register Constant NO Constant General Register NO Constant Memory Location NO Constant Segment Register NO Constant Constant NO Legal Combinations of Operands for ADD & SUB Destination Operand Source Operand Legal General Register General Register YES General Register Memory Location YES General Register Constant YES Memory Location General Register YES Memory Location Memory Location NO Memory Location Constant YES References Chapter 3, Ytha Yu and Charles Marut, “Assembly Language Programming and Organization of IBM PC” Chapter 3,Assembly Language for Intel Based-Computers 20