Lecture 02 Introduction to Assembly Language PDF

Introduction to Assembly Language  Introduction to assembly language Assembly language program consist of statements, one per line. Each statement call instruction, which the assembler translates into machine code, or an assembler directive. which tells the assembler to perform some specific ta...

Introduction to Assembly Language  Introduction to assembly language Assembly language program consist of statements, one per line. Each statement call instruction, which the assembler translates into machine code, or an assembler directive. which tells the assembler to perform some specific task. Both instruction and directives consists of four fields: 2 Dr. Hassanein Shaban Ahmed Introduction to assembly language Statements in assembly is either  Instruction: one per line and assembler translates it into machine code  Assembler directive : instructs assembler to perform some specific task Most (but not all) instructions are made up of three characters and have an operand then a comma then another operand. Syntax: Instruction operand1, [operand2]  operand1 can be a register, or memory location.  operand2 can be a register, memory location, or immediate value. 3 Dr. Hassanein Shaban Ahmed  Introduction to assembly language In General, instruction in assembly consists of four fields: name operation operand1,[operand2 ] ;comment An example of an instruction is start: MOV CX,5 ;initialize counter An example of an assembler directive is MAIN PROC 4 Dr. Hassanein Shaban Ahmed  Introduction to assembly language Name Field: The name field is used for instruction labels, procedure name, and variables names: the assembler translates names into memory addresses.  It consists of letters, digits, and the special characters ?. @ _ $ %  It is long from 1 to 31 characters  It must be the first character  Not begin with a digit 5 Dr. Hassanein Shaban Ahmed  Introduction to assembly language Operation Field: For an instruction, the operation field contains a symbolic operation code (opcode) for example MOV,ADD. In an assembler directive, the operation field contains a pseudo-operation code. Pseudo-ops are not translated machine code; they simply tell assembler to do something. such as PROC is used to create a procedure. 6 Dr. Hassanein Shaban Ahmed  Introduction to assembly language Operands Field: For an instruction, the operand field specifies the data that are to be acted on by the operation. An instruction may have zero, one, or two operands for example: NOP INC AX ADD word1,2 operand1 can be a register, or memory location. It is the destination operand  operand2 can be a register, memory location, or immediate value (constant). It is the source operand For an assembler directive, the operand field contains more information about the directive 7 Dr. Hassanein Shaban Ahmed  Introduction to assembly language Comment Field: The comment field of a statement is used by the programmer to say something about what the statement does. A semicolon marks the beginning of this field, and the assembler ignores anything typed after the semicolon.  It starts by a semi colon  It is optional  The assembler ignore it  It is used by the programmer to say what the statement does 8 Dr. Hassanein Shaban Ahmed  Program Data The processor “machine language” operates only on binary data. In assembly language program we may express data as binary, decimal, or hex, and even as characters. Numbers A binary number is written as a bit string followed by letter “B” or “b”. For example 1010B A decimal number is a string of decimal digit, ending with an optional letter “D” or “d” A hex number must be begin with a decimal digit and end with the letter “H” or “h”. For example 0ABCH (the reason for this is that the assembler would be unable to tell whether a symbol such as “ABCH” represents the variable name “ABCH” or the hex number ABC Characters Characters and character string must be enclosed in single or double quotes for example, “A” or ‘hello’ 9 Dr. Hassanein Shaban Ahmed  Variables We use assembler directive to define a variable. We use one of the following pseudo op.  DB : define Byte  DW : Define Word 10 Dr. Hassanein Shaban Ahmed  Variables: DB and DW Directive The DB and DW are directive that define data. The byte variable defines as the following : name DB initial_value For Example: ALPHA1 DB 4 ALPHA2 DB ? Notice that:The question mark “?” means the variables uninitialized. 11 Dr. Hassanein Shaban Ahmed  DB and DW Directive The word variable defines as the following : name DW initial_value For Example: BETA1 DW 329 12 Dr. Hassanein Shaban Ahmed  Program Structure The machine language programs consists of code, data, and stack. Each part occupies a memory segment. This organization is reflected in an assembly language program. This time, the code, data, and stack are structured as program segments. Each program segment is translated into a memory segment by assembler. Memory Models: The size of code and data of a program can be determined by specifying a memory model using the.MODEL directive. The syntax is.MODEL memory model Most frequently memory models are SMALL, MEDIUM, COMPACT, LARGE, and Huge. In general the appropriate model is small. 13 Dr. Hassanein Shaban Ahmed  Memory Models Model Description Code in one segment SMALL data in one segment Code in more than one segment MEDIUM data in one segment Code in one segment COMPACT data in more than one segment Code in more than one segment LARGE data in more than one segment No array larger than 64kb Code in more than one segment HUGE data in more than one segment Arrays may be larger than 64kb The.MODEL directive should come before any segment definition 14 Dr. Hassanein Shaban Ahmed  Segment Definition Data Segment A program’s data segment contains all variables definitions and constant. To declare a data segment, we use the directive.DATA, followed by variable and constant declarations. For example,.DATA WORD1 DW 2 WORD2 DW 5 MSG DB ‘THIS IS A MESSAGE’ 15 Dr. Hassanein Shaban Ahmed Cont… A program containing data segment should begin with two instructions: MOV AX, @DATA MOV DS,AX @DATA is the name of data segment defined by.DATA 16 Dr. Hassanein Shaban Ahmed  Segment Definition Stack Segment The purpose of the stack segment declaration is to set aside a block of memory (the stack area) to store the stack. The declaration syntax is.STACK size Where size is an optional number that specifies the stack size in bytes. For example.STACK 100H This means that the stack area is 100H bytes. 17 Dr. Hassanein Shaban Ahmed  Segment Definition Code Segment The code segment contains a program instructions. The declaration syntax is.CODE name Where name is the optional name of the segment (there is no need for a name in a SMALL program, because the assembler will generate an error Inside a code segment, instructions are organized as procedures. 18 Dr. Hassanein Shaban Ahmed  Segment Definition The simplest procedure definition as the following Name Proc ; body of procedure Name EndP Where Name is the name of the procedure Proc and EndP are assembler directive (more on defined procedure in chapter 8). An example of a code segment definition:.CODE MAIN PROC ;main procedure instructions MAIN ENDP ; other procedure go here 19 Dr. Hassanein Shaban Ahmed  Putting it together A form of assembly programs We can construct the general Form of a SMALL model program. This form may be used in most applications:.MODEL SMALL.STACK 100H.DATA ;variables & constants definitions here.CODE MAIN PROC ; instructions go here MAIN ENDP ; other procedure go here END MAIN The last line in the program should be the END directive, followed by name of the main procedure 20 Dr. Hassanein Shaban Ahmed  The MOV Instruction The MOV instruction transfers data of the second operand2 (source) to the first operand1(destination). The general format for MOV is [label:] MOV operand1,operand2 comment [label:] MOV register/memory, register/memory, immediate Or simplified format is MOV operand1,operand2 Both operands must agree in size: (both must have the same size) Byte, Word,…. With Emu8086, each hex. Number having the size ” Byte” should start by 0. 21 Dr. Hassanein Shaban Ahmed  Example: MOV AL,0B2h; put B2h into AL register Instruction Operand 1 Operand 2 Comment Emu8086 doesn’t accept : MOV AL, B2 22 Dr. Hassanein Shaban Ahmed  The MOV Instruction Example MOV DX,AX ;register-to-register MOV data1,DH ;register-to-memory, direct MOV [DI],BX ;register-to-memory, indirect MOV CX,40 ;immediate-to-register MOV data1,40 ;immediate-to-memory MOV CH,data1 ;memory-to-register MOV AL, 0FFh ; Hex. Number move MOV AH, 255 ; Dec. Number move MOV DL, 01011111b; Binary Number move MOV BL, AH ; Register value move MOV CL, ‘A’ ; set CL to ASCII code of 'A', it is 41h. 23 Dr. Hassanein Shaban Ahmed  The XCHG exchange Instruction The XCHG instruction performs another type of data transfer. It exchanges the contents of 2 registers or register and memory location. The general format for XCHG is [label:] XCHG register/memory, register/memory Destination Operand Source Operand General Register Memory Location General Register Yes Yes Memory Location yes no Example WORDQ DW ? …… XCHG CL,BH ; exchange the contents two ; registers XCHG CX,WORDQ ; exchange the contents register ; and memory 24 Dr. Hassanein Shaban Ahmed  Sample assembly Programs.model small.stack 64.DATA data1 DB 52h data2 DB 29h sum DB ?.CODE MAIN PROC FAR MOV AX,@DATA MOV DS,AX MOV AL,data1 MOV BL,data2 ADD AL,BL MOV sum,AL MOV AH,4CH INT 21H MAIN ENDP END MAIN 25 Dr. Hassanein Shaban Ahmed  INC and DEC Instruction  INC is used to add 1 to register or memory location  Dec is used to subtract one from register or memory location  The general format for DEC/INC is [label:] INC/DEC register, memory Note that INC and DEC require only one operand  Example:  INC word1  DEC b1  INC AX 26 Dr. Hassanein Shaban Ahmed  ADD and SUB instruction The ADD and SUB instructions are used to add or substrate the contents of two registers, a register and memory location, or to add (subtract) a number to (from) a register or memory location. The general format for ADD/SUB is ADD destination, source SUB destination, source Illegal Combinations of operands for ADD and SUB Destination Source Register Example Memory Example Register Yes ADD AX,AX Yes ADD WORD,AX Memory Yes ADD AX,WORD NO ADD WORD1,WORD2 Constant Yes ADD AX,5 Yes ADD WORD,5 27 Dr. Hassanein Shaban Ahmed ADD instruction- Addition Operation: operand1 = operand1 + operand2 Examples: MOV AL, 5 ; AL = 5 ADD AL, -3 ; AL = 2 MOV AL,08h ADD AL,07h ; AL=0Fh MOV AL,0FFh ADD AL,01h ; AL=00h and AH =00h MOV AL,0FFh ADD AX,01h ; AL=00h and AH =01h 28 Dr. Hassanein Shaban Ahmed  SUB instruction- Subtraction Operation: operand1 = operand1 - operand2 Examples: MOV AL, 5 ; AL = 5 SUB AL, 4 ; AL = 1 MOV AL,0Ah SUB AL,07h ; AL=03h MOV AL,0FFh SUB AL,01h ; AL=FEh 29 Dr. Hassanein Shaban Ahmed  ADD and SUB instruction EX.4.3.MODEL SMALL.STACK 64.DATA xx1 Dw 0AFh yy1 Dw 96h zz1 Dw ?.CODE MAIN PROC FAR mov ax,@data mov ds,ax mov ax,xx1 add ax,yy1 mov zz1,ax mov ax,4c00h int 21h MAIN ENDP END MAIN 30 Dr. Hassanein Shaban Ahmed  NEG instruction The NEG instruction is used to negate the contents of the destination. NEG does this by replace the contents by its two’s complement. The general format for NEG is [label:] NEG register, memory Ex4.4.MODEL SMALL.STACK 64.DATA xx1 Dw 0FFFEh.CODE MAIN PROC mov ax,@data mov ds,ax mov ax,xx1 NEG ax mov ah,4ch int 21h MAIN ENDP END MAIN 31 Dr. Hassanein Shaban Ahmed