Assembly Language Lab #5

Transcription

1 Islamic University of Gaza Computer Engineering Department 2009 Assembly Language Lab #5 Eng. Tahani Z. Fourah Islamic University of Gaza

2 Lab 5 Addressing Modes The addressing modes are different ways to find the physical memory address( how cpu can access data in memory). There is seven addressing modes used in assembly programming with 80x86 family,these modes are. 1. Register Operand AM Immediate operand AM mov ax,5 2. Immediate operand AM mov ax,bx 3. Direct: [200],op1 In this mode the 16 bit Offset is taken directly from the instruction..data START DB 23H mydouble DD H.CODE MOV CX, START MOV START,BL MOV CX, [START+1] MOV CX, [200] Mov al, Mydouble ;error because the operand size do not match Mov al, byte PTR mydouble Mov al, word PTR mydouble Note : The PTR operator is used to override the default size of an operand. 4. Register Indirect: [bx],[si], [di],[bp] In this mode the Offset is specified in either a pointer register or an index register. The pointer register can be either base register(bx) or base pointer(bp) and Index register can either be Source index(si) or Destination index(di) register. Example: mov bx,100h 33

3 mov ax,[bx] This AM is useful to address an array since we can't use direct addressing to address all the elements in an array. 5. Based AM (Addressing Mode) In this mode EA( effective address or physical address ) is obtained by adding a displacement (offset ) value in of BX or BP to the segment registers used are DS & SS. When Memory is accessed, the 20 bit physical address is computed from BX and DS. On the other hand, when the stack is accessed, the 20 bit physical address is computed from BP and SS. MOV AX,[BX] MOV AX,[BX+5] MOV AX,5[BX] MOV AX, [BX]+5 ; Register added to a constant to form the offset MOV AX,[BP] MOV AX,START[BP] ; Register added to an offset 6. Indexed AM The same as the Base AM but, the EA( effective address or physical address ) is obtained by adding a displacement (offset ) value in of SI or DI to the segment registers used are DS. MOV AL,[SI] MOV AL,[DI+5] MOV AL,5[DI] MOV AL, [DI]+5 MOV AX,[SI] 7. Base-indexed: MOV AX, [bx+si] MOV AX, [bx+si+2] ; Base-Indexed with displacment MOV AX, START[bx+si] MOV AX,START [bx][si] 34

4 Instructions that deals with adresses like mov takes this address as operand there is many types of operands as: - Register operand: mov ax,bx - Immediate operand: mov ax,20h - Direct operand: msg DB 10 : mov ax,msg Segment Defaults: The default segment is when the offset is used to point to a memory location. bx,si,di Data segment bp Stack segment - Based indexed operands: mov ax,[bx+si] * mov ax,[bp+bx] ; error, two base registers * mov ax,[si+di] ; error, index base registers Example: SI = 100H DI = 200H BP = 100H BX= 1 MOV AX,[SI] MOV CL,[DI] MOV CH,[BP] MOV CH,DS:[BP] MOV AH,SS :[SI] MOV AL,[SI]+1 ; AX=0705H ; CL= 0AH ; CH=10H ; CH=05H ;AH=10H ;AL=07 35

5 MOV AL,1 [SI] MOV AL,[SI+1] MOV AL,[SI+BX] MOV AL,DS:1[BP+SI] ;AL=07 ;AL=07 ;AL=07 ;AL= the content of the memory location with the address DS:201H Lab Work: lets write, assemble, link, run and debug the following programs Example 1: Write an assembly program that determines if two strings are equal or not Dosseg.model small.data msg1 db 'Hello','$' msg2 db 'Helloo','$' msg3 db 'Equal','$' msg4 db 'NotEqual','$'.code main: mov mov ds,ax mov si,0 x: mov cl,[msg1+si] mov ch,[msg2+si] cmp cl,ch jne Exit1 cmp cl,'$' je Exit2 inc si jmp x Exit1: mov dx,offset msg4 36

6 jmp exit Exit2: mov dx,offset msg3 exit: mov ah,4ch end main Example 2: Write an assembly program that allows the user to enter two strings and determines if they are equal or not. Dosseg.model small.data msg1 db 100 DUP(?) msg2 db 100 DUP(?) msg3 db 'Equal','$' msg4 db 'NotEqual','$'.code main: mov mov ds,ax mov si,0 first: mov ah,1 mov [msg1+si],al cmp al,'$' je second inc si jmp first 37

7 second: mov dl,0ah mov dl,0dh mov bx,0 K: mov ah,1 mov [msg2+bx],al cmp al,'$' je X inc bx jmp K x: mov si,0 H: mov cl,[msg1+si] mov ch,[msg2+si] cmp cl,ch jne Exit1 cmp cl,'$' je Exit2 inc si jmp H Exit1: mov dl,0ah mov dl,0dh mov dx,offset msg4 38

8 jmp exit Exit2: mov dl,0ah mov dl,0dh mov dx,offset msg3 exit: mov ah,4ch end main Example 3: Write an assembly program that allows the user to enter 5-digits Number, then prints it in reversed-order digits. Dosseg.model small.data msg1 db 'Enter five-digit Number: ','$' msg2 db 'Reversed Number: ','$' Rev db?.code main: 39

9 mov mov ds,ax mov dx,offset msg1 mov cx,5 mov si,4 X: mov ah,1 mov [Rev+si],al dec si loop X mov dl,0ah mov dl,0dh mov dx,offset msg2 mov bx,0 ; to print contents of index 0,1,2,3,4 K: mov dl,[rev+bx] inc bx cmp bx,5 je exit jmp K exit: mov ah,4ch end main 40