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ICS312 Set 6

ICS312 Set 6. Operands. Basic Operand Types (1). Register Operands. An operand that refers to a register. MOV AX, BX ; moves contents of register BX to register AX. Immediate Operands. A constant is an immediate operand. MOV AX, 5 ; moves the numeric value 5 into AX

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ICS312 Set 6

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  1. ICS312 Set 6 Operands

  2. Basic Operand Types (1) • Register Operands. An operand that refers to a register. • MOV AX, BX ; moves contents of register BX to register AX. • Immediate Operands. A constant is an immediate operand. • MOV AX, 5 ; moves the numeric value 5 into AX • MOV AX, @DATA ; moves the constant represented by @data into AX • Direct Operands.  A variable name that represents a memory address • is a direct (memory) operand. • MOV BX, NUM ; moves the contents of the memory variable • ; NUM into BX

  3. Basic Operand Types (2) • A variable name +/- a numeric offset • MOV BX, NUM+2 ; moves 2nd byte following NUM into BX • Useful for array addressing • OFFSET operator: returns the 16-bit offset (address) of a memory • variable. • MOV BX, OFFSET NUM ; puts offset address of NUM into BX • LEA instruction: stores the address of a memory variable in a • general register. Same effect as instruction above obtained by: • LEA BX, NUM ; puts offset address of NUM into BX

  4. Basic Operand Types (3) Examples. Identify the types of ALL operands used in each line of the following code: .DATA CRLF  EQU 0AH, 0DH VAR1  DW  1200H, 56H, 0FFFFH MSG   DB  'The answer is: ', '$‘ .CODE MOV AX, @DATA            MOV DX, AX MOV AX, OFFSET VAR1 ADD BH, 15 LEA BX, VAR1+2 MOV DX, OFFSET MSG MOV AH, 9 INT 21H MOV AX, 4C00H INT 21H

  5. An indirect operand is an offset of a variable, which is contained in a register. The register acts as a pointer to the variable involved. • The 16-bit registers used for indirect addressing are:  SI, DI, BX, and BP • Any of the general purpose 32-bit registers may be used. • Default segments:  For SI, DI, and BX the variable involved is assumed to be in the Data segment. For BP, the variable involved is assumed to be in the stack segment. • Segment Overrides:  Use a Segment Override Prefix to designate  which segment to use when the default segment is not appropriate: • mov al, cs:[si]            ; uses SI as an offset into the Code segment, ; instead of the Data segment • mov dx, ds:[bp]            ; uses BP as an offset into the Data segment,                            ; instead of the Stack segment.

  6. An example of the use of an indirect operand is: mov ax, [BX] Note that BX is enclosed within square brackets. If BX contains e.g. 4, then the no. moved into ax is not 4, but the value of the word located at offset 4 in the data segment. E.g. if the data segment was: .DATA x dw 100 ; x is at offset 0 y dw 250 ; y is at offset 2 z dw 300 ; z is at offset 4 k dw 99 ; k is at offset 6 then the no. moved into ax would be 300.

  7. Example: To add up the items in List & store the answer in sum. .data sum db 0 • List    db    10h, 20h, 30h, 40h • Method 1:  increment bx to advance to each value .code lea    bx, List ;in this example, sets bx to 1 • mov    al, [bx]           ; al = 10h inc    bx                   ; bx is 2 and so points to 20h add    al, [bx]             ; al = 30h inc    bx                   ; bx points to 30h add    al, [bx]             ; al = 60h inc    bx                    ; bx points to 40h add    al, [bx]             ; al = 0A0h mov sum, al Indirect Operands (2)

  8. .data Sum db 0 List db 10h, 20h, 30h, 40h Method 2:  use bx with displacements to access each value .code lea    bx, List mov    al, [bx]            ; AL = 10h add    al, [bx+1]          ; AL = 30h add    al, [bx+2]          ; AL = 60h add    al, [bx+3]          ; AL = 0A0h mov    sum, al           ; store sum in next memory location (sum)

  9. Examples of different indirect operand formats mov ax, [bx]         mov dx, [bx+10]      mov cx, array[bx]   • ; same as mov cx [bx+array] • ; array is a constant equal to the • ; array’s offset   mov ax, [di]         mov dx, [di + 2*5]   mov cx, [array+si]  

  10. Example:  code to display contents of an array (message) • using only function 2 and the loop instruction: .data message    db "Hello, World!", 0Ah, 0Dh count      equ $-message                     ; length of message .code    mov    si, 0        ; initialize offset to 0    mov    ah, 2        ; function to display a character on screen    mov    cx, count    ; cx = message length L_top:    mov    dl, message[si]    ; move character into dl    int    21h                ; display character    inc    si                 ; set si to offset of next characterloop   L_top              ; repeat until count = 0 • Note: instead of: mov si,0 … mov dl,message[si], • we could have used: lea si,message … mov dl,[si]

  11. Examples requiring use of PTR ByteVal    DB    05h, 06h, 08h, 09h, 0Ah, 0Dh, '$'WordVal   DW   5510h, 6620h, 0A0Dh, '$' LEA    BX, WordVal ; or: MOV BX, OFFSET WordVal ; BX points to WordVal MOV WORD PTR [BX+2], 7 ; Replaces 6620h by 0007h The PTRoperator is used to explicitly state the size of an operand that would otherwise be ambiguous, in particular when the first operand is indirect, and the second is immediate LEA BX, ByteVal+2 ;Puts offset of the 08h into BX MOV BYTE PTR [BX], 2 ; Replace 08h by 02h INC BYTE PRT [BX] ; Increments the 02H to 03H

  12. Textbook Reading (Jones): • Chapter 10  Arrays

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