CODEWITHSUNDEEP
assemblyconditional-statementsx86-16
Ansh
Ansh

Reputation: 175

Program in 8086 assembly language to compare two numbers

The following is my code in assembly language to compare two numbers and print a test character to confirm if it's written correctly.

DATA SEGMENT

    NUM1 DB 50
    NUM2 DB 45
DATA ENDS

CODE SEGMENT

    ASSUME CS: CODE, DS: DATA
START: MOV AX, DATA

    MOV DS, AX
    MOV AL, NUM1
    MOV BL, NUM2
    CMP AL, BL
    JLE TAG
    TAG: MOV DL, AL
         MOV AH, 02H
         MOV DL,  'T'
         INT 21H
    MOV AX, 4CH
    INT 21H
    CODE ENDS
    END START

My assumption is, CMP will compare AL to BL. If AL is smaller, JLE will be true and the code in the part 'TAG' will be executed. As you can see AL is not smaller, still, TAG is executed.

Upvotes: 3

Views: 26659

Answers (2)

Sep Roland
Sep Roland

Reputation: 39166

My assumption is, CMP will compare AL to BL. If AL is smaller, JLE will be true and the code in the part 'TAG' will be executed. As you can see AL is not smaller, still, TAG is executed.

I was a bit worried when I read this. I hope you know that the JLE mnemonic stands for jump if less OR EQUAL. If you only need to decide about smaller (and this seems to be the case) then you would better use the jl instruction (jump if less).

The real problem then with your code (and you already solved this yourself) is that with a construct like:

    jcc label
label:
    ...

the code at label is always executed because

  • if the condition is true the execution jumps to the code at label
  • if the condition is false the execution falls thru in the code at label

A simple way to solve this would be to insert an unconditional jump right before the label, so that in the event the condition wasn't true the code at label could be jumped over:

    cmp al, bl
    jl  label
    jmp beyond
label:
    mov ah, 02h    ;Executed only is AL is smaller than BL
    mov dl, 'T'
    int 21h
beyond: 
    ...

An even simpler way to solve this issue is to just bypass the code at label by using the opposite conditional jump. There's also no longer need for the label itself.

For jl the opposite conditional jump is jge (jump if greater or equal)
For jle the opposite conditional jump is jg (jump if greater) 

    cmp al, bl
    jge beyond
    mov ah, 02h    ;Executed only is AL is smaller than BL
    mov dl, 'T'
    int 21h
beyond:
    ...

A note on the numbers involved.

If you treat the numbers as signed quantities, then using jl (jump if less) and jg (jump if greater) is the correct way.
On the other hand if the numbers are to be treated as unsigned quantities you need to use the jb (jump if below) and ja (jump if above) instructions.

Upvotes: 1

Ansh
Ansh

Reputation: 175

I solved it.

In assembly language. It goes top-down and will come across the code in the TAG section regardless of the condition being met or not. Simply adding a JMP command (before the TAG section begins) will make it go to termination directly after checking the condition to ensure it gives a logically correct answer.

Upvotes: 2

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