CODEWITHSUNDEEP
x86masmirvine32
billy bob
billy bob

Reputation: 11

program is performing conditional jump and I don't know why

I'm new to assembly language and I have no idea why my conditional statement is not performing how I want it to. How I think it should work is when I use cmp on eax and -1 the program should jump to the label nonnegative when input is 0 or greater and should exit the program if less than 0, but no matter the input, the program always jumps to the label nonNegative and performs the code assigned to that label. I am using Irvine32. 

INCLUDE Irvine32.inc
.data
testing BYTE    "you have entered a non-negative number! ", 0
.code
main PROC


call    ReadInt
cmp     eax, -1
JG      nonNegative
nonNegative:
mov     edx, OFFSET testing
call    WriteString
call    CrLf
exit
main ENDP
END main

I expect program to print "you have entered a non-negative number!" for any non-negative number and I expect the program to exit for any negative number. instead the program always prints "you have entered a non-negative number!" and then exits.

Upvotes: 0

Views: 42

Answers (1)

zx485
zx485

Reputation: 29022

You have this code:

call    ReadInt              ; Read the value to EAX
cmp     eax, -1              ; determine JMP condition
JG      nonNegative          ; JMP if EAX > -1
nonNegative:                 ; Jump @target
mov     edx, OFFSET testing  ; Print message
call    WriteString          ; ...
call    CrLf                 ; Print NextLine

The problem is that you correctly compare the returned value in EAX to -1 with

cmp     eax, -1              ; determine JMP condition

And you correctly execute a conditional JMP with

JG      nonNegative          ; JMP if EAX > -1

But your mistake is that the JMP target of this jump is the next line:

nonNegative:                 ; Jump @target

So either if the JUMP is taken (condition fulfilled) or not taken (condition not fulfilled), the following instructions are executed:

mov     edx, OFFSET testing  ; Print message
call    WriteString          ; ...
call    CrLf                 ; Print NextLine

Hence you always get the same result on the console:

you have entered a non-negative number!

To provide you a correct alternative, have a look at this code:

.data
   negNumber   BYTE  "you have entered a negative number! ", 0
   zeroOrAbove BYTE  "you have entered a non-negative number! ", 0
.code
  call    ReadInt                 ; Read the value to EAX
  cmp     eax, -1                 ; determine JMP condition
  JG      nonNegative             ; JMP if EAX > -1
  ; this execution path was missing in your solution
  mov     edx, OFFSET negNumber   ; Print message
  call    WriteString             ; ...
  call    CrLf                    ; Print NextLine
  jmp     fin
nonNegative:                      ; Jump target
  mov     edx, OFFSET zeroOrAbove ; Print message
  call    WriteString             ; ...
  call    CrLf                    ; Print NextLine
fin:

Upvotes: 1

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