CODEWITHSUNDEEP
assemblymasmirvine32
Mitch
Mitch

Reputation: 3418

Run time error if last array value is negative

I have this short piece of code, its purpose is to sum negative values and count positive ones, the vector is initialized else where 

;sum all negative values and count positive values 
    ;let eax hold sum and ebx hold number of positives 
    CALL crlf
    mov ecx, VectorSize
    mov eax, 0
    mov ebx, 0
    mov edx, 0
    sumMe: 
        cmp Vector[edx * TYPE Vector], 0
        jge pos
        ;neg
        add eax, Vector[edx * TYPE Vector]
        INC edx
        loop sumMe 
        pos:
            INC ebx 
            INC edx
            loop sumMe

The piece of code works correctly except when a negative value is the last value in the array, even something like -1 -1 -1 -1 5 works. However if I have a negative value in the last location the program crashes.

Any help would be appreciated

Upvotes: 0

Views: 78

Answers (1)

Peter Cordes
Peter Cordes

Reputation: 364483

Assuming there's a ret or something at the end:

What does loop do when ecx reaches zero: execution falls through. If the last element was negative, the loop instruction that makes ecx zero is followed by pos:.

When loop runs with ecx=0 to start with, ecx wraps to 0xFFFFFFFF and the jump is taken. loop is like the bottom of a do{}while(--ecx);.

All of this would be obvious if you used a debugger to single-step that last iteration, because you already know the problem is at the end of the array with a negative element.

Upvotes: 3

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