What am I doing wrong? (Simple Assembly Loop)

Question

It won't let me post the picture. Btw, Someone from Reddit.programming sent me over here. So thanks!

TITLE MASM Template

; Description
;
; Revision date:

INCLUDE Irvine32.inc
.data
myArray BYTE 10, 20, 30, 40, 50, 60, 70, 80, 90, 100

.code
main PROC
    call Clrscr
    mov esi, OFFSET myArray
    mov ecx, LENGTHOF myArray
    mov eax, 0
L1:
    add eax, [esi]
    inc esi
    loop L1
    call WriteInt
    exit
main ENDP
END main

Results in:

-334881242

Answer 1

You have to cast the value to a byte pointer instead of 32 bit pointer.

Change

add eax, [esi]

to

add eax, byte ptr [esi]

Answer 2

You're trying to get a DWORD from a byte table, so masm gives you an error.Forcing byte size here using BYTE PTR is gonna get you a masm error (invalid operand or something alike), because you can't directly add a byte to a DWORD.

But there are still several ways to do what you want. Here is one that costs an extra register (EDX):

(...)
    mov edx, 0         ; We want the upper 3 bytes to zero.
    mov eax, 0

L1:
    mov dl, [esi]      ; Inject one byte from the table,
    add eax, edx       ;  upper bytes still zero, so EDX has your byte value.
(...)

Answer 3

Okay here's the thing:

My guess is that WriteInt expects a 32-bit signed value in EAX. Therefore you could do:

movzx eax, BYTE PTR [esi] inc esi call WriteInt

loop L1 -- or -- dec ecx jnz L1

Or if you are sure that WriteInt do not touch EAX you can do:

xor eax,eax ; clear EAX L1: lodsb ; loads a byte into AL and increments ESI for you call WriteInt loop L1

Answer 4

INCLUDE Irvine32.inc
.data
myArray BYTE 10, 20, 30, 40, 50, 60, 70, 80, 90, 100

.code
main PROC
    call Clrscr
    mov esi, OFFSET myArray
    mov ecx, LENGTHOF myArray
    xor eax, eax
L1:
    add eax, byte ptr [esi]
    inc esi
    loop L1
    call WriteInt
    exit
main ENDP
END main

I'm assuming, WriteInt is taking a parameter in EAX to print out the results, since the code failed because you were using a 32bit pointer to the offset into the data within the ESI register, change that to a byte ptr in order to get the proper 8 bits (a byte). Also, use of XOR would be faster than a MOV instruction for the EAX register, then the code should work...

Hope this helps, Best regards, Tom.