CODEWITHSUNDEEP
arraysloopsassemblyx86
Ron Dankner
Ron Dankner

Reputation: 33

Assembly x86 putting values into array with loop

I wanted to put numbers in an array of length 10, but each number is 1 bigger than the last number. It means: myArray = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

I have tried this:

IDEAL
MODEL small
STACK 100h

DATASEG

intArray db 10 dup (0)
index db 1

CODESEG

start:
mov ax, @DATA
mov ds, ax
loopArray:
mov al, [index]
add [intArray+index], al ; here is the problem
inc [index]
cmp [index], 11
jb loopArray
exit:
mov ax, 4c00h
int 21h
END start

But I can't add the index to [intArray + index], so I tried to to add it to [intArray+al], and that doesn't work either.

How can I add the index to the next array's value each time?

Upvotes: 2

Views: 38280

Answers (1)

Sep Roland
Sep Roland

Reputation: 39676

myArray = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.

These are the numbers that you want your array to contain. But since you initialized the index variable (that you will be using for both indexing and storing) as 1 (using index db 1) this will lead to another result.
Just setup the index with:

index db 0

There's another reason to set it up this way! In the notation [intArray+index] the index part is meant to be an offset in the array. Offsets are always zero based quantities. The way you wrote your program, it will write the 10th value behind the array.

add [intArray+index], al ; here is the problem

You're right that this is the problem. Some assemblers won't compile this, and others will just add the addresses of both these variables. Neither suits your purpose. What you need is putting the content of the index variable in a register and use that combination of operands.

    intArray db 10 dup (0)
    index    db 0
    ...
loopArray:
    movzx    bx, [index]
    mov      [intArray+bx], bl ;Give the BX-th array element the value BL
    inc      [index]
    cmp      [index], 10
    jb       loopArray

With this code the index will start at 0 and then the loop will continu for as long as the index is smaller than 10.

Of course you could write this program without using an index variable at all.

    intArray db 10 dup (0)
    ...
    xor      bx, bx            ;This make the 'index' = 0
loopArray:
    mov      [intArray+bx], bl ;Give the BX-th array element the value BL
    inc      bx
    cmp      bx, 10
    jb       loopArray

Given that the array was initially filled with zeroes, you can replace the:

    mov      [intArray+bx], bl ;Give the BX-th array element the value BL

with:

    add      [intArray+bx], bl ;Give the BX-th array element the value BL

Do remember this can only work if the array was filled with zeroes beforehand!

Upvotes: 3

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