Arrays em assembly (80x86)

Um array (vetor) em assembly pode ser visto como um bloco sequencial de memoria. Uma forma de acessar um componente (celula) do array e' por meio do modo de enderecamento indexado, calculando o deslocamento do elemento desejado a partir do endereco de inicio do array.

Por exemplo, para acessar o terceiro elemento do array (assumindo que cada elemento possui dois bytes):

mov ax, [array + 3 * 2]

Assim, de forma geral para acessar o elemento em uma posicao n qualquer do array:

mov ax, [inicio_do_array + N * tamanho_do_elemento]

Um array multidimensional (matriz) e' representado na memoria de forma equivalente a um vetor, porem e' utilizada uma forma diferente para calcular os indices e acessar os elementos. Formula para um array de duas dimensoes:

(linha * tamanho_da_linha + coluna ) * tamanho_do_elemento

Abaixo uma duvida que surgiu em uma lista de discussao sobre o uso de arrays bi-dimensionais (matrizes) usndo assembly com NASM:

http://pub1.ezboard.com/fpatorjksmessageboards33340frm5.showMessage?topicID=6.topic

Moving through a 2D array in Assembly

I'm writing a C program that calls a function written in Assembly. The assembly function takes in a 2D array and I need to loop through it. I've been working for a while but I can't seem to figure it out. I'm also using NASM. Here's how it's being called from C:

int the_array[100][100], r = 10, c 20;
.
.
. some code here...
.
// we only want to deal with the first 10 rows and 20 columns
i = the_function(the_array, r, c);

In the .data section of the assembler I have this variable:

themat dd 0

In the code section this is the basics of what I'm doing to use the variable:

; getting the array
mov eax, [esp + 4]
mov [themat], eax

; here's how I'm getting the element from the array: (problem area)
; esi = for looping through the columns
; edx = for looping through the rows
mov eax, [themat + 4*esi + edx]
mov eax, [eax]

Re: Moving through a 2D array in Assembly

If both esi and edx equal zero, eax will equal the first element in the array, which is correct. But I'm not sure how to set the other parts up so they work correctly. I also know r and c are being passed in with their correct values. I've been working on it all afternoon but I can't seem to make any progress, any suggestions? (note: that statement is inside of a for loop that is inside of a for loop)

the_array[y][x] in 1-D terms is the_array[(y*c)+x] where c is the number of columns.

i wrote this in masm, but you should be able to adapt it:

r        dd 10
cc        dd 20 (c is a reserved word in masm)
...

mov ecx, 200
mov edi, 0 ;column
mov esi, 0 ;row
mov ebx, themat
_loop:
        mov eax, esi ;eax=y
        mul cc ;y*20
        mov edx, edi ;edx = x
        add eax, edx ;eax = y*20+x
        mov edx, [ebx+eax*4]

        inc edi
        cmp edi, cc
        jne @F
                inc esi
                mov edi, 0
        @@:

        ;PrintDec edx ;debug output stuff

        dec ecx
        jnz _loop