CODEWITHSUNDEEP
c++arraysassemblyx86masm
Meph1k
Meph1k

Reputation: 90

x86 MASM - passing and accessing a 2D array

I'm currently doing my project in assembly for my university.
The goal is to write the exact same application in C/C++ and asm. The part with C++ was easy.
The problem starts when I want to access a 2D array in asm and the internet is very scarce in this type of situation.

In my main part of application I have:

extern "C" int _stdcall initializeLevMatrix(unsigned int** x, DWORD y, DWORD z);

and my asm function:

initializeLevMatrix PROC levTab: PTR DWORD, len1: DWORD, len2: DWORD
  xor eax, eax
  mov DWORD PTR [levTab], eax ; I want to pass 0 to the first element
  mov ebx, eax
  mov ecx, len1
init1:
  cmp eax, ecx ; compare length of a row with a counter
  jge init2 ; jump if greater or the same
  inc eax ; increment counter
  mov ebx, eax ; index
  imul ebx, ecx ; multiply the index and the length of a row
  imul ebx, 4 ; multiply by the DWORD size
  mov DWORD PTR [levTab + ebx], eax ; move the value to a proper cell
  jmp init1
init2:
  ret
initializeLevMatrix ENDP

The function is incomplete, because I decided to fix the current problem before building it further.

The problem is that I can't get or set values.
The function should initialize the matrix as follows: 

levTab[0][0..n] = 0..n

Yet I guess my poor indexing is wrong or the way I pass the parameters is wrong.

Thank you a lot for your help.

Upvotes: 2

Views: 1438

Answers (1)

Sep Roland
Sep Roland

Reputation: 39691

Based on your comment "I just want to initialize the first row", it's not correct to treat len1 as the length of a row like you've written in the program. It's to be seen as the number of elements in each column.

Bring the pointer to the matrix in a register first. I suggest EDI:

mov  edi, levTab
xor  eax, eax
mov  [edi], eax           ; I want to pass 0 to the first element

Use scaled indexed addressing

mov  ebx, eax             ; index
imul ebx, ecx             ; multiply the index and the length of a column
mov  [edi + ebx * 4], eax ; move the value to a proper cell

Upvotes: 3

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