Arrays in MASM Assembly (very confused beginner)

I have a pretty basic question: How do you populate arrays in assembly? In high level programming languages you can use a for-loop to set a value to each index, but I'm not sure of how to accomplish the same thing assembly. I know this is wrong, but this is what I have:

ExitProcess PROTO .data warray WORD 1,2,3,4 darray DWORD ? .code main PROC mov edi, OFFSET warray mov esi, OFFSET darray mov ecx, LENGTHOF warray L1: mov ax, [edi] ;i want to move a number from warray to ax movzx esi,ax ;i want to move that number into darray. add edi, TYPE warray ;this points to the next number? loop L1 call ExitProcess main ENDP END 

Each time the loop runs, ax will be overwritten with the value of the array's index, right? Instead how do I populate darray with the array elements from warray? Any help would be very much appreciated. I'm pretty confused.

2 Answers 2

There are more than one way to populate an array and your code is almost working. One way is to use counter in the indirect address so you don't have to modify destination and source array pointers each loop:

ExitProcess PROTO .data warray WORD 1,2,3,4 darray DWORD 4 dup (?) ; 4 elements .code main PROC mov edi, OFFSET warray mov esi, OFFSET darray xor ecx, ecx ; clear counter L1: mov ax, [edi + ecx * 2] ; get number from warray movzx [esi + ecx * 4], ax ; move number to darray inc ecx ; increment counter cmp ecx, LENGTHOF warray jne L1 call ExitProcess main ENDP END 

Of course this code could be modified to fill the array backwards to possibly save couple of bytes like you probably meant to do in your original code. Here is another way that has more compact loop:

ExitProcess PROTO .data warray WORD 1,2,3,4 darray DWORD 4 dup (?) ; 4 elements .code main PROC mov edi, OFFSET warray mov esi, OFFSET darray mov ecx, LENGTHOF warray - 1 ; start from end of array L1: mov ax, [edi + ecx * 2] ; get number from warray movzx [esi + ecx * 4], ax ; move number to darray loop L1 ; get and set element zero separately because loop terminates on ecx = 0: mov ax, [edi] movzx [esi], ax call ExitProcess main ENDP END 

You should also note that when working with arrays of the same type you can do simple copy very efficiently using repeat prefix with instructions like MOVSD:

ExitProcess PROTO .data array1 DWORD 1,2,3,4 array2 DWORD 4 dup (?) .code main PROC mov esi, OFFSET array1 ; source pointer in esi mov edi, OFFSET array2 ; destination in edi mov ecx, LENGTHOF array1 ; number of dwords to copy cld ; clear direction flag so that pointers are increasing rep movsd ; copy ecx dwords call ExitProcess main ENDP END