accessing array's element in assembly language (windows)

I've a problem in assembly language that i got to access element of an array... suppose an array contains weeks day... like sun,mon,tues,wed.... i have to access 2nd index of the array... How can I do this?

1 Answer

Indexing in assembly is basically the same as C/C++ except for one difference: you have to know the size of your data elements.

For example, to loop through an array of bytes (or characters in a string) in assembly, you could do the following:

mov eax, 0 mov ecx, 0 loop_start: cmp ecx, ARRAY_LENGTH jge loop_end add eax, BYTE PTR myArray[ecx] add ecx, 1 jmp loop_start loop_end: 

As you can see, the array is looped through one element at a time by incrementing ecx (which I'm using as a counter). Each element is added to eax which contains the sum at the end of the loop. Notice I had to add "BYTE PTR" when referencing the array to tell the assembler what type of data I'm using.

Now take a look at this code which does the same thing for DWORD data (4 bytes):

mov eax, 0 mov ecx, 0 loop_start: cmp ecx, ARRAY_LENGTH jge loop_end add eax, myArray[ecx*4] add ecx, 1 jmp loop_start loop_end: 

Only two things changed: I no longer needed to use "BYTE PTR" here because, unless told otherwise, the assembler assumes you are using 32-bit data types on a 32-bit machine; I also needed to change the index of the array to "ecx*4" because each element in the array is 4 bytes long. Most data types used on 32-bit machines are 32 bits in size so the later example will be more common.

To answer your specific question, here is one way to loop through an array of strings and display them:

.data sunday db "Sun",0 monday db "Mon",0 tuesday db "Tues",0 wednesday db "Wed",0 thursday db "Thurs",0 friday db "Fri",0 saturday db "Sat",0 daysOfWeek dd OFFSET sunday, OFFSET monday, OFFSET tuesday OFFSET wednesday dd OFFSET thursday, OFFSET friday, OFFSET saturday .code mov ecx, 0 loop_start: cmp ecx, 7 jge loop_end mov eax, daysOfWeek[ecx*4] ; eax now contains the pointer to the ; next element in the array of days add ecx, 1 jmp loop_start loop_end: 

Because pointers on a 32-bit machine are 32 bits wide, treat them like DWORDs as in the second example.