Accessing array in MASM

Question

I've got the address of my array (passed as a pointer to the function) in esi register. How can I access a particular cell of the array? i.e:

my_array[a + b * c]

where c is constant.

Answer 1

Look up instructions such as LEA

Think of it this way:

LEA edx,[esi+ebx*4]

Answer 2

You could directly move it as follows also:

MOV EDX, [ESI + 4*EBX]

For a static array, you can actually use two registers at once to index it, but using the array base address as the 32-bit displacement in the addressing mode. This only works for non-position-independent code.

MOV EDX, my_array[ECX + 4*EBX]

Instead of using two different registers, you can use the same register twice to simulate a scale factor of 3 (ebx + ebx*2), 5, or 9.