Question

I’m writing an assembly program which I want to be able to do the (basic) following:

x = 100;
y = int[x]

E.g. the size of y depends on the value of x.

NOTE: I am using NASM instruction set on a 64 bit Ubuntu system.

In assembly I know that the size of an array needs to be declared in the data section of the file e.g.

myvariable resq 1000

The problem is I won’t know how big to make it till I have done a previous calculation.
What I really want is something like:

mov rax, 100
myvariable resq rax

But that’s not allowed right? Just having some confusion over array access/declarations in assembly.

Any pointers appreciated!

Answer 1

Your C example is only possible if you declare the array on the stack or if you pull the memory from the heap with malloc or similar. For small values it’s perfectly fine (and faster) to use the stack:

mov rax, 100   # 100 elements
shl rax, 3     # multiply with 8, the size of an element
sub rsp, rax   # rsp now points to your array

# do something with the array
mov rbx, [rsp]    # load array[0] to rbx
mov [rsp+8], rbx  # store to array[1]

add rsp, rax   # rsp points to the return address again