Lets say I have a PROC in My assembly code like so:
.CODE
PROC myProc
MOV EAX, 00000001
MOV EBX, 00001101
RET
ENDP myProc
I want to MOV 1, into the EAX register, and move 13 into the EBX register in my procedure, however I want to create two variables local to my PROC, assigning var a the value of 1, and var b the value of 13, and from there MOVing [a] into EAX, and [b] into EBX. I have had many ideas about this before, perhaps creating space on the stack for the variables, or something like:
.CODE
PROC myProc
PUSH ESP
PUSH EBP
MOV ESP, 00000001
MOV EBP, 00001101
MOV EAX, [ESP]
MOV EBX, [EBP]
ENDP myProc
But this still really isn't dynamic variable creation, I am just writing and reading data back and forth between registers. So in essence I am trying to figure out how to create variable in assembly at run-time. I would appreciate any help.
Variables are a high-level concept. An asm implementation of a C function will typically have a variable live in a register for some of the time, but maybe at other times it's live in a different register, or in memory at some location once it's no longer needed (or you ran out of registers).
In asm you don't really have variables (other than static storage), except by using comments to keep track of what means what. Just move data around and produce a meaningful result.
Avoid memory whenever possible. Look at C compiler output: any decent compiler will keep everything in registers as much as possible.
int foo(int a, int b) {
int c = a + 2*b;
int d = 2*a + b;
return c + d;
}
This function compiles to the following 32-bit code with gcc6.2 -O3 -fverbose-asm (on the Godbolt compiler explorer). Notice how gcc attaches variable names to registers with comments.
mov ecx, DWORD PTR [esp+4] # a, a
mov edx, DWORD PTR [esp+8] # b, b
lea eax, [ecx+edx*2] # c,
lea edx, [edx+ecx*2] # d,
add eax, edx # tmp94, d
ret