What exactly are the variables in assembly? - windows

Question

I am new to x86 assembly and have been doing some experiments lately using nasm and running the program on a windows 10 machine.
I Have this code:
global _start
extern _GetStdHandle#4
extern _WriteFile#20
extern _ExitProcess#4
section .data
message db "1234"
section .text
_start:
call print
call _ExitProcess#4
print:
; DWORD bytes;
mov ebp, esp
sub esp, 4
; hStdOut = GetstdHandle( STD_OUTPUT_HANDLE)
push -11
call _GetStdHandle#4
mov ebx, eax
; WriteFile( hstdOut, message, length(message), &bytes, 0);
push 0
lea eax, [ebp-4]
push eax
push 4
push message
push ebx
call _WriteFile#20
mov esp, ebp
ret
; ExitProcess(0)
That I assemble it using the following commands:
nasm -f win32 out.asm
link out.obj /entry:start /subsystem:console "C:\Program Files (x86)\Windows Kits\10\Lib\10.0.18362.0\um\x86\kernel32.lib"
and when running it on cmd it outputs "1234" as expected
Now when assembling and running the following code, where instead of pushing message the program pushes "1234" directly
global _start
extern _GetStdHandle#4
extern _WriteFile#20
extern _ExitProcess#4
section .data
message db "1234"
section .text
_start:
call print
call _ExitProcess#4
print:
; DWORD bytes;
mov ebp, esp
sub esp, 4
; hStdOut = GetstdHandle( STD_OUTPUT_HANDLE)
push -11
call _GetStdHandle#4
mov ebx, eax
; WriteFile( hstdOut, message, length(message), &bytes, 0);
push 0
lea eax, [ebp-4]
push eax
push 4
push "1234"
push ebx
call _WriteFile#20
mov esp, ebp
ret
It outputs nothing
Why? What information does message have that "1234" doesn't? When pushing message, does the program just push the address of the memory that is storing "1234"? If so, can I store "1234" somewhere else, and than push its address without creating a variable?

Answer 1

A variable is a logical construct — variables have lifetimes, some short, some long.  They can come into being and disappear.
By contrast, registers and memory are physical constructs — in some sense, they are always there.
In assembly programming, by a human or generated by compiler, we make mappings from logical variables needed by our C code, algorithms, and pseudo code, to physical storage available in the processor.  When a variable's lifetime ends, we can reuse the physical storage that it was using for another purpose (another variable).
Assembly language supports global variables (full process lifetime), and local variables — which can be either in memory on the stack, or CPU registers.  CPU registers, of course, do not have addresses, so cannot be passed by (memory) reference.  CPU registers also cannot be indexed, so to index an array requires memory.

Answer 2

I would make a local variable on the stack, like this:
print:
; DWORD bytes;
mov ebp, esp
sub esp, 12
; hStdOut = GetstdHandle( STD_OUTPUT_HANDLE)
push -11
call _GetStdHandle#4
mov ebx, eax
lea ecx, [ebp-12]
mov dword ptr [ecx], “1234”
; WriteFile( hstdOut, message, length(message), &bytes, 0);
push 0
lea eax, [ebp-4]
push eax
push 4
push ecx
push ebx
call _WriteFile#20
mov esp, ebp
ret
Note: the syntax mov ..., “1234” may or may not do what you want, depending on the assembler. I don’t remember how the Microsoft assembler handles it. If it doesn’t translate to 0x34333231, then use that constant instead.