Consider the set up of two local variables
func:
push ebp
mov ebp,esp
sub esp,8
virtual at esp
fd dd ?
filled dd ?
end virtual
mov [filled],200h
; blabla
mov esp,ebp
pop ebp
ret

This works. But what is the preferred way to preset 'filled'. I expected
filled dd 200h
but then 'filled' is not set to 200h.

Best Regards,
Ton

The data defined as "virtual" is not placed in the output file, therefore any initialization done there has no effect (except for the use with "load" directive, but that's a different topic) - it only defines labels for you. To initialize data addressed by those labels correctly at run time it's the programmer's responsibility, and generally it should be done just like you did: "mov [filled],200h" etc. Since you have allocated the space for those variables manually (with "sub esp,8"), it's obvious that you have fill them with the right values manually, too.

It is possible (with clever use of "virtual" and "load" directives) to make some macros to automatically allocate and initialize local variables with the defined values, but that's a different story.

Thanks a lot for your fast reply.
load loads from the compiled source or file. Thus somehow I need to set a value. Something like :

virtual at esp
xor eax,eax
add eax, 200h
load dword filled from eax
end virtual

How is this clever use really done?

--
Ton

I mean something like:

But to make this automated, some very complicated macro solutions would have to be applied, not sure if it's worth it.

Clever !

I stick to the initial approach

If your code section is writeable/readable why not just have your value as part of the function rather than the stack

Yes, that works as well, but then the executable is a bit longer.

that's not true.
SUB ESP,8 takes up more bytes than a arbitrary dword value 200h after a function return.

SUB takes 3 bytes, dword value 4 bytes:

83EC 08 sub esp,8

But you still have to initialize that dword with at least one more instruction, don't you?

Its actually
SUB ESP,8 <==This way used more often
MOV ESP,200h <==Because people are funny
Versus
Label: dd 200h <==This way shorter

MazeGen, I forgot the MOV because the space you make on the stack has to be initialized.

So using the Label: DD 200h after the return is less bytes

$ more try.asm
format elf executable
entry main

main:
mov eax, 1
mov dword ebx,[a]
int 80h
a: dd 5
$ more try2.asm

format elf executable
entry main

main:
mov eax, 1
mov dword ebx,5h
int 80h
$ ll try try2
-rwxrwxr-x 1 tonl tonl 101 Mar 21 08:53 try
-rwxr-xr-x 1 tonl tonl 96 Mar 21 08:55 try2


For now I think mov dword esp,200h is shorter.

Best Regards,
Ton

For your consideration:


Seven bytes to allocate space for, and initialize both variables. If you don't care about initializing "fd", then use "push eax" instead of "push 0", and it's down to six bytes. For small initialization values (-128 to +127) it's even less -- "push 127 / push eax" takes three bytes.