Initialise .bss variable assembly

Question

I have declared a variable:

section .bss
var    resb    1

I want to initialise this in my program to the value 255.

mov    [var], 255 ;error on this line

When compiling I get the error below:

program.asm:123: error: invalid size for operand 1

What am I doing wrong here?

Works fine in `nasm` (which you've tagged your question with). Which exact assembler are you using, and which options are you passing to it? — Michael, Mar 27 '14 at 12:58
I am using yasm (which I thought was a branch of nasm, although could be wrong). Command line args are yasm -f elf -g dwarf2 -m amd64 ascii.asm @Michael — ojhawkins, Mar 27 '14 at 13:22
Michael's version works fine for me with yasm v1.1.0.2352. But note that `.bss` doesn't, I had to use `section .bss` instead (unrelated to the error you posted). — Jester, Mar 27 '14 at 13:47
So if you're initializing it to a constant anyway, why not put it in the .data segment and let it be done automatically? — harold, Mar 27 '14 at 14:44
I want to be able to change the value of of it as I discover lower numbers elsewhere in the code. @harold — ojhawkins, Mar 27 '14 at 15:41

score -1 · Answer 1 · answered Nov 05 '15 at 19:49

-1

it works this way, you have to use a register to assign data to the location addressed by the symbol [myVar] :

mov eax,255
mov [myVar],eax

PRINT_DEC 2,myVar

answered Nov 05 '15 at 19:49

Saman Sadeghy

1

You can assign an immediate value (like 255) to a memory location without going through a register. OPCODE information `C6 /0 MOV r/m8,imm8 ` | `Move imm8 to r/m8.` . See the x86 [instruction set reference](http://x86.renejeschke.de/html/file_module_x86_id_176.html). Move an 8 bit value to an 8 bit object in memory can be done with something like `mov byte [myVar],255` . `byte` says what the size of the memory object that `myVar` points to. The original poster wants to move an 8 bit value to `myvar` your code incorrectly tries to move 32 bits to a variable declared as 1 byte. – Michael Petch Nov 05 '15 at 20:26

1 Answers1