Reading from a file in assembly

you must declare your buffer in bss section and the bufsize in data

section .data
   bufsize dw      1024

section .bss
   buf     resb    1024

Ohh, this is going to be fun.

Assembly language doesn't have variables. Those are a higher-level language construct. In assembly language, if you want variables, you make them yourself. Uphill. Both ways. In the snow.

If you want a buffer, you're going to have to either use some region of your stack as the buffer (after calling the appropriate stack-frame-setup instructions), or use some region on the heap. If your heap is too small, you'll have to make a SYSCALL instruction (another INT 80h) to beg the operating system for more (via sbrk).

Another alternative is to learn about the ELF format and create a global variable in the appropriate section (I think it's .data).

The end result of any of these methods is a memory location you can use. But your only real "variables" like you're used to from the now-wonderful-seeming world of C are your registers. And there aren't very many of them.

The assembler might help you out with useful macros. Read the assembler documentation; I don't remember them off the top of my head.

Life is tough down there at the ASM level.

After the call to open, the file handle is in eax. You rightfully move eax it to ebx, where the call to read will look for it. Unfortunately, at this point you have already overwritten it with 3, the syscall for reading.