file io in nasm

I need to open two files and compare byte by byte each file. I was
wondering, once I have the file descriptor in eax after I've opened
the file, what is the best way to access the file? I mean, how do I
use the fd?

section .data
f1:     db      "filename",0


        xor     ecx, ecx        ; 0 RD_ONLY
        lea     ebx, f1
        mov   al, 0x5           ;open syscall
        int     0x80

Is the fd just a pointer to the file in memory? Can I just use the fd
as an offset to cycle through the files contents??