Where are the system codes coming from in x86-64 A

Question

I am looking through some demos of assembly (using NASM on a Mac, I am new to assembly) and seeing things like this:

; read a byte from stdin
mov eax, 3     ; 3 is recognized by the system as meaning "read"
mov ebx, 0     ; read from standard input
mov ecx, variable    ; address to pass to
mov edx, 1     ; input length (one byte)
int 0x80             ; call the kernel

I am beginning to understand that eax, ebx, etc. are "general registers", which is where you store common things. Still have more to learn there but I get the gist of it.

But I am confused as to where the values like 3 (recognized by the system as meaning "read") and 0 (read from standard input) are coming from. How do you know that 0 means "standard input"? Is there a list of such integer values, or some book or standard reference?

Answer 1

You're conflating system call numbers with the system call arguments.

The system call numbers (e.g. "3 = read") are OS-specific (well, kernel-specific), and sometimes version-specific. For example, see the system call numbers for Linux on x86 here and on x86_64 here. How the arguments are passed, how the system call is invoked, and what the system call numbers mean are all architecture- and kernel-specific.

The number "0" for "standard input" on the other hand is a UNIX standardized value, STDIN_FILENO.