So I am still kind of a beginner in NASM x64. I am writing a rock paper scissors program and I decided that instead of getting a random number I should just get the last digit of milliseconds. I already know how to get the time in seconds:

section .text:
    global _start
_start:
    mov rax, 201
    xor rdx, rdx      ; if rdx is empty, the time value will go to rax
    syscall
    ; exit
    mov rax, 60
    mov rdi, 0
    syscall

I've tested it out and it works. How can I do the exact same thing, but rax will hold the value in milliseconds? (I am not saying multiply by 1000, I want the real value)

I have looked for a really long time: went through the syscall table, searched it up, look at a university's course plan. I even asked an AI. So I'm at my last resort asking you guys. Any help would be greatly appreciated.

CodePudding user response：

There is the gettimeofday system call which gives you microseconds, or clock_gettime that gives you nanoseconds. They are a little more work to call, since they return the time in a buffer rather than in a register, but you can allocate a temporary buffer on the stack. If you specifically care about getting milliseconds, then divide by 1000 or 1000000 accordingly.

If you're going to go to the trouble to make a system call, though, why not do it right and call getrandom(2) for some more truly random bytes?

Untested example of getrandom:

    sub rsp, 16    ; make some stack space, maintain alignment
    mov rdi, rsp   ; point rdi at our stack buffer
    mov esi, 1     ; read one byte
    xor edx, edx   ; no flags needed
    mov eax, 318   ; see https://chromium.googlesource.com/chromiumos/docs/ /HEAD/constants/syscalls.md#x86_64-64_bit
    syscall
    ; check for errors here; left as an exercise
    movzx eax, byte [rsp]  ; load the byte from stack into eax, zero the high bits
    add rsp, 16    ; clean up the stack