How to Access System Time Using Nasm

How can I access system time using NASM?

On bare metal (in a custom OS), or in a DOS program:

%define RTCaddress  0x70
%define RTCdata     0x71

;Get time and date from RTC

.l1:    mov al,10           ;Get RTC register A
    out RTCaddress,al
    in al,RTCdata
    test al,0x80            ;Is update in progress?
    jne .l1             ; yes, wait

    mov al,0            ;Get seconds (00 to 59)
    out RTCaddress,al
    in al,RTCdata
    mov [RTCtimeSecond],al

    mov al,0x02         ;Get minutes (00 to 59)
    out RTCaddress,al
    in al,RTCdata
    mov [RTCtimeMinute],al

    mov al,0x04         ;Get hours (see notes)
    out RTCaddress,al
    in al,RTCdata
    mov [RTCtimeHour],al

    mov al,0x07         ;Get day of month (01 to 31)
    out RTCaddress,al
    in al,RTCdata
    mov [RTCtimeDay],al

    mov al,0x08         ;Get month (01 to 12)
    out RTCaddress,al
    in al,RTCdata
    mov [RTCtimeMonth],al

    mov al,0x09         ;Get year (00 to 99)
    out RTCaddress,al
    in al,RTCdata
    mov [RTCtimeYear],al

    ret

This uses NASM, and is from here.

This will not work under a normal OS like Linux that stops user-space processes from directly accessing hardware. You could maybe get this to work as root, with an ioperm(2) system call to allow access to that I/O port. Linux only updates the BIOS/hardware RTC to match the current system time during shutdown, not continuously, so don't expect it to be perfectly in sync, especially if the motherboard battery is dead.

Assemble-time read the value of a data variable

That's not how assembly language works. It assembles bytes into the output file, and there's no way to read back those bytes at assemble time. "Variable" is a high-level concept which you can implement in asm, but isn't natively supported¹.

If you want to have multiple things depend on the same value,

use foo equ 123 to define an assemble-time constant you can use later in multiple places.

To assemble some bytes in data memory with that value, use bar: dw foo.

It's an assemble-time constant so you can do things like resb foo*512 or

times (512*foo-511)-($-begin_main_program)

If you only did bar: dw 123, there'd be no way to get at the 123 while assembling other lines. (The bar: label is separate from the dw 123 or db 123, 0 or whatever you choose to put before or after it. It gives you a way to refer to that address from elsewhere, e.g. dw bar to assemble a pointer, or mov ax, [bar] to assemble an instruction that at run-time will load from that absolute address.)

Footnote 1: Except in NASM's macro language, with stuff like %assign i i+1 - useful inside a %rep 10 / %endrep block.

https://nasm.us/doc/nasmdoc4.html#section-4.1.8

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