linux - How can I access system time using NASM?

Question

How can I access the system time using NASM, on Linux?

(Editor's note: the accepted answer is for 16-bit DOS with direct hardware access; it would work inside DOSBox. The other answers are actually for Linux.)

Answer 1

Using 32-bit code under Linux:

mov  eax, 13         ; call number = __NR_time
xor  ebx, ebx        ; tloc = NULL
int  0x80
; 32-bit time_t in EAX

This is a system call to time(2) (system call number 13), and it returns the signed 32-bit time_t in EAX.

(Unlike other system calls, return values >= -4095U (MAX_ERRNO) are still successes, and simply small negative numbers that represent times just before Jan 1, 1970. With a NULL pointer arg, time(2) can't fail. See the man page for details.)