DOS ain't dead

modern timers in DOS (32bit) (Developers)

posted by Japheth , Germany (South), 06.05.2022, 08:06

Absolutely true!

> If you only want to measure time in a very fine-grained way, use the TSC.
> Every Intel CPU since at least Pentium had a time stamp counter and most
> other brands introduced one not much later. The time stamps typically have
> a very high resolution, for example CPU clock cycles, and processors may
> even compensate the counts for you when changing clock speeds in power
> saving modes.

If you need a resolution of 0.1 to 1 ms, just use the normal, unmodified int 8 timer and read the values of the timer at the events you are interested in. Here's my standard code:


;--- this returns timer value in ms

GetTimerValue proc uses es bx

    mov es, [_0000H]
tryagain:
    mov edx,es:[46ch]
    mov al,0C2h     ;read timer 0 status + value low/high
    out 43h, al
    xchg edx, edx
    in al,40h
    mov cl,al       ;CL = status
    xchg edx, edx
    in al,40h
    mov ah, al      ;AH = value low
    xchg edx, edx
    in al,40h       ;AL = value high

    test cl,40h     ;was latch valid?
    jnz tryagain
    cmp edx,es:[46ch]   ;did an interrupt occur in the meantime?
    jnz tryagain        ;then do it again!

    xchg al,ah
;--- usually (counter mode 3) the timer is set to count down *twice*!
;--- however, sometimes counter mode 2 is set!
    mov ch,cl
    and ch,0110B    ;bit 1+2 relevant
    cmp ch,0110B    ;counter mode 3?
    jnz @F
;--- in mode 3, PIN status of OUT0 will become bit 15
    shr ax,1
    and cl,80h
    or ah, cl
@@:
;--- now the counter is in AX (counts from FFFF to 0000)
    neg ax
;--- now the count is from 0 to FFFF
    ret
GetTimerValue endp

;--- get timer value in ms in eax

gettimer proc

    call GetTimerValue

;--- the timer ticks are in EDX:AX, timer counts down
;--- a 16bit value with 1,193,180 Hz -> 1193180/65536 = 18.20648 Hz
;--- which are 54.83 ms
;--- to convert in ms:
;--- 1. subticks in ms: AX / 1193
;--- 2. ticks in ms: EDX * 55
;--- 3. total 1+2

    push edx
    movzx eax,ax    ;step 1
    cdq
    mov ecx, 1193
    div ecx
    mov ecx, eax
    pop eax         ;step 2
    mov edx, 55
    mul edx
    add eax, ecx    ;step 3
    ret
gettimer endp


---
MS-DOS forever!

Complete thread:

• modern timers in DOS (32bit) - LowLevel, 05.05.2022, 11:11 (Developers)
  • modern timers in DOS (32bit) - bretjohn, 05.05.2022, 19:05
    • modern timers in DOS (32bit) - mceric, 05.05.2022, 20:53
      • modern timers in DOS (32bit) - Japheth, 06.05.2022, 08:06
        • modern timers in DOS (32bit) - LowLevel, 06.05.2022, 08:37
      • modern timers in DOS (32bit) - LowLevel, 06.05.2022, 08:29
      • modern timers in DOS (32bit) - RayeR, 18.05.2022, 22:40
    • modern timers in DOS (32bit) - LowLevel, 06.05.2022, 08:08
      • modern timers in DOS (32bit) - bretjohn, 06.05.2022, 20:01
        • modern timers in DOS (32bit) - tkchia, 08.05.2022, 17:57
  • modern timers in DOS (32bit) - Laaca, 05.05.2022, 20:56
    • modern timers in DOS (32bit) - LowLevel, 06.05.2022, 08:30