1. Forum
  2. Usenet
  3. COMP.OS.MSDOS.PROGRAMMER

  • is there a high-frequency timer available on the PC? (besides the PIT)

    From Mateusz Viste@21:1/5 to All on Wed Aug 17 12:01:58 2022
    Hi,

    I am playing a little with feeding the DAC of my SoundBlaster card with
    raw samples that I am trying to time accurately.

    The "obvious" way to do such thing is to use DMA transfers and
    let the SoundBlaster chip control the sampling. That's not what I am
    doing for the moment. I use the SoundBlaster DAC directly and write 8
    bit samples to it in real time. Yes, it is not CPU-efficient, does not
    allow to play things in background, etc. but please indulge me and
    consider this a thought exercise.

    So here's the question: Is there any kind of timer available on the PC
    that would allow to generate some 8 kHz samples? I mean, besides the
    obvious PIT of course. Some hidden VGA register maybe? A way to abuse a
    timer in the SoundBlaster or Adlib hardware? Some creative
    polling of a port or keyboard controller?

    So far, the timers I know about are those:
    - VGA VSYNC (tied to screen's refresh rate, ie. 50-60 Hz or so)
    - calibrated busy loop (meh)
    - PIT (18.2 Hz default, changing it doesn't work under Windows)
    - BIOS "system clock" (sourced by the PIT, really)
    - TSC (requires a Pentium+ CPU)
    - APIC (modern CPUs only)
    - CMOS RTC clock (ports 0x70 & 0x71, fixed at 1024 Hz)*

    Anything else?

    Mateusz

    *a pretty nice article about abusing the RTC clock here: https://web.archive.org/web/20150514074601/http://www.nondot.org/sabre/os/files/MiscHW/CMOSTimer.html

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Johann Klammer@21:1/5 to Mateusz Viste on Wed Aug 17 19:02:14 2022
    On 08/17/2022 12:01 PM, Mateusz Viste wrote:
    Hi,

    I am playing a little with feeding the DAC of my SoundBlaster card with
    raw samples that I am trying to time accurately.

    The "obvious" way to do such thing is to use DMA transfers and
    let the SoundBlaster chip control the sampling. That's not what I am
    doing for the moment. I use the SoundBlaster DAC directly and write 8
    bit samples to it in real time. Yes, it is not CPU-efficient, does not
    allow to play things in background, etc. but please indulge me and
    consider this a thought exercise.

    So here's the question: Is there any kind of timer available on the PC
    that would allow to generate some 8 kHz samples? I mean, besides the
    obvious PIT of course. Some hidden VGA register maybe? A way to abuse a
    timer in the SoundBlaster or Adlib hardware? Some creative
    polling of a port or keyboard controller?

    So far, the timers I know about are those:
    - VGA VSYNC (tied to screen's refresh rate, ie. 50-60 Hz or so)
    - calibrated busy loop (meh)
    - PIT (18.2 Hz default, changing it doesn't work under Windows)
    - BIOS "system clock" (sourced by the PIT, really)
    - TSC (requires a Pentium+ CPU)
    - APIC (modern CPUs only)
    - CMOS RTC clock (ports 0x70 & 0x71, fixed at 1024 Hz)*

    Anything else?

    Mateusz

    *a pretty nice article about abusing the RTC clock here: https://web.archive.org/web/20150514074601/http://www.nondot.org/sabre/os/files/MiscHW/CMOSTimer.html

    some have what they call a HPET, but Idunno if it's available on your specific box. or
    in portspace.
    https://en.wikipedia.org/wiki/High_Precision_Event_Timer

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Ross Ridge@21:1/5 to mateusz@xyz.invalid on Fri Aug 19 15:25:13 2022
    Mateusz Viste <mateusz@xyz.invalid> wrote:
    So here's the question: Is there any kind of timer available on the PC
    that would allow to generate some 8 kHz samples?

    Yes, the PIT. You can't generate a 8000 kHz timer exactly, but you can
    come close enough (8008 Hz).

    - PIT (18.2 Hz default, changing it doesn't work under Windows)

    Changing it works just fine under Windows, as demonstrated by countless
    MS-DOS games. Try Star Control 2 for example, it uses tracker based
    music, mixed in real-time on the CPU on most soundcards. Probably a
    lot like what you're trying to do.

    One important thing do is to chain the original BIOS handler at the rate
    it expects. The easest way to do this it add your PIT reload value
    (149 for 8kHz if I've done my math correctly) to a 16-bit variable on
    every interrupt and then call the old handler when it overflows.

    --
    l/ // Ross Ridge -- The Great HTMU
    [oo][oo] rridge@csclub.uwaterloo.ca
    -()-/()/ http://www.csclub.uwaterloo.ca:11068/
    db //

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Ross Ridge@21:1/5 to mateusz@xyz.invalid on Fri Aug 19 19:44:41 2022
    Mateusz Viste <mateusz@xyz.invalid> wrote:
    - PIT (18.2 Hz default, changing it doesn't work under Windows)

    Ross Ridge <rridge@csclub.uwaterloo.ca> wrote:
    Changing it works just fine under Windows, as demonstrated by countless >MS-DOS games.

    Digging a little deaper, while Windows 9x does let MS-DOS applications
    change the PIT timer interval, it doesn't let you set a timer frequency
    faster than 1 kHz. Still this is probably your best bet if you want a
    wide range of hardware support and don't mine sacrificing Windows support.

    Otherwise, your next best option is the Pentium TSC, but in addition to
    not working on earlier processors, using it on certain newer processors
    can be tricky. You wouldn't have to worry about different cores of
    a multi-core CPU keeping different counts, but it might break an CPUs
    with dynamically changing frequencies and that don't support the newer frequency invariant TSC.

    Anything else is likey to not work on Windows and/or not as as wide of
    a range of hardware as the PIT or TSC.

    --
    l/ // Ross Ridge -- The Great HTMU
    [oo][oo] rridge@csclub.uwaterloo.ca
    -()-/()/ http://www.csclub.uwaterloo.ca:11068/
    db //

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Mateusz Viste@21:1/5 to Ross Ridge on Fri Aug 19 22:16:07 2022
    2022-08-19 at 19:44 -0000, Ross Ridge wrote:
    Digging a little deaper, while Windows 9x does let MS-DOS applications
    change the PIT timer interval, it doesn't let you set a timer
    frequency faster than 1 kHz.

    Not very useful for audio generation. :)

    Anything else is likey to not work on Windows and/or not as as wide of
    a range of hardware as the PIT or TSC.

    Thanks, that's indeed what I think as well. I was silently hoping for
    some exotic "under the hood" tricks like "reading port x or y takes
    between 33-37 microseconds on all PCs" or something alike. Yes, wishful thinking, I know.

    I did some archaeology and it appears that so far whenever people
    wanted or needed to generate real-time audio data on PCs, they were
    simply resorting to PIT reprogramming.

    I tested it and it works as expected, although results are quite
    jittery due to the relatively poor precision of interrupts firing. DMA transfers are vastly superior in this regard.

    Mateusz

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Alexei A. Frounze@21:1/5 to Mateusz Viste on Fri Aug 19 21:20:27 2022
    On Friday, August 19, 2022 at 1:16:09 PM UTC-7, Mateusz Viste wrote:
    ....
    I did some archaeology and it appears that so far whenever people
    wanted or needed to generate real-time audio data on PCs, they were
    simply resorting to PIT reprogramming.

    I tested it and it works as expected, although results are quite
    jittery due to the relatively poor precision of interrupts firing. DMA transfers are vastly superior in this regard.

    At one point I looked deeper into it and discovered that part of the
    jitter was due to SMIs.

    Alex

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)