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> DOS > Good alternative for high-resolution timing?Goto page 1, 2 Next |
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windwakr 08 Dec 2009, 15:51
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08 Dec 2009, 15:51 |
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rCX 09 Dec 2009, 00:51
Before the days of windows and rtdsc, many people used the PIT timer described in detail here.
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It runs at 1.193MHz. This means that it is 65536 times faster than the int 1A clock. I may be wrong but I think the time in Linux and Windows is based on this clock. |
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09 Dec 2009, 00:51 |
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windwakr 09 Dec 2009, 01:04
rCX wrote: Before the days of windows and rtdsc, many people used the PIT timer described in detail here. To use channel 0 for high precision counting, you would need to reprogram it first. By default it has a divisor of 65535 or 65536(depending on the BIOS) and runs at 18.2065 Hz. But changing channel 0 would mess up the system time, wouldn't it? |
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09 Dec 2009, 01:04 |
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LocoDelAssembly 09 Dec 2009, 01:21
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It would afect this Code: ----------M0040006C-------------------------- MEM 0040h:006Ch - TIMER TICKS SINCE MIDNIGHT Size: DWORD Desc: updated approximately every 55 milliseconds by the BIOS INT 08 handler SeeAlso: MEM 0040h:0070h,INT 08"IRQ0",INT 1A/AH=00h But you can fix it by calling the original interrupt handler at the appropriate intervals. |
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09 Dec 2009, 01:21 |
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windwakr 09 Dec 2009, 01:51
LocoDelAssembly wrote:
Oh ya, I remember now. I read about it here a long time ago, but forgot. That's a very useful article for PIT stuff. The code in it is in pascal, but is very easy to understand. |
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09 Dec 2009, 01:51 |
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Cas 09 Dec 2009, 03:13
Thank you, guys! The RDTSC looks like the best to use now. On the other hand, old games must have used the PIT, then. I'm surprised of how they still produce reasonable sound output under emulators. My knowledge on the PIT was not very deep before, I see. I will study both technologies.
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09 Dec 2009, 03:13 |
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sinsi 09 Dec 2009, 03:20
Do you want to measure time or clocks? The RDTSC reads the number of clock cycles, not time.
You can easily program the PIT for 1000Hz (milliseconds) even on an 8086. |
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09 Dec 2009, 03:20 |
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edfed 09 Dec 2009, 04:38
i think that the timer tick after midnight register is not a critical value.
if not updated, it is not a problem isn't it? |
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09 Dec 2009, 04:38 |
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LocoDelAssembly 09 Dec 2009, 05:13
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Perhaps some TSRs and DOS itself may need it (for file datetimes?). It depends on how much control you have over the environment, but to be sure you won't screw up your neighbors you better keep it running and at the expected rate. I don't remember if TSR were officially allowed to hook the INT 08 but in case they can, not calling the original handler at the expected intervals would make the TSR to stop working properly. |
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09 Dec 2009, 05:13 |
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sinsi 09 Dec 2009, 05:35
INT 1C is supposed to be the way to hook the timer (called by the BIOS INT 08 handler).
AFAIK DOS uses the ticks and midnight flag if there is no RTC. The BIOS handles that flag, that's why you chain to the original INT 08 |
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09 Dec 2009, 05:35 |
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DOS386 11 Dec 2009, 01:49
> Oh ya, I remember now. I read about it http://www.qzx.com/pc-gpe/pit.txt
Good link, check also: http://www.sat.dundee.ac.uk/~psc/pctim003.txt And the FAQ : http://board.flatassembler.net/topic.php?t=9473 "15. How to use timers / cause a delay ?" > I know that, especially when in protected mode and at a ring > other than 0, it may become very messy to deal with the timer NO. The timer is accessible from any Ring (still not from NTVDM  )
> need an accurate way to measure time with high resolution, and > even in real mode, many times the timer does not do. Why ??? Rise the PIT frequency 
> how did some old games and applications manage to sample WAV files through the PC speaker? 4 ways: - delay loop - delay using the PIT - high frequency interrupt fired by the PIT 
- RDTSC (and WRTSC) (not 80486 and below) _________________ Bug Nr.: 12345 Title: Hello World program compiles to 100 KB !!! Status: Closed: NOT a Bug |
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11 Dec 2009, 01:49 |
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bitshifter 12 Dec 2009, 05:46
Ooh, a delay loop
Code: repeat 8 jmp $ + 2 end repeat _________________ Coding a 3D game engine with fasm is like trying to eat an elephant, you just have to keep focused and take it one 'byte' at a time. |
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12 Dec 2009, 05:46 |
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Asmix 13 Dec 2009, 12:40
CMOS REAL-TIME CLOCK
Int 70h is called when the real-time clock chip generates an alarm or periodic interrupt. The periodic interrupt by default occurs 1024 times per second. May be masked by setting bit 0 on I/O port A1h details here http://www.compuphase.com/int70.txt _________________ 2 beer or not 2 beer? |
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13 Dec 2009, 12:40 |
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edfed 13 Dec 2009, 19:00
Quote: RDTSC (and WRTSC) (not 80486 and below) WRTSC is not fasm compilable. i wonder if it exists in IA architecture. |
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13 Dec 2009, 19:00 |
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Alphonso 13 Dec 2009, 20:22
Maybe as a macro for
Code: mov ecx,10h wrmsr |
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13 Dec 2009, 20:22 |
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DOS386 15 Dec 2009, 02:51
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15 Dec 2009, 02:51 |
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LocoDelAssembly 15 Dec 2009, 04:31
Yep, pretty bad documentation, AMD's nor Intel's manuals mention such instruction.
Intel - Vol 3B wrote: The RDMSR and WRMSR instructions read and write the time-stamp counter, treating |
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15 Dec 2009, 04:31 |
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Cas 17 Dec 2009, 02:59
Wow! All those alternatives sound very useful!
For those who asked or mentioned... these are some points I wanted to state: - DOS386... yes, you can use it on any ring, but I guess you can't change the frequency from a ring other than 0, because you have to access ports. Am I right? - One concern about using high frequency PIT is that, if not in pure DOS, there is a chance that the code behind the INT08 is HUGE! If that is the case, a high frequency may cause the interrupt to be requested more often than it is possible, because it takes so long to return from it. Of course, the programmer of that HUGE code could have been good enough to make a good dispatcher and STI as soon as possible, but... you never know... This would make the system unstable and the time measure inaccurate - I'm interested in measuring time, not clocks, but I know that, with one loop, I can calculate the proportion and then use the TSC to measure time as well. Unless I'm using an incredibly high resolution, this should not create considerable errors, would it? Still, when it regards to old games sampling through the PC speaker, I understand they couldn't possibly have used this method - One example of when I would need this high resolution is this: suppose I want a routine to run at the start of every vertical retrace, but my board, as many others, does not support interrupt triggering. I have no option but to poll, but then, if I'm busy doing other things, when my polling tells me there's a VR going on, it may be just about to end... so I have to "sit and wait" for it! What if I'm busy doing other things? - Another example... suppose I'm recording audio in CD resolution to a buffer and I want my application to react immediately if a sound of a certain type (i.e.: a square wave of a given frequency) enters the buffer. I would need to be checking the buffer freq/2 times, that is, at 22KHz. Of course, if the "reaction" is only meant to be directed to the human user, I understand that 25Hz would be enough, as the conscious part of the brain understands as "immediate" anything that happens within about 1/25th of a second, but that does not have to be the case. QUESTION: Can I have different frequencies for the different PIT channels and.... while not using the PC speaker (having the output off all time)... use its channel for other purposes? This would be a good background counter, compatible with very old hardware, and not calling interrupts all the time, so it would be stable as well |
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17 Dec 2009, 02:59 |
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revolution 17 Dec 2009, 03:15
Cas wrote: yes, you can use it on any ring, but I guess you can't change the frequency from a ring other than 0, because you have to access ports. Am I right? |
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17 Dec 2009, 03:15 |
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