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modern timers in DOS (32bit) (Developers)

posted by mceric, Germany, 05.05.2022, 20:53

> > I would like to use a modern timer in DOS (to be able to get a 5-10kHz
> > timer interrupt), I know that I have to dig into ACPI
No you do not. If you do not spend too much CPU time in your handler, even ancient 386 already allowed you to create timer interrupts at a rate of many 1000 per second, using the classic https://en.wikipedia.org/wiki/Intel_8253 chip. This was used in audio players for people without sound cards :-)

Actually 18.2 Hz is the SLOWEST possible speed (1.19MHz / 64k) which is used by the BIOS to use as little CPU time for wall clock updates as possible. Everybody is free to change to a different speed as long as they take care of keeping the 40:6c clock run at the same speed, for example by only calling the original int 8 handler once for one in every X interrupts generated by the 8253. There are plenty of tutorials for such things from the 1990s :-)

You can also use the RTC timer interrupt or ask the BIOS to wait for N microseconds. The latter is usually implemented by the BIOS activating a 1024 Hz RTC interrupt, but you can also get 2 Hz and 8192 Hz RTC interrupt rates.

The nice thing about using RTC interrupt 0x70 is that it is independent from int 0x08 and int 0x1c, so you do not have to worry about whether and how often to call previous interrupt 0x08 handlers. You will probably have the RTC interrupt all for yourself on many systems, it is not widely used otherwise.

The disadvantage is that you only have a choice of 2, 1024 or 8192 Hz. If you want more flexibility, use good old 8253 int 8, which can have any frequency between 18.2 Hz and 0.6 MHz, but you better limit yourself to 1/10 or 1/100 of that to avoid wasting too much CPU time. On a 386, even 10 kHz already means a significant CPU load even with optimized handler code.

Of course, if you feel masochistic, you could use modern timers such as HPET or similar. Modern computers are likely to even contain various extra microcontrollers for maintenance stuff or I/O interfaces. Who knows what sorts of abuse can be implemented using those :-p

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. TSC is easy to use, you just need a little helper to fetch the value, usually written in Assembly.

More DOS-friendly C compilers may already have the helper prepared for you. Protected mode OS may not let you access the real counter, so I guess your mileage may vary with a DOS application running in Windows. But that would certainly not let you manipulate HPET or ACPI stuff either. I wonder whether Windows offers a simulated 8253 to DOS boxes, does anybody know? In any case, before using RDTSC, make sure that you can actually access it.

https://www.felixcloutier.com/x86/rdtsc

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FreeDOS / DOSEMU2 / ...

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