First, I program for Vintage computer groups. What I write is specifically for MS-DOS and not windows, because that's what people are running. My current program is for later systems and not the 8086 line, so the plan was to use IRQ 8. This allows me to set the interrupt rate in binary values from 2 / second to 8192 / second (2, 4, 8, 16, etc...)
Only, for some reason, on the newer old systems (ok, that sounds weird,) it doesn't seem to be working. In emulation, and the 386 system I have access to, it works just fine, but on the P3 system I have (GA-6BXC MB w/P3 800 CPU,) it just doesn't work.
The code
setting up the interrupt
disable();
oldrtc = getvect(0x70); //Reads the vector for IRQ 8
settvect(0x70,countdown); //Sets the vector for
outportb(0x70,0x8a);
y = inportb(0x71) & 0xf0;
outportb(0x70,0x8a);
outportb(0x71,y | _MRATE_); //Adjustable value, set for 64 interrupts per second
outportb(0x70,0x8b);
y = inportb(0x71);
outportb(0x70,0x8b);
outportb(0x71,y | 0x40);
enable();
at the end of the interrupt
outportb(0x70,0x0c);
inportb(0x71); //Reading the C register resets the interrupt
outportb(0xa0,0x20); //Resets the PIC (turns interrupts back on)
outportb(0x20,0x20); //There are 2 PICs on AT machines and later
When closing program down
disable();
outportb(0x70,0x8b);
y = inportb(0x71);
outportb(0x70,0x8b);
outportb(0x71,y & 0xbf);
setvect(0x70,oldrtc);
enable();
I don't see anything in the code that can be causing the problem. But it just doesn't seem to make sense. While I don't completely trust the information, MSD "does" report IRQ 8 as the RTC Counter and says it is present and working just fine. Is it possible that later systems have moved the vector? Everything I find says that IRQ 8 is vector 0x70, but the interrupt never triggers on my Pentium III system. Is there some way to find if the Vectors have been changed?
It's been a LONG time since I've done any MS-DOS code and I don't think I ever worked with this particular interrupt (I'm pretty sure you can just read the memory location to fetch the time too, and IRQ0 can be used to trigger you at an interval too, so maybe that's better. Anyway, given my rustiness, forgive me for kinda link dumping.
http://wiki.osdev.org/Real_Time_Clock the bottom of that page has someone saying they've had problem on some machines too. RBIL suggests it might be a BIOS thing: http://www.ctyme.com/intr/rb-7797.htm
Without DOS, I'd just capture IRQ0 itself and remap all of them to my own interrupt numbers and change the timing as needed. I've done that somewhat recently! I think that's a bad idea on DOS though, this looks more recommended for that: http://www.ctyme.com/intr/rb-2443.htm
Anyway though, I betcha it has to do with the BIOS thing:
"Notes: Many BIOSes turn off the periodic interrupt in the INT 70h handler unless in an event wait (see INT 15/AH=83h,INT 15/AH=86h).. May be masked by setting bit 0 on I/O port A1h "