Path: utzoo!attcan!uunet!mcvax!hafro!krafla!rispa2!orn
From: orn@rsp.is (Orn E. Hansen)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Precise loop timing on PC?
Summary: Interrupts on a PC
Message-ID: <111@.rsp.is>
Date: 10 Feb 89 20:27:02 GMT
References: <5930024@hpcupt1.HP.COM>
Organization: National Hospital of Iceland
Lines: 16

In article <5930024@hpcupt1.HP.COM>, dclaar@hpcupt1.HP.COM (Doug Claar) writes:
> This has probably been discussed before, but is it possible for the PC
> to execute something precisely and repeatedly, at or near the clock
> speed? I have a little 75 clock loop that I would like to take 75
> clocks EVERY time...no interrupts. It is important that the I/O port
> referenced is accessed in uniform time slices. I tried doing a CLI
> and also setting bit 7 of port 0A0H and 070H (I saw differing docs
> on which one was supposed to control NMI), but these steps didn't
> help. Can this be done?

NMI stands for Non Maskable Interrupt, and as the name implies this
actually cannot be masked.  The system uses NMI for Parity Check
and that can be masked with $A0.  So, if Shadowed Interrupts occurr
on the NMI Line, you can not mask them out.

Have you tried to mask out interrupts on the 8259A Controller???