Can you do this in any UNIX?

[Robert Hartman]

In article <1991Jan30.234153.13055 at cbnewsm.att.com> gregk at cbnewsm.att.com (gregory.p.kochanski) writes:
> ... Basically, I need to be able to read a single word
>from some hardware in 50 microseconds or less (and write in the same
>smount of time).  Can any unix do this, perhaps by locking processes into
>the CPU and allowing some end-run around device driver overheads?
>That and C++, too?

I've never written a device driver, but I have written a device-driver
manual.  I'm only speculating, but it seems to me that a virtual driver
could easily be written to lock down memory and queue all signals until
it's done.  A driver is privileged, so it may be that the driver can
ignore the clock interrupt too, in which case you're home free; once
you drop into driver code, the driver may well run to completion.

If the driver can't override the clock interrupt, then the user process
will do a context switch when it hits.  In this case you might still
get the results you want by running the application in single-user
mode.  Running single-user does not guarantee that the process will not
time slice, but if it's the only active process when the clock
interrupt hits, it will schedule itself to run again with a minimum of
delay.  I don't know whether this would be good enough for your
purposes, but if it's not, then perhaps you could run the time-critical
program in the monitor.  I know this sort of thing can be done on a Sun.

-r