From: "Tony Ingenoso" 

It doesn't necessarily slow everything else down.

If the DOS app is keyboard polling, as is typical with DOS apps, there are
idle detection heuristics in the scheduling to detect this lack of real
work and "idle" the app.  It will get an occasional short time
slice and chance to redeem itself if declared idle. Usually DOS apps are
run at a somewhat lower priority than normal Windows apps due to this CPU
burning nature, so idled DOS apps are really minimal impact on system
performance.  There are a few pathological DOS apps that have apparent
activity in their keyboard polling loops that can fool the idle detection
logic though and make it look like they're doing real work.  Those few bad
actors will indeed be genuine CPU burners...

The 100% pegged appearance is because the system's idle loop doesn't run
much when there's a DOS app running - if there's nothing else of non-idle
time priority to run, then the DOS app really has something to do - i.e. it
runs its keyboard polling loop.  The CPU monitor is a lowest priority app
that sucks up CPU cycles otherwise wasted on the scheduler's idle loop (ex.
a Seti client will peg the CPU monitor too)

So, DOS app running typically will have little degradation impact on normal
apps, but may keep the lowest priority idle-time things from running much
(but they are by definition the lowest priority things in the system run
only when there's nothing else to do, so this is no great loss).  Properly
designed apps wouldn't rely on the lowest idle time threads to ever
complete anything anyway because depending on the job mix they may truly
never run much all.  ex - Something like relying on a very low priority
thread to release some semaphore in a timely manner is inviting disaster
and can result in what amounts to a defacto deadlock as long as other
higher priority jobs are in the system.

"Ellen K."  wrote in message
news:qb9efvorkbcvjorusevo01qhplik6kvs0r{at}4ax.com...
>
> 1.  If you open a DOS program, the CPU zooms up to 100% (ergo slowing
> everything else down) until you close it.

--- BBBS/NT v4.01 Flag-4