rec.autos.simulators

Cheers

Mark
Reading, UK

Cutting to the chase...

I believe that this setting will only have an effect on the computer
while you're running a program that is using DirectInput to read the
joystick.  So you don't need to worry about it messing up Quicken or
Excel or whatever.  While running games, having this box unchecked can
cause a couple of anomalies...
(1) It's possible that your system clock may run a little slowly while
running the game.
(2) If the game uses any devices that require frequent and/or timely
attention by the CPU (the serial ports are most notorious for this),
these devices may not operate very well while the box is unchecked.  So
if you find you're now having more disconnects, or jumpy on-line game
performance, you may need to adjust your serial port settings.

Now, some background...

The computer reads a standard PC gameport joystick by timing how long
it takes to charge a capacitor on each axis.  When you move the
joystick, you alter the resistance on the axis, and thus how long it
takes for the capacitor to charge to a certain level.  A compliant
joystick will use variable resistors (potentiometers - a.k.a. "pots")
that yield a maximum charge time of about one millisecond.  This may
seem to be almost instantaneous, but on a 400MHz machine, it's an
eternity!

To read the joystick, the CPU will tell the gameport "Discharge your
capacitors now, and start recharging them."  The CPU will then sit in a
loop that tests each capacitor, noting when each one has recharged.
The "when" here may be "how many times have I tested the capacitors so
far?", or "How many clock cycles have passed since I told the
capacitors to discharge?", or some such.

While the CPU is sitting in its loop waiting for the capacitors to
recharge, some other device in the computer may generate an interrupt.
For example, the serial port might say "Hey! You need to feed me more
characters to transmit!", or "Hey! I've received a bunch of characters,
and my buffer is getting full!  You better read them out of my buffer
soon, or I'm not going to have anyplace to put new characters as I
receive them!", etc.

If this happens, and if the joystick reader hasn't turned off
interrupts, the computer will go off and service the interrupt.  So,
the joystick reader won't necessarily have an accurate idea of when
each capacitor has recharged, giving you "jumpy" joystick reads.

They will be particularly jumpy if the reader is using the "how many
times have I tested the capacitors so far?" method of timing.  In this
case, its timing will be thrown off if the interrupt came in any time
after it discharged the capacitors.  If it's using the "How many
machine cycles have passed..." method, then it will only be thrown off
if the interrupt comes in just as a capacitor is about to become
recharged.  Either way, by disallowing interrupts while reading the
joystick, you give the joystick reader a better chance of coming up
with an accurate read.

Randy