On Sun, Jul 22, 2001 at 09:06:43AM -0400, Russell Hires wrote:
<snip>
> Then, after all that, I thought I would just attempt to get some basic
> information on COM and IRQ, but I don't know where to start. I don't want to
> spend another $20-$40 on " a great book by O'Reilly," that's for sure.
>
<snip>
Okay. IRQs and I/O Addresses, etc. First, get little booklet called
"Pocket PCRef". This has everything you ever wanted to know about PC
hardware. Standard pinouts for things, ASCII charts, IRQ lists, lists of
old DOS commands, specs for every hard drive known to Man, phone numbers
for manufacturers, etc. Cost is about $20, I believe, and it fits in a
shirt pocket (smaller than a paperback novel).
Now, IRQs. The AT-class PC has two interrupt controllers, chained to
each other. Each controls 8 interrupts for a total of 16. (Yes, it's
woefully inadequate, which is why we have USB and such now.) A list is
below:
IRQ Function
0 timer interrupt
1 keyboard interrupt
2 chained to second interrupt controller (can be used, but beware)
3 interrupt for second serial port (COM2 under DOS)
4 interrupt for first serial port (COM1 under DOS)
5 interrupt for second parallel port (usually isn't one, so free)
6 floppy disk controller interrupt
7 interrupt for first parallel port (LPT1 in DOS)
8 real time clock interrupt
9 chained to first interrupt controller
10 free
11 free
12 free
13 math coprocessor interrupt (dunno if used on Pentium class)
14 hard drive controller (IDE)
15 hard drive controller (IDE)
The "free" interrupts above (and IRQ 5) are only "free" if something
else isn't using them. Sound cards use interrupts, video controllers use
interrupts, NIC cards use interrupts, etc. PnP is designed to circumvent
some of this, but IMHO, it sucks badly. With NICs, I _always_ buy NICs
that have a setup program that allows me to _set_ the IRQ the way I
want, usually IRQ 5.
I/O addresses are another story, and there are one or more for each
interrupt used. Likewise DMA channels, though there are far fewer of
them and not all devices use them.
I'm not really a hardware geek, but I've done a lot of system building
and repairing over the years. If you ask other specific questions, I'll
do what I can to help.
Paul
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