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Serial COM Port Memory and I/O Allocation
Master OCW1 ($21)
Bit IRQ Enabled Device Function
2 IRQ2 Slave PIC
1 IRQ1 Keyboard
0 IRQ0 System Timer
Slave OCW1 ($A1)
Bit IRQ Enabled Device Function
7 IRQ15 Reserved
6 IRQ14 Hard Disk Drive
5 IRQ13 Math Co-Processor
4 IRQ12 PS/2 Mouse
3 IRQ11 PCI Devices
2 IRQ10 PCI Devices
1 IRQ9 Redirected IRQ2 Devices
0 IRQ8 Real Time Clock
Assuming that we want to turn on IRQ3 (typical for the serial port COM2), we would use
the following software:
Port[$21] := Port[$21] and $F7; {Clearing bit 3 for enabling IRQ3}
And to turn it off we would use the following software:
Port[$21] := Port[$21] or $08; {Setting bit 3 for disabling IRQ3}
If you are having problems getting anything to work, you can simply send this command in
your software:
Port[$21] := 0;
which will simply enable everything. This may not be a good thing to do, but will have to
be something for you to experiment with depending on what you are working with. Try not
to take short cuts like this as not only is it a sign of a lazy programmer, but it can have
side effects that your computer may behave different than you intended. If you are working
with the computer at this level, the goal is to change as little as possible so you don't cause
damage to any other software you are using.
3.5 Serial COM Port Memory and I/O Allocation
Now that we have pushed through the 8259 chip, lets move on to the UART itself. While the
Port I/O addresses for the PICs are fairly standard, it is common for computer manufacturers
to move stuff around for the serial ports themselves. Also, if you have serial port devices
that are part of an add-in card (like an ISA or PCI card in the expansion slots of your
computer), these will usually have different settings than something built into the main
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