Monday January 29 1996 14:33, Robert Bilton wrote to Peter Watney:

 RB> Yeah, i have a few books on assembler, although most are old, ie the
 RB> highest baud rate they include in the text is 14400... but i haven't found
 RB> anything on the irq's yet...

Fortunately for me I have always been able to use other people's comms
drivers, so I have never actually had to program the beastly things.

The cascaded 8259s that control the precedence and manner in which the
programmable interrupts are handled are programmed using system ports:
020 - 03F for Interrupt Controller No 1; and
0A0 - 0Bf for Interrupt controller No 2

I think I am right in saying that ports 020 and 0A0 are the 'command ports'
for the chips, and that ports 021 & 0A1 are the ports to which the Mask
Interrupt bits are sent that enable/disable the various IRQs, which are
something like:
in the Interrupt Controller No 1:
IRQ 4 (COM1:);
IRQ 3 (COM2:);
IRQ 2 (to Interrupt Controller No 2);

In the Interrupt Controller No 2:
IRQ 4 (COM3:);
IRQ 3 (COM4:).

Because IRQs 5 and 7 traditionally control the parallel ports and the fixed
disks, and lately have been looking after such things as sound cards, you
need the extra knowledge that the BOOKS I mention later can impart before
you start playing with them.

Depending on what you have enabled/disabled with the mask that you have
sent to the appropriate port/s these chips control the priority with which
hardware interrupts from the various devices are handled.

Control of the serial ports is handled through the appropriate I/O ports:
2E8 - 2EF COM4:;
2F8 - 2FF COM2:;
3E8 - 3EF COM3:;
3F8 - 3FF COM1:;

So for successful communications you have to both control the manner in
which the Proggrammable Interrupt Controller (PIC or 8259) handles
interrupts from the Serial Port vis-a-vis the timers, the disks, etc, but
you also have to control the way in which the particular Com Port that you
have connected to the serial device is set (speed, stop bits, etc, etc) and
handles the data passing through it in both directions.

There are BOOKS written on the subject!

Regards,
Peter

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