Re: Why no interrupt priorities?

Michael Frank
2004-02-27T13:24:37Z

On Fri, 27 Feb 2004 12:37:25 +0100 (CET), Etienne Lorrain <etienne_lorrain@yahoo.fr> wrote:

> Russell King wrote:
>> Michael Frank wrote:
>> > Is this to imply that edge triggered shared interrupts
>> > are used anywhere?
>>
>> It is (or used to be) rather common with serial ports. Remember that
>> COM1 and COM3 were both defined to use IRQ4 and COM2 and COM4 to use
>> IRQ3.
>
> Some unusual hardware could handle two ports on the same IRQ (the
> same card mixing itself the IRQ of the two COM port present on the
> same ISA card) but the mixing could not be reliably handled at the
> ISA level. Using a resistor and/or a diode instead of a jumper to
> select the IRQ could also do the trick. So the software was ready
> to handle two interrupts from two UART on the same IRQ - but you
> had to have special hardware.

I used those cheapy 8 Channel 8250 serial cards and modified them
by oring 8 interrupts to irq 3.

The IRQ was set to level triggered and on an IRQ one polled the
status regs of all 8 channels. That was _just_ OK on a 8MHz 286
at 9600 baud in a custom designed low latency application
(no FIFOS here).

>
> Some people had success having a modem on COM1 and a serial _mouse_
> on COM3 (or the other way around) because the COM1/modem was winning
> the control of the ISA IRQ and the COM3/modem was loosing but because
> of 1200 bauds transmission speed the timer interrupt was recovering the
> char received. So they could not tell the difference and did not know
> why inverting the two serial plug did not work.

:-(

Regards
Michael