Re: Why no interrupt priorities?

Etienne Lorrain
2004-02-27T11:37:28Z

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.

If you are talking of old ISA serial cards, the IRQ were "shared" in
the sense that the IRQ pin was not connected to the microprocessor
when the device was not open (under Linux).
So you could reliably have COM1 and COM3 on IRQ4 if you did not use
the two devices at the same time - or use one of them in the polling
mode (IRQ 0: handle by the timer at low speed).

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.

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.

Now if you are talking of newer motherboard with COM port integrated,
you may have been able to share (but unusual) - if you are talking
of PCI card serial ports then that is another story.

Etienne.

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