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'Important revision for 16C5x or in fact; IRQ shari'
1998\07\06@184550 by paulb

Caisson wrote:

>> As for serial ports this is not the case... you can purchase 4-8-16
>> or even 32 port serial boards that only use 1 interrupt for all of
>> the ports on the board.

> I'ts not the problem of the Hardware ... but the Software.  I'm still
> amazed about the fact that a mouse & a modem can't seem to share a
> Interrupt-handling routine.  The software is blind.

 Of course it's the software, though the original design of the PC was
part of the limitation.  That can be fixed by modifying ISA boards with
a diode pointing from IRQ driver to bus paralleled by a 1K resistor on
the first such card per IRQ.  Such hardware provision as the original
IBM COM port definition made for sharing IRQs was subsequently written
out by bodgie card manufacturers in the name of "not used/ not needed".

 What went wrong with the software was that there were no IRQ drivers
defined.  Use of each IRQ was left entirely to individual programs, so
any *single* program could share an IRQ among as many devices as it
wished.  Without a driver/ interface however, application programs
cannot share.  The mouse program is written separately to the modem
program.  If you write a program to service both, you can share easily.

 Linux and (...) later, NT have drivers to handle *all* IRQs, and so
naturally can share IRQs amongst virtually unlimited devices.  The
Motorola (/ Synertek etc.) programmers amongst us find this a perfectly
natural concept.
       Paul B.

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