Re: Question: Multiple Super Serial Card ][s in //e?

[pausch@saaf.se (Paul Schlyter)]

In article <1eqhqtp.r40mpibk68n2N%dempson@actrix.gen.nz>,
David Empson <dempson@actrix.gen.nz> wrote:
 
> Paul Schlyter <pausch@saaf.se> wrote:
> 
>> Finally: isn't it possible to turn on an interrupt signal on the
>> Super Serial Card?  Now try to do that on more than one of those
>> seven SSC's, and you'll immediately run into a severe problem: the
>> Apple II only has one IRQ line, not 8 or 16 as the Wintel machines
>> have....
> 
> No problem, as long as the software is set up correctly.  The IRQ line
> on the 6502 is designed to be shared between multiple peripherals.
 
Well, I would rather say that the Apple II bus was designed to let
multiple boards send an IRQ to the single IRQ pin on the 6502.  And
it's really a misfeature of the PC's ISA bus that it's not
implemented that way -- the only way two PC devices can share the
same IRQ is if they both reside on the same I/O card and the I/O card
implements interrupt sharing appropriately.
 
Of course interrupt handling got flakey on the Apple II anyway: since
the RWTS would damage a disk it was writing to, if interrupted,
interrputs were turned off when RWTS was entered.  Now, if a SSC
received characters while the Apple was writing to disk with
interrupts turned off ..... I guess you get the idea.
 
 
BTW a bug in Apple DOS 3.1 was that interrupts were NOT turned off
while RWTS was executing.  So if the 6502 received an interrupt
while the Apple was writing to disk, the disk got damaged.  This
was fixed in DOS 3.2.
 
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