Linux-kernel mailing list archive 1999-34,: Re: problem with edge and level triggered interrupts

# Re: problem with edge and level triggered interrupts

Steve Underwood (steveu@netpage.com.hk)
Tue, 24 Aug 1999 15:14:02 +0000

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"Richard B. Johnson" wrote:

> [....]
> The 8259 cannot be used with level interrupts. This is because when
> an interrupt line goes true, it is latched internally.

In the original 8259A data sheet the weird behaviour of the screwed up logic
on the interrupt lines was too hard to explain, because it didn't actually
make any sense. Answer? They drew out the logic, and let everyone see how
stupid it was. This circuit was either designed by someone totally
incompetent, or by some competent having an _extremely_ bad day. Either way
its a pain we are still suffering from that blunder 20 years later.

In the late 70s you only ever used an Intel chip with an A or B at the end of
its number. The original was certain not to function usefully. The 8251 USART
and 8255 parallel port both had unusable interrupt facilities, which the
8251A and 8255A fixed. The 8259 had lots of problems. The 8259A fixed most.
Why the hell didn't they make an 8259B that fixed the rest?

Steve

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