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From: Marko_Mäkelä
Date: 2000-08-15
Subject: Re: Interrupts again

On Tue, 15 Aug 2000, Zsolt Prievara wrote:

> And if the IRQ routine does not acknowledge IRQ (IRQ_LINE is still 'low'),
> after the IRQ routine the CPU will restart the IRQ sequence. Right?

Almost. It'll execute the instruction immediately following the CLI, RTI or PLP instruction that clears the Interrupt flag, since the Status Register won't be affected until the following instruction is already being fetched.

> And how does TED set IRQ line to high state? Which bit(s) of $FF09
> makes this? Only the 7th bit of $FF09?

I don't know, but you should be able to measure this yourself with a simple test program. I assume that the TED derives the status of the IRQ line from an AND operation between the "interrupt enable" and "interrupt occurred" registers. Levente or someone else, can you answer this? (My C= collection has been packed in a closet for almost two years now.)

Marko