Re: RE: Nested interrupts in a 15x1

From: André Fachat (
Date: 2006-12-18 11:24:33

Hi Ruud,

the controller routine does not have a SEI instruction, because when going into the interrupt routine, the CPU automatically sets the interrupt bit, disabling all new interrupts. You would rather have to look for CLI instructions in the controller code to see if it allows other interrupts to occur during the execution of the controller code.


-------- Original-Nachricht --------
Datum: Mon, 18 Dec 2006 10:33:48 +0100
Betreff: RE: Nested interrupts in a 15x1

> Can it happen that on a 15x1 drive another IRQ interrupt occurs during the
> time that a previous one still is handled?

> But I'm afraid I can answer my own question: looking at the sources again
> for the xxxth time I realise that it is possible IMHO. There are a lot of
> SEI instructions in the sources but IMHO an ATN can trigger the IRQ again
> during the time the 6502 is handling the IRQ routine for the disk controller
> as that routine does not contain a SEI.

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