Date   : Sat, 23 Feb 2008 22:06:47 +0000 (GMT)
From   : info@... (Sprow)
Subject: BBC FPGA Boots to BASIC... almost...

In article <080223195504@...>,
   Jonathan Graham Harston <jgh@...> wrote:
> I've occasionally pondered why the recommended way for the IRQ
> routine to determine if it's a hardware or software IRQ (ie BRK)
> is to do:
>  
> PLA:AND #&10
>  
> which requires A to be preserved first, as in the BBC MOS:
>  
> .IRQHandler
> STA &FC:PLA:AND #&10 = 3+4+2
>  
> .IRQhandler
> PHA:PHP:PLA:AND #&10 = 3+3+4+2

3 cycles longer? Doesn't overflow the stack if the stack is full?
Sprow.