Date   : Wed, 10 Mar 2004 10:29:33 +0000
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: CLI then SEI

tom@... wrote:
> But on a real BBC, it seems there's some kind of pipelining-like
> behaviour going on. After a bit of playing around, I found that the
> interrupt doesn't happen until after the instruction following the
> CLI has run! So a CLI/SEI pair won't cause an infinite chain of
> interrupts, though a CLI/NOP/SEI will. (The address on the stack on
> entry to the IRQ routine is the instruction after the instruction
> after the CLI.)
> 
> Very strange.
> 
> Should I have known this already?! :)
 
Only you have Z80 programming experience where this behavious is clearly
and repeatedly documented.  I have never seen this documented for the
6502, but I have often seen the CLI/SEI sequence used to allow one
interupt in.
 
-- 
J.G.Harston (JGH BBC PD Library) 70 Camm Street, Walkley, SHEFFIELD S6 3TR
jgh@...                - Running on BBCs & Masters with SJ MDFS FileServer
Z80+6502/CoPro+Tubes/Econet+SJ - - - - - - - http://www.mdfs.net/User/JGH/