Date   : Fri, 07 Aug 2009 18:06:32 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Acknowledging 1770 NMI - was BBC B+ Econet

Rick Murray wrote:
> Michael Firth wrote:
> > The question now is whether anyone can point me in the direction of the
> > minimum code needed to clear / acknowledge an interrupt from the disk
> > controller so that it won't hold the NMI line active on power on.
> 
> What I am wondering is if it is normal for it to want to interrupt on
> start-up?
 
The notes for DFS 2.40 say "Extra 1770 reads at reset to clear
spurious NMIs" (Master Operating System, Atherton, p182).
 
On service call &01 you could do:
 
PHA
LDA &FE84 ; Read BBC B/B+ 1770 Status register
LDA &FE84 ; Read BBC B/B+ 1770 Status register
PLA
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
NOBODY expects the MACRO ARGUMENT INQUISITION!  Our chief weapon is
unexpected addressing modes... unexpected addressing modes and local
branches out of range.  Our two weapons are unexpected addressing modes and
local branches out of range and quoted argument lists.  Our *three* weapons
are unexpected addressing modes, local branches out of range, quoted
argument lists, and an almost fanatical devotion to the pope.  *Amongst* our
weapons are such elements as unexpected addressing modes, local branches out
of range... I'll come in again.