Date   : Tue, 04 Nov 2003 09:29:28 GMT
From   : Chris Johns <chris.johns@...>
Subject: NMI Handlong on the Electron

 I've made some progress with the Electron Econet project, I've managed
to talk to a fileserver, and *Stations from my RiscPC now includes  "127
Acorn Electron" - however I've been bitten ny the NMI handling in MODEs 0-3.

As I understand it, when the NMI occurs, it steals the CPU from the ULA,
and causes the ULA to stop accessing the RAM - giving the "snow" effect.

When you've finished handling the NMI, you're meant to set the NMI clear
bit (bit 7 of &FE05) which tells the ULA it can have the RAM back.

I've tried setting this bit on exit from the NMI handler - however this
causes various strange errors or lockups, usually it hangs on a break if
the clock is connected, if the clock is disconnected then it getrs as
far as the BASIC prompt, and prints crap over the screen when you plug
the clock back in.

If I don't set this bit, it all works fine, however the screen is a mess
(as would be expected).

Anyone any ideas?
-- 
 <chris.johns@...             >