Date   : Wed, 15 Aug 2012 13:44:50 +0100
From   : splodge@... (Richard Gellman)
Subject: Programatically =22reset=22 a Beeb?

  

Hi, 

I'm having trouble with a "reset" process. Hopefully someone
here can point out the Bleeding Obvious(tm) to me. 

I've build a board
for my M128 with two ROM sets, the original M128 set and the Model B
OS1.2/Basic2 ROMs. Switching between them is under software control.


Obviously, one do not simply change ROMs. So my procedure is thus:


Disable interrupts (SEI) 

Store 0 at ROMSEL (fe30) and ACCCON (fe34)
to enable B-like memory mapping. 

Store the bit in the ROM set select
register (1MHz bus) 

JMP (&FFFC) 

Except, it doesn't quite work. It
changes ROM set sure enough, but the start sequence is interrupted and
scrolls uncontrollably, sometimes with characters on screen. 

I tried
blanking the 32K RAM beforehand, setting it all to &FF, and use of
*FX200,3 but none worked. I'm clearly missing a clue on how to reset the
state of the machine so it can start properly. 

Any suggestions as to
what I'm missing? 

Thanks 

-- Richard