Date   : Sat, 03 Dec 2005 21:39:12 +0000
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: ?Shadow RAM point

Peter Craven <cjl_craves@...> wrote:
>   If have Shadow RAM on BBC/ Master as being displayed, what happens if you
> are entering a BASIC program that goes over &3000, as the computer would
have
> to swap the 'Non-Shadow' User memory back in to store the program?????
 
If a shadow mode is being displayed, then the computer (BASIC,
WORD, whatever) can use everything up to &8000. The shadow memory
is only every accessed by the VDU drivers. The VDU code in the MOS
switches the shadow memory in temporarily just to write
information to the screen memory.
 
The VDU hardware displays the screen memory continously as it is
accessing it seperately from the CPU. The CPU can only "see"
normal memory at &0000-&7FFF. The VDU hardware can only "see"
shadow memory at &0000-&7FFF (when shadow mode selected). These
are two seperate bits of memory.
 
It's the same concept as BASIC and DFS both being in the same part
of the address map at &8000-&C000. When BASIC is running, BASIC is
switched into &8000-&C000. When a call to some DFS code is made,
DFS is switched in at &8000-&C000 until DFS returns to BASIC, when
the MOS switches BASIC back in again.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
The most perfect world is an imperfect world as the imperfections give
people a reason to strive to change it.