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OSBYTE &6C (108) - Main/Shadow RAM access

 On entry, Master:
   X=0  Main memory appears from &3000 to &7FFF
   X<>0 Shadow memory appears from &3000 to &7FFF
 On entry, Integra-B on BBC B:
   X=0  Non-display memory appears from &3000 to &7FFF
   X<>0 Display memory appears from &3000 to &7FFF
 On exit:
   X    preserved

This has immediate effect and allows the user to access the shadow display memory, for instance, to load or save it or manipulate it directly. The memory that is accessed by the Master VDU drivers and displayed can set with OSBYTE &70 and OSBYTE &71.

Shadow RAM on the BBC B with the Integra-B system is also paged in with OSBYTE &6C. Note that the Integra-B functions subtly differently to the Master. If a shadow screen mode is being displayed the Integra-B functions the same as the Master. If a non-shadow screen mode is being displayed, the action is the opposite to the Master.

The Aries and Watford systems use OSBYTE &6F.

On the BBC B+ the displayed memory is always the bank selected with MODE or OSBYTE &72, and the user can only access the shadow memory by calling OSRDSC.

See also

Jgharston (talk) 01:12, 14 January 2016 (UTC)