OSBYTE &6C

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

 On entry:
   X=0  Master:    Main memory appears from &3000 to &7FFF
        Integra-B: Non-display memory appears from &3000 to &7FFF
   X<>0 Master:    Shadow memory appears from &3000 to &7FFF
        Integra-B: Display memory appears from &3000 to &7FFF
 On exit:
   X    master:    preserved
        Integra-B  returns previous state, &00 or &01

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.

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.

The Aries and Watford systems use OSBYTE &6F.

See also

• OSBYTE &49 - Check for Integra-B shadow memory
• OSBYTE &6C - Select Master/Integra-B shadow memory
• OSBYTE &6F - Select Aries/Watford shadow memory
• OSBYTE &70 - Select memory for Master VDU access
• OSBYTE &71 - Select memory for Master video display
• OSBYTE &72 - Select future use of shadow memory
• OSBYTE &73 - Select shadow bank
• OSBYTE &74 - Check for Solidisk shadow memory
• Paging in video memory
• http://mdfs.net/Docs/Comp/BBC/Osbyte00

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