OSBYTE &6F

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OSBYTE &6F (111) - Read/Write Aries/Watford Shadow RAM Switch

   X b7=0 no stack operation
     b7=1 pop state from stack (reading), push state on stack (writing)
     b6=0 write switch state
     b6=1 read switch state
     b0=0 select video RAM
     b0=1 select program RAM
   On exit, X contains the previous switch state:
     b0=0 video RAM
     b0=1 program RAM
   
   This gives the following entry and return values:
   X=&00 - Select video RAM
     &01 - Select program RAM
     &40 - Read current RAM state
     &41 - Read current RAM state
     &80 - Save current RAM state and select video RAM
     &81 - Save current RAM state and select program RAM
     &C0 - Pop and select stacked RAM state
     &C1 - Pop and select stacked RAM state

This call is implemented by the Watford and Aries shadow memory systems for the BBC B computer, and compatible systems. The B+ and Master series use different shadow selection calls.

See Also

WikiSysop (talk) 13:31, 8 March 2015 (UTC)