OSBYTE &6F
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
- OSBYTE &6C - Select Master shadow memory
- OSBYTE &6F - Select BBC B 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
