Paging in video memory

The BBC Master, the Aries-B20 and B-32, the Integra-B and the Watford 32 RAM card have shadow screen memory. The following code (taken from HADFS and HostFS) will page in or out the video memory transparently using the appropriate OSBYTE call to do so.

 \ Screen selection routines
 \ =========================
 \ On entry, A=0 - select main memory
 \           A=1 - select video memory
 \ On exit,  all registers corrupted
 \
 .vramSelect
 PHA:TAX                   :\ A=0 main RAM, A=1 video RAM
 LDA #108:JSR OSBYTE       :\ Attempt to select Master/Integra-B vRAM
 PLA:INX:BNE vramOk        :\ X<>255, successful
 EOR #1:TAX                :\ A=1 main RAM, A=0 video RAM
 LDA #111:JMP OSBYTE       :\ Attempt to select Aries/Watford vRAM
 .vramOk
 RTS

Using this code, filing system or other code can selectively access main or video memory according to the standard &FFFFxxxx, &FFFExxxx and &FFFDxxxx address ranges.

 \ Decide what local memory to transfer data to/from
 \ -------------------------------------------------
 \ On entry, DADDR+0...DADDR+3=data transfer address
 \                           A=&Fx - read (load) data
 \                           A=&Ex - write (save) data
 \
 LDX &27A:BPL TransIO      :\ No Tube
 LDX DADDR+3               :\ Check transfer address &XX------
 INX:BNE TransTube         :\ Tube present, ADDR<&FFxxxxxx
 .TransIO
 AND #&F0:PHA              :\ Push transfer flag, b7=1 for IO transfer
 LDX DADDR+2               :\ Get address &--XX----
 INX:BEQ TransIOGo         :\ &FFFFxxxx - current IO memory
 INX:BEQ TransIODisplay    :\ &FFFExxxx - use current display memory
 INX:BEQ TransIOShadow     :\ &FFFDxxxx - shadow memory
 BNE TransIOGo             :\ Not &FFFDxxxx - use current IO memory
 :
 .TransIODisplay
 LDA #&84:JSR OSBYTE
 TYA:BPL TransIOGo         :\ Not shadow screen displayed
 .TransIOShadow
 LDA #1:JSR vramSelect     :\ Page in shadow memory
 PLA:ORA #1:PHA            :\ Set b0 for screen, page in video RAM
 :
 .TransIOGo
 \ At this point, the byte on the stack holds
 \   &E0 - load is to current memory
 \   &E1 - load is to screen memory
 \   &F0 - save is from current memory
 \   &F1 - save is from screen memory
 \ ie b7=1 - IO memory, b4=save/load, b0=main/video memory
 \
 \
 \ Do IO memory transfer here
 \
 \
 .TransExitDone
 PLA:BPL TransExitRelease  :\ Pop transfer flag, b7=0 - Tube release
 ROR A:BCS TransExitScreen :\ b0=1, Screen release
 RTS
 .TransExitRelease
 JMP TubeRelease           :\ Release Tube, return
 .TransExitScreen
 LDA #0:JMP vramSelect     :\ Page in main memory, return
 \
 .TransTube
 CLC:ADC #&10:ROL A        :\ Cy=1/0 for load/save
 LDA #0:ADC #0:PHA         :\ A=1/0 for load/save
 JSR TubeAction            :\ Claim Tube and start transfer
 \ At this point, the byte on the stack holds
 \   &1 - load is to Tube
 \   &0 - save is from Tube
 \ ie b7=0 - Tube memory
 \
 \
 \ Do Tube transfer here
 \
 \
 JMP TransExitDone         :\ Jump to release Tube

Electron

This is untested at present, but this appears to be the equivalent code for the Electron

\ Electron screen selection routine
\ =================================
\ On entry, A=0 - select main memory
\           A=1 - select video memory
\ On exit,  A corrupted, X,Y preserved
\
 .vramSelect
ROR A                      :\ Move selection in bit 0 to Carry
LDA &027F:BPL vramOk       :\ No shadow RAM present
ROR A:STA &FC7F            :\ Move Carry A to bit 7, set paging flag
.vramOk
RTS

See also

• OSBYTE &6C - Select Master/Integra-B shadow memory
• OSBYTE &6F - Select BBC B shadow memory
• OSBYTE &84 - Read top of user memory
• Paging in video memory
• Extended addressing
• http://mdfs.net/Docs/Comp/BBC/Osbyte00

References

Jgharston 22:25, 25 December 2011 (UTC) Jgharston (talk) 20:38, 3 April 2015 (UTC)