Paging in video memory

The BBC Master, the Aries-B20, the Aries B-32 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, Y=0 - select main memory
 \           Y=1 - select video memory
 \
 .vramSelect
 TYA:PHA:AND #1:PHA:TAX     :\ A=0 main RAM, A=1 video RAM
 LDA #108:JSR OSBYTE        :\ Attempt to select Master video RAM
 PLA:INX:BNE vramOk         :\ X<>255, successful
 EOR #1:LDA #111:JSR OSBYTE :\ Attempt to select Aries/Watford RAM
 .vramOk
 PLA:TAY: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, DADR+0...DADR+3=data transfer address
 \                         A=&Fx - read (load) data
 \                         A=&Ex - write (save) data
 \
 LDX &27A:BPL WaitTransIO     :\ No Tube
 LDX DADR+3                   :\ Check transfer address
 INX:BNE WaitTransTube        :\ Tube present, ADDR<&FFxxxxxx
 .WaitTransIO
 AND #&F0:TAY                 :\ Y=transfer flag with b7=1 for IO transfer
 LDX DADR+2:INX:BEQ WaitIOGo  :\ &FFFFxxxx - current IO memory
 LDA &D0                      :\ Get VDU status, bit 4=shadow screen
 INX:BEQ WaitIOScreen         :\ &FFFExxxx - use current display memory according to b4
 INX:BNE WaitIOGo             :\ Not &FFFDxxxx - use current IO memory
 LDA #16                      :\ &FFFDxxxx - shadow screen memory, force b4=1
 .WaitIOScreen
 AND #16:BEQ WaitIOGo         :\ Non-shadow screen displayed, jump with Y=&E0/&F0
 INY:JSR vramSelect           :\ Set b0 for screen, page in video RAM
 .WaitIOGo
 TYA:PHA                      :\ Save IO and screen selection flags
 \ 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 if from screen memory
 \ ie b7=1 - IO memory, b0=main/video memory
 \
 \
 \ Do IO memory transfer here
 \
 \
 .WaitExitDone
 PLA:BPL WaitExitRelease      :\ Pop transfer flag, b0=0 - Tube release
 ROR A:BCS WaitExitScreen     :\ b0=1, Screen release
 RTS
 .WaitExitRelease
 JMP TubeRelChk               :\ Release Tube, return
 .WaitExitScreen
 LDY #0:JMP vramSelect        :\ Page in main memory, return
 \
 .WaitTransTube
 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 WaitExitDone             :\ Jump to release Tube

Jgharston 22:25, 25 December 2011 (UTC)