Difference between revisions of "Paging in video memory"

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(Optimised vramSelect, better method of detecting shadow screen.)
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[[Category:Programming]]
 
[[Category:Programming]]
 
[[Category:6502]]
 
[[Category:6502]]
The BBC Master, the Aries-B20, the Aries B-32 and the Watford 32 RAM card
+
The BBC Master, the Aries-B20 and B-32, the Integra-B and the Watford 32 RAM
have shadow screen memory. The following code (taken from
+
card have shadow screen memory. The following code (taken from
 
HADFS<ref>[http://mdfs.net/hadfs mdfs.net/hadfs]</ref> and
 
HADFS<ref>[http://mdfs.net/hadfs mdfs.net/hadfs]</ref> and
 
HostFS<ref>[http://mdfs.net/hostfs mdfs.net/hostfs]</ref>) will page in or
 
HostFS<ref>[http://mdfs.net/hostfs mdfs.net/hostfs]</ref>) will page in or
out the video memory transparently using the appropriate OSBYTE call to do so.
+
out the video memory transparently using the appropriate OSBYTE call to do
 +
so.
  
 
   \ Screen selection routines
 
   \ Screen selection routines
 
   \ =========================
 
   \ =========================
   \ On entry, Y=0 - select main memory
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   \ On entry, A=0 - select main memory
   \          Y=1 - select video memory
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   \          A=1 - select video memory
 +
  \ On exit,  all registers corrupted
 
   \
 
   \
 
   .vramSelect
 
   .vramSelect
   TYA:PHA:AND #1:PHA:TAX   :\ A=0 main RAM, A=1 video RAM
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   PHA:TAX                 :\ A=0 main RAM, A=1 video RAM
   LDA #108:JSR OSBYTE      :\ Attempt to select Master video RAM
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   LDA #108:JSR OSBYTE      :\ Attempt to select Master/Integra-B video RAM
 
   PLA:INX:BNE vramOk      :\ X<>255, successful
 
   PLA:INX:BNE vramOk      :\ X<>255, successful
 
   EOR #1:TAX              :\ A=1 main RAM, A=0 video RAM
 
   EOR #1:TAX              :\ A=1 main RAM, A=0 video RAM
   LDA #111:JSR OSBYTE      :\ Attempt to select Aries/Watford RAM
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   LDA #111:JMP OSBYTE      :\ Attempt to select Aries/Watford video RAM
 
   .vramOk
 
   .vramOk
   PLA:TAY:RTS
+
   RTS
  
 
Using this code, filing system or other code can selectively access main or
 
Using this code, filing system or other code can selectively access main or
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   \ Decide what local memory to transfer data to/from
 
   \ Decide what local memory to transfer data to/from
 
   \ -------------------------------------------------
 
   \ -------------------------------------------------
   \ On entry, DADR+0...DADR+3=data transfer address
+
   \ On entry, DADDR+0...DADDR+3=data transfer address
   \                         A=&Fx - read (load) data
+
   \                           A=&Fx - read (load) data
   \                         A=&Ex - write (save) data
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   \                           A=&Ex - write (save) data
 
   \
 
   \
 
   LDX &27A:BPL WaitTransIO    :\ No Tube
 
   LDX &27A:BPL WaitTransIO    :\ No Tube
   LDX DADR+3                   :\ Check transfer address
+
   LDX DADDR+3                 :\ Check transfer address
 
   INX:BNE WaitTransTube        :\ Tube present, ADDR<&FFxxxxxx
 
   INX:BNE WaitTransTube        :\ Tube present, ADDR<&FFxxxxxx
 
   .WaitTransIO
 
   .WaitTransIO
   AND #&F0:TAY                 :\ Y=transfer flag with b7=1 for IO transfer
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   AND #&F0:PHA                 :\ Push transfer flag with b7=1 for IO transfer
   LDX DADR+2:INX:BEQ WaitIOGo :\ &FFFFxxxx - current IO memory
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   LDX DADDR+2:INX:BEQ WaitIOGo :\ &FFFFxxxx - current IO memory
  LDA &D0                      :\ Get VDU status, bit 4=shadow screen
+
   INX:BEQ WaitIODisplay        :\ &FFFExxxx - use current display memory according to b4
   INX:BEQ WaitIOScreen        :\ &FFFExxxx - use current display memory according to b4
+
   INX:BEQ WaitIOShadow        :\ &FFFDxxxx - shadow memory
   INX:BNE WaitIOGo             :\ Not &FFFDxxxx - use current IO memory
+
  BNE WaitIOGo                 :\ Not &FFFDxxxx - use current IO memory
   LDA #16                      :\ &FFFDxxxx - shadow screen memory, force b4=1
+
  :
   .WaitIOScreen
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  .WaitIODisplay
   AND #16:BEQ WaitIOGo        :\ Non-shadow screen displayed, jump with Y=&E0/&F0
+
   LDA #&84:JSR OSBYTE
   INY:JSR vramSelect          :\ Set b0 for screen, page in video RAM
+
  TYA:BPL WaitIOGo            :\ Not shadow screen displayed
 +
   .GetShadow
 +
   LDA #1:JSR vramSelect        :\ Page in shadow memory
 +
   PLA:ORA #1:PHA              :\ Set b0 for screen, page in video RAM
 +
  :
 
   .WaitIOGo
 
   .WaitIOGo
  TYA:PHA                      :\ Save IO and screen selection flags
 
 
   \ At this point, the byte on the stack holds
 
   \ At this point, the byte on the stack holds
 
   \  &E0 - load is to current memory
 
   \  &E0 - load is to current memory
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   \  &F0 - save is from current memory
 
   \  &F0 - save is from current memory
 
   \  &F1 - save is from screen memory
 
   \  &F1 - save is from screen memory
   \ ie b7=1 - IO memory, b0=main/video memory
+
   \ ie b7=1 - IO memory, b4=save/load, b0=main/video memory
 
   \
 
   \
 
   \
 
   \
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   JMP TubeRelChk              :\ Release Tube, return
 
   JMP TubeRelChk              :\ Release Tube, return
 
   .WaitExitScreen
 
   .WaitExitScreen
   LDY #0:JMP vramSelect        :\ Page in main memory, return
+
   LDA #0:JMP vramSelect        :\ Page in main memory, return
 
   \
 
   \
 
   .WaitTransTube
 
   .WaitTransTube
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   \
 
   \
 
   JMP WaitExitDone            :\ Jump to release Tube
 
   JMP WaitExitDone            :\ Jump to release Tube
 +
 +
== See also ==
 +
* [[Extended addressing]]
  
 
== References ==
 
== References ==
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[[User:Jgharston|Jgharston]] 22:25, 25 December 2011 (UTC)
 
[[User:Jgharston|Jgharston]] 22:25, 25 December 2011 (UTC)
 +
[[User:Jgharston|Jgharston]] ([[User talk:Jgharston|talk]]) 20:38, 3 April 2015 (UTC)

Revision as of 21:38, 3 April 2015

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[1] and HostFS[2]) 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 video RAM
 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 video RAM
 .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 WaitTransIO     :\ No Tube
 LDX DADDR+3                  :\ Check transfer address
 INX:BNE WaitTransTube        :\ Tube present, ADDR<&FFxxxxxx
 .WaitTransIO
 AND #&F0:PHA                 :\ Push transfer flag with b7=1 for IO transfer
 LDX DADDR+2:INX:BEQ WaitIOGo :\ &FFFFxxxx - current IO memory
 INX:BEQ WaitIODisplay        :\ &FFFExxxx - use current display memory according to b4
 INX:BEQ WaitIOShadow         :\ &FFFDxxxx - shadow memory
 BNE WaitIOGo                 :\ Not &FFFDxxxx - use current IO memory
 :
 .WaitIODisplay
 LDA #&84:JSR OSBYTE
 TYA:BPL WaitIOGo             :\ Not shadow screen displayed
 .GetShadow
 LDA #1:JSR vramSelect        :\ Page in shadow memory
 PLA:ORA #1:PHA               :\ Set b0 for screen, page in video RAM
 :
 .WaitIOGo
 \ 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
 \
 \
 .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
 LDA #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

See also

References

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