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 | + | 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, | + | \ On entry, A=0 - select main memory |
| − | \ | + | \ A=1 - select video memory |
| + | \ On exit, all registers corrupted | ||
\ | \ | ||
.vramSelect | .vramSelect | ||
| − | + | PHA:TAX :\ A=0 main RAM, A=1 video RAM | |
| − | LDA #108:JSR OSBYTE :\ Attempt to select Master video RAM | + | 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: | + | LDA #111:JMP OSBYTE :\ Attempt to select Aries/Watford video RAM |
.vramOk | .vramOk | ||
| − | + | 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 | ||
| Line 27: | Line 29: | ||
\ Decide what local memory to transfer data to/from | \ Decide what local memory to transfer data to/from | ||
\ ------------------------------------------------- | \ ------------------------------------------------- | ||
| − | \ On entry, | + | \ 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 &27A:BPL WaitTransIO :\ No Tube | ||
| − | LDX | + | LDX DADDR+3 :\ Check transfer address |
INX:BNE WaitTransTube :\ Tube present, ADDR<&FFxxxxxx | INX:BNE WaitTransTube :\ Tube present, ADDR<&FFxxxxxx | ||
.WaitTransIO | .WaitTransIO | ||
| − | AND #&F0: | + | AND #&F0:PHA :\ Push transfer flag with b7=1 for IO transfer |
| − | LDX | + | LDX DADDR+2:INX:BEQ WaitIOGo :\ &FFFFxxxx - current IO memory |
| − | + | INX:BEQ WaitIODisplay :\ &FFFExxxx - use current display memory according to b4 | |
| − | INX:BEQ | + | INX:BEQ WaitIOShadow :\ &FFFDxxxx - shadow memory |
| − | INX:BNE WaitIOGo | + | BNE WaitIOGo :\ Not &FFFDxxxx - use current IO memory |
| − | LDA # | + | : |
| − | . | + | .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 | .WaitIOGo | ||
| − | |||
\ 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 | ||
| Line 51: | Line 56: | ||
\ &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 |
\ | \ | ||
\ | \ | ||
| Line 64: | Line 69: | ||
JMP TubeRelChk :\ Release Tube, return | JMP TubeRelChk :\ Release Tube, return | ||
.WaitExitScreen | .WaitExitScreen | ||
| − | + | LDA #0:JMP vramSelect :\ Page in main memory, return | |
\ | \ | ||
.WaitTransTube | .WaitTransTube | ||
| Line 80: | Line 85: | ||
\ | \ | ||
JMP WaitExitDone :\ Jump to release Tube | JMP WaitExitDone :\ Jump to release Tube | ||
| + | |||
| + | == See also == | ||
| + | * [[Extended addressing]] | ||
== References == | == References == | ||
| Line 85: | Line 93: | ||
[[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)
