Simple ROM selection command code
Sideways ROMs usually need to recognise a command to start up or to select a simple filing system. The following code provides the simplest way of doing this. It simply matches the name of the ROM itself.
\ -------------------- \ SERVICE 4 - *Command \ -------------------- .Serv4 TYA:PHA:DEY:LDX #&FF .Serv4Lp INX:INY:LDA (&F2),Y CMP #ASC".":BEQ Serv4Dot CMP #ASC"!":BCC Serv4End CMP &8009,X:BEQ Serv4Lp :\ Match with ROM title EOR #&20 :\ Change case CMP &8009,X:BEQ Serv4Lp :\ Match with ROM title .Serv4Quit PLA:TAY:LDA #4:RTS :\ Restore A,Y and return unclaimed .Serv4End LDA &8009,X:BNE Serv4Quit .Serv4Dot LDA #142:LDX &F4:JMP OSBYTE :\ Select myself as a language or JSR SelectFS:PLA:LDA #0:RTS :\ Select myself as a filing system
Note that X does not need to be restored on exiting a service call handler as the MOS immediately fetches X from &F4 on return from the call.
A better way of recognising a minimal command to start a filing system is to actually match the string in the filing system information block.
\ -------------------- \ SERVICE 4 - *Command \ -------------------- .Serv4 TYA:PHA:DEY:LDX #&FF .Serv4Lp INX:INY:LDA (&F2),Y CMP #ASC".":BEQ Serv4Dot CMP #ASC"!":BCC Serv4End CMP Serv25Info,X:BEQ Serv4Lp :\ Match with FS Info Block EOR #&20 :\ Change case CMP Serv25Info,X:BEQ Serv4Lp :\ Match with FS Info Block .Serv4Quit PLA:TAY:LDA #4:RTS :\ Restore A,Y and return unclaimed .Serv4End LDA &8009,X:BNE Serv4Quit .Serv4Dot JSR SelectFS:PLA:LDA #0:RTS :\ Select myself as a filing system \ ---------------------------- \ SERVICE &25 - Return FS Info \ ---------------------------- .Serv25 LDX #0 .Serv25lp LDA Serv25Info,X:STA (&F2),Y :\ Copy FS Info to MOS workspace INY:INX:CPX #11:BNE Serv25lp :\ Copy 11 bytes LDA #&25:RTS .Serv25Info EQUS "MyFS " :\ Filing system name EQUB lowchannel :\ Lowest channel number EQUB highchannel :\ Highest channel number EQUB fsnumber :\ Filing system number
It can also be convenient for other code that refers to filing system numbers and channels to reference the Filing System Info directly, for example:
.fsc07 :\ Request filing system handle range LDX Serv25Info+8 :\ Lowest channel number LDY Serv25Info+9 :\ Highest channel number RTS .args00 :\ Return filing system number LDA Serv25Info+10:RTS .SelectFS LDY Serv25Info+10 :\ Set Y=my filing system number .Serv12 CPY Serv25Info+10 BEQ SelectMyFS:RTS :\ If not my FS number, return .SelectMyFS ...
