Some useful 6502 routines.
Generate error from sideways ROM
When running code in a service ROM you can't generate an error with BRK as the current language will be in another ROM and won't be able to read it. The standard method is to copy the error to the bottom of the stack and execute it there.
\ Generate inline error \ ===================== .MkError PLA:STA &FD:PLA:STA &FE :\ Pop address after JSR LDY #0 :\ Index into the error block .MkErrorLp INY:LDA (&FD),Y:STA &100,Y :\ Copy error block to stack BNE MkErrorLp :\ Loop until terminating &00 byte STA &100:JMP &100 :\ Store &00 as BRK and jump to it
It is called with:
JSR MkError EQUB errornumber:EQUS "error message":EQUB 0
The error number must not be zero as that will be matched as the terminator byte. &FD/E are allowed to be used as they are the error pointer anyway and will be overwritten immediately after the JMP &100 when the BRK handler sets them up.
\ Print inline text \ ================= \ Corrupts A,Y .PrText PLA:STA &F6:PLA:STA &F7 LDY #1 .PrTextLp LDA (&F6),Y:BEQ PrTextEnd JSR OSASCI:INY:BNE PrTextLp .PrTextEnd CLC:TYA:ADC &F6:TAY LDA #0:ADC &F7:PHA TYA:PHA:RTS
This is called with:
JSR PrText EQUS "text message" EQUB 0 \ execution continues here
&F6/7 are allowed to be used as they are only otherwise used when calling OSRDRM and when ROMFS makes sideways ROM calls. As you can't *SPOOL text to ROMFS as it is a read-only filing system printing text won't result in a ROMFS filing system call that uses &F6/7.