Detokenising in 6502 machine code
BBC BASIC programs are tokenised, that is, BASIC keywords are stored as one-byte values This results in programs which execute faster and are more compact. BASIC has code to detokenise program code when listing, but it is unwise to try to call it directly as it is in different locations in different versions, and also depends on workspace locations containing values that BASIC expects which will not be the case if trying to use the routines when BASIC is no the current language.
The following code, which must run in the I/O processor, will search for the token table in any version of BBC BASIC - even in non-6502 BASIC ROMs.
\ Detokenise BASIC tokens \ ======================= \ needs, eg tbase=&70:tptr=&72 if calling from BASIC \ eg tbase=&A8:tptr=&AA if calling as a *command \ \ ---------------------------------- \ TokenInit - Find BASIC token table \ ---------------------------------- \ On entry, BASIC ROM must be paged in \ On exit, tbase=>Start of token table \ A,Y,tptr corrupted, X preserved \ .TokenInit LDY #0:STY tptr:LDA #&80:STA tptr+1 :\ tptr=>ROM start .TokInitLp LDA #1:CLC:ADC tptr:STA tptr :\ Inc. tptr LDA #0:TAY:ADC tptr+1:STA tptr+1 LDA (tptr),Y:CMP #&80:BEQ TokInit2:DEY :\ Found &80 .TokInit2 INY:LDA (tptr),Y:CMP #ASC"A":BNE TokInitLp :\ Not "A" INY:LDA (tptr),Y:CMP #ASC"N":BNE TokInitLp :\ Not "AN" INY:LDA (tptr),Y:CMP #ASC"D":BNE TokInitLp :\ Not "AND" .TokInitOk LDA tptr:STA tbase:LDA tptr+1:STA tbase+1 RTS : : \ ----------------------------- \ PrToken - Print a BASIC token \ ----------------------------- \ On entry, A=token byte \ BASIC ROM must be paged in \ tbase=>Start of token table, set by TokenInit \ On exit, A,Y,tptr corrupted, X preserved \ .TokenPrint PHA:LDA tbase:STA tptr :\ Point to start token table LDA tbase+1:STA tptr+1 .TokPrLp1 LDY #&FF .TokPrLp2 INY:LDA (tptr),Y:BPL TokPrLp2 :\ Loop until token byte found PLA:CMP (tptr),Y:BEQ TokPrFound :\ Found matching token PHA:TYA:BNE TokPrStep :\ Step to next token .TokPrLp3 INY:LDA (tptr),Y:BPL TokPrLp3 :\ Find next token byte DEY:DEY .TokPrStep INY:TYA:SEC:ADC tptr:STA tptr :\ Step past this token string LDA #0:ADC tptr+1:STA tptr+1 BNE TokPrLp1 :\ Loop to keep searching .TokPrFound TYA:BEQ TokPrNxt:LDY #0 :\ Skip past leading token .TokPrLp3 LDA (tptr),Y:BMI TokPrEnd :\ Token byte, end CMP #32:BCC TokPrEnd:JSR OSWRCH :\ Flag byte, end .TokPrNxt INY:BNE TokPrLp3 :\ Loop back for next character .TokPrEnd RTS
You can test the code with the following:
CALL TokenInit FOR A%=&80 TO 255:PRINT ;A%;": ";:CALL TokenPrint:PRINT:NEXT
Note that the code requires the BASIC ROM to be paged in, and also needs to run in the I/O processor - where the ROMs are. This would normally be done in a *command utility with code such as the following:
LDA &F4:PHA :\ Save current ROM LDA &24B:CMP #&FF:BEQ errNoBASIC :\ Get BASIC ROM, error if no BASIC AND #&127:JSR SelectROM :\ Page in BASIC ROM ... JSR TokenInit ... JSR TokenPrint ... PLA :\ Restore previous ROM .SelectROM STA &F4:STA &FE30:RTS :\ Page in ROM
This code has been tested with 6502 BASIC 1.00-4.00, 4.32, 4.40, 4.86, Z80 BASIC and PDP-11 BASIC.
See also
Jgharston 03:09, 22 November 2010 (UTC)
