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Elite on the BBC Micro

Text: DETOK2 (6502SP version)

Name: DETOK2 [View in context] Type: Subroutine Category: Text Summary: Print an extended text token (1-255) Deep dive: Extended text tokens
Arguments: A The token to be printed (1-255) Returns: A A is preserved Y Y is preserved V(1 0) V(1 0) is preserved Other entry points: DTS Print the single letter pointed to by A, where A is an address within the extended two-letter token tables of TKN2 and QQ16
.DETOK2 CMP #32 \ If A < 32 then this is a jump token, so skip to DT3 to BCC DT3 \ process it BIT DTW3 \ If bit 7 of DTW3 is clear, then extended tokens are BPL DT8 \ enabled, so jump to DT8 to process them \ If we get there then this is not a jump token and \ extended tokens are not enabled, so we can call the \ standard text token routine at TT27 to print the token TAX \ Copy the token number from A into X TYA \ Store Y on the stack PHA LDA V \ Store V(1 0) on the stack PHA LDA V+1 PHA TXA \ Copy the token number from X back into A JSR TT27 \ Call TT27 to print the text token JMP DT7 \ Jump to DT7 to restore V(1 0) and Y from the stack and \ return from the subroutine .DT8 \ If we get here then this is not a jump token and \ extended tokens are enabled CMP #'[' \ If A < ASCII "[" (i.e. A <= ASCII "Z", or 90) then BCC DTS \ this is a printable ASCII character, so jump down to \ DTS to print it CMP #129 \ If A < 129, so A is in the range 91-128, jump down to BCC DT6 \ DT6 to print a randomised token from the MTIN table CMP #215 \ If A < 215, so A is in the range 129-214, jump to BCC DETOK \ DETOK as this is a recursive token, returning from the \ subroutine using a tail call \ If we get here then A >= 215, so this is a two-letter \ token from the extended TKN2/QQ16 table SBC #215 \ Subtract 215 to get a token number in the range 0-12 \ (the C flag is set as we passed through the BCC above, \ so this subtraction is correct) ASL A \ Set A = A * 2, so it can be used as a pointer into the \ two-letter token tables at TKN2 and QQ16 PHA \ Store A on the stack, so we can restore it for the \ second letter below TAX \ Fetch the first letter of the two-letter token from LDA TKN2,X \ TKN2, which is at TKN2 + X JSR DTS \ Call DTS to print it PLA \ Restore A from the stack and transfer it into X TAX LDA TKN2+1,X \ Fetch the second letter of the two-letter token from \ TKN2, which is at TKN2 + X + 1, and fall through into \ DTS to print it .DTS CMP #'A' \ If A < ASCII "A", jump to DT9 to print this as ASCII BCC DT9 BIT DTW6 \ If bit 7 of DTW6 is set, then lower case has been BMI DT10 \ enabled by jump token 13, {lower case}, so jump to \ DT10 to apply the lower case and single cap masks BIT DTW2 \ If bit 7 of DTW2 is set, then we are not currently BMI DT5 \ printing a word, so jump to DT5 so we skip the setting \ of lower case in Sentence Case (which we only want to \ do when we are already printing a word) .DT10 ORA DTW1 \ Convert the character to lower case if DTW1 is \ %00100000 (i.e. if we are in {sentence case} mode) .DT5 AND DTW8 \ Convert the character to upper case if DTW8 is \ %11011111 (i.e. after a {single cap} token) .DT9 JMP DASC \ Jump to DASC to print the ASCII character in A, \ returning from the routine using a tail call .DT3 \ If we get here then the token number in A is in the \ range 1 to 32, so this is a jump token that should \ call the corresponding address in the jump table at \ JMTB TAX \ Copy the token number from A into X TYA \ Store Y on the stack PHA LDA V \ Store V(1 0) on the stack PHA LDA V+1 PHA TXA \ Copy the token number from X back into A ASL A \ Set A = A * 2, so it can be used as a pointer into the \ jump table at JMTB, though because the original range \ of values is 1-32, so the doubled range is 2-64, we \ need to take the offset into the jump table from \ JMTB-2 rather than JMTB TAX \ Copy the doubled token number from A into X LDA JMTB-2,X \ Set DTM(2 1) to the X-th address from the table at STA DTM+1 \ JTM-2, which modifies the JSR DASC instruction at LDA JMTB-1,X \ label DTM below so that it calls the subroutine at the STA DTM+2 \ relevant address from the JMTB table TXA \ Copy the doubled token number from X back into A LSR A \ Halve A to get the original token number .DTM JSR DASC \ Call the relevant JMTB subroutine, as this instruction \ will have been modified by the above to point to the \ relevant address .DT7 PLA \ Restore V(1 0) from the stack, so it is preserved STA V+1 \ through calls to this routine PLA STA V PLA \ Restore Y from the stack, so it is preserved through TAY \ calls to this routine RTS \ Return from the subroutine .DT6 \ If we get here then the token number in A is in the \ range 91-128, which means we print a randomly picked \ token from the token range given in the corresponding \ entry in the MTIN table STA SC \ Store the token number in SC TYA \ Store Y on the stack PHA LDA V \ Store V(1 0) on the stack PHA LDA V+1 PHA JSR DORND \ Set X to a random number TAX LDA #0 \ Set A to 0, so we can build a random number from 0 to \ 4 in A plus the C flag, with each number being equally \ likely CPX #51 \ Add 1 to A if X >= 51 ADC #0 CPX #102 \ Add 1 to A if X >= 102 ADC #0 CPX #153 \ Add 1 to A if X >= 153 ADC #0 CPX #204 \ Set the C flag if X >= 204 LDX SC \ Fetch the token number from SC into X, so X is now in \ the range 91-128 ADC MTIN-91,X \ Set A = MTIN-91 + token number (91-128) + random (0-4) \ = MTIN + token number (0-37) + random (0-4) JSR DETOK \ Call DETOK to print the extended recursive token in A JMP DT7 \ Jump to DT7 to restore V(1 0) and Y from the stack and \ return from the subroutine using a tail call