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Elite on the BBC Micro and NES

Text: DETOK

[BBC Micro disc version, Docked]

Name: DETOK [Show more] Type: Subroutine Category: Text Summary: Print an extended recursive token from the TKN1 token table Deep dive: Extended text tokens
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * BRIS calls DETOK * BRP calls DETOK * CATS calls DETOK * DELT calls DETOK * DETOK2 calls DETOK * GTDRV calls DETOK * GTNMEW calls DETOK * HME2 calls DETOK * MT17 calls DETOK * MT28 calls DETOK * PDESC calls DETOK * STATUS calls DETOK * SVE calls DETOK * TITLE calls DETOK * TT102 calls DETOK * TT210 calls DETOK * TT214 calls DETOK * DETOK3 calls via DTEN

Arguments: A The recursive token to be printed, in the range 1-255
Returns: A A is preserved Y Y is preserved V(1 0) V(1 0) is preserved
Other entry points: DTEN Print recursive token number X from the token table pointed to by (A V), used to print tokens from the RUTOK table via calls to DETOK3
.DETOK PHA \ Store A on the stack, so we can retrieve it later 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 LDA #LO(TKN1) \ Set V to the low byte of TKN1 STA V LDA #HI(TKN1) \ Set A to the high byte of TKN1, so when we fall \ through into DTEN, V(1 0) gets set to the address of \ the TKN1 token table .DTEN STA V+1 \ Set the high byte of V(1 0) to A, so V(1 0) now points \ to the start of the token table to use LDY #0 \ First, we need to work our way through the table until \ we get to the token that we want to print. Tokens are \ delimited by #VE, and VE EOR VE = 0, so we work our \ way through the table in, counting #VE delimiters \ until we have passed X of them, at which point we jump \ down to DTL2 to do the actual printing. So first, we \ set a counter Y to point to the character offset as we \ scan through the table .DTL1 LDA (V),Y \ Load the character at offset Y in the token table, \ which is the next character from the token table EOR #VE \ Tokens are stored in memory having been EOR'd with \ #VE, so we repeat the EOR to get the actual character \ in this token BNE DT1 \ If the result is non-zero, then this is a character \ in a token rather than the delimiter (which is #VE), \ so jump to DT1 DEX \ We have just scanned the end of a token, so decrement \ X, which contains the token number we are looking for BEQ DTL2 \ If X has now reached zero then we have found the token \ we are looking for, so jump down to DTL2 to print it .DT1 INY \ Otherwise this isn't the token we are looking for, so \ increment the character pointer BNE DTL1 \ If Y hasn't just wrapped around to 0, loop back to \ DTL1 to process the next character INC V+1 \ We have just crossed into a new page, so increment \ V+1 so that V points to the start of the new page BNE DTL1 \ Jump back to DTL1 to process the next character (this \ BNE is effectively a JMP as V+1 won't reach zero \ before we reach the end of the token table) .DTL2 INY \ We just detected the delimiter byte before the token \ that we want to print, so increment the character \ pointer to point to the first character of the token, \ rather than the delimiter BNE P%+4 \ If Y hasn't just wrapped around to 0, skip the next \ instruction INC V+1 \ We have just crossed into a new page, so increment \ V+1 so that V points to the start of the new page LDA (V),Y \ Load the character at offset Y in the token table, \ which is the next character from the token we want to \ print EOR #VE \ Tokens are stored in memory having been EOR'd with \ #VE, so we repeat the EOR to get the actual character \ in this token BEQ DTEX \ If the result is zero, then this is the delimiter at \ the end of the token to print (which is #VE), so jump \ to DTEX to return from the subroutine, as we are done \ printing JSR DETOK2 \ Otherwise call DETOK2 to print this part of the token JMP DTL2 \ Jump back to DTL2 to process the next character .DTEX 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 PLA \ Restore A from the stack, so it is preserved through \ calls to this routine RTS \ Return from the subroutine