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Copy protection: doPROT1

[BBC Micro cassette version, Loader]

Name: doPROT1 [Show more] Type: Subroutine Category: Copy protection Summary: Routine to self-modify the loader code
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * Elite loader (Part 2 of 6) calls doPROT1

This routine modifies various bits of code in-place as part of the copy protection mechanism. It is called with A = &48 and X = 255.
.doPROT1 LDY #&DB \ Store &EFDB in TRTB%(1 0) to point to the keyboard STY TRTB% \ translation table for OS 0.1 (which we will overwrite LDY #&EF \ with a call to OSBYTE later) STY TRTB%+1 LDY #2 \ Set the high byte of V219(1 0) to 2 STY V219+1 STA PROT1-255,X \ Poke &48 into PROT1, which changes the instruction \ there to a PHA LDY #&18 \ Set the low byte of V219(1 0) to &18 (as X = 255), so STY V219+1,X \ V219(1 0) now contains &0218 RTS \ Return from the subroutine