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Elite on the BBC Micro

Version analysis of doPROT1

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

Name: doPROT1 Type: Subroutine Category: Copy protection Summary: Routine to self-modify the loader code
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

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STA PROT1-255,X \ Poke &48 into PROT1, which changes the instruction \ there to a PHA
CMP swine-5,X \ This part of the loader has been disabled by the \ crackers, by changing an STA to a CMP (as this is an \ unprotected version)
 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

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