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Copy protection: BEGIN

[BBC Micro disc version, Loader 1]

Name: BEGIN [Show more] Type: Subroutine Category: Copy protection Summary: Decrypt the loader code using a rolling EOR that uses the decryption routine itself to seed the decryption
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
.BEGIN \ Note that the following copy protection code is \ skipped in the unprotected version built here, as the \ binaries are not encrypted and therefore do not need \ to be decrypted \ \ Instead, the execution address for the ELITE2 binary \ points to ENTRY rather than BEGIN LDX p1c+1 \ Set X to the comparison value from the CMP instruction \ below, which has the value p1d - BEGIN, which we use \ as the offset of the first byte to decrypt (so we \ decrypt from p1d onwards) .p1 LDA BEGIN \ Fetch the first byte from BEGIN to act as the initial \ seed for the rolling EOR process (this address gets \ modified by the following to work through the whole \ decryption routine) .p1a EOR BEGIN,X \ Set A = A EOR the X-th byte of the loader STA BEGIN,X \ Store the decrypted byte in the X-th byte the loader INX \ Increment the pointer BNE p1a \ Loop back until we have decrypted to the end of the \ page .p1b INC p1+1 \ Increment the low byte of the argument to the LDA \ instruction at p1 above, so this would change it from \ LDA BEGIN to LDA BEGIN+1, for example (so the next \ time we do the EOR, we choose the next byte of the \ decryption routine as the initial seed) BEQ p1d \ If it equals zero (so the LDA BEGIN has worked itself \ to LDA BEGIN+255 and round again to LDA BEGIN), jump \ down to p1d as we have finally finished decrypting \ the code LDA p1+1 \ Fetch the newly incremented low byte of the argument \ to the LDA instruction at p1 above .p1c CMP #(p1d - BEGIN) \ If we have used all the bytes from the decryption BEQ p1b \ routine as seeds, skip one byte and then continue on \ to keep using seeds until we have done the whole page. \ This means we decrypt using the following seeds: \ \ * The contents of BEGIN \ * The contents of BEGIN+1 \ * The contents of BEGIN+2 \ ... \ * The contents of p1d-1 \ * The contents of p1d+1 \ ... \ * The contents of BEGIN+254 \ * The contents of BEGIN+255 \ \ and then we are done JMP BEGIN \ Otherwise look up to BEGIN to do another decryption \ run, but using the next byte of the decryption routine \ as the seed .p1d \ From this point on, the code is encrypted by \ BIT BYTEV+1 \ If the high byte of BYTEV does not have bit 7 set (so BPL BEGIN \ BYTEV is less than &8000, i.e. it's pointing to user \ RAM rather than the ROM routine), then jump up to \ BEGIN to hang machine, as it has presumably been \ changed by someone trying to crack the game \ Otherwise fall through into the now-decrypted loader \ code to get on with loading the game