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Elite on the BBC Micro

Copy protection: DEEOR (6502SP version)

Name: DEEOR [View in context] Type: Subroutine Category: Copy protection Summary: Decrypt bytes between &1300 and &9FFF by EOR'ing them with their page offset
In the original source, the bytes between &1300 and &9FFF are EOR'd by the first call to SC in the Big Code File, though in the BeebAsm version they are EOR'd by The original 6502 assembly language version of the SC routine can be found in the elite-checksum.asm file.
.DEEOR LDY #0 \ Set (X Y) = SC(1 0) = &1300 STY SC LDX #&13 .DEEL STX SC+1 \ Set SC+1 = X, so now SC(1 0) = (X 0) TYA \ Set A = contents of (SC(1 0) + Y) EOR Y EOR &75 EOR (SC),Y \ = contents of ((X 0) + Y) EOR Y EOR &75 EOR #&75 \ = contents of (X Y) EOR Y EOR &75 IF _REMOVE_CHECKSUMS NOP \ If we have disabled checksums, then don't update (X Y) NOP \ with the result, and just move on to the next byte ELSE STA (SC),Y \ Store the EOR'd value in SC(1 0) + Y, i.e. (X Y) ENDIF DEY \ Decrement the loop counter to process the next byte BNE DEEL \ Loop back until we have done the whole page INX \ Increment the page counter to point to the next page CPX #&A0 \ Loop back to do the next page until X = &A0, when BNE DEEL \ (X Y) = &A000 JMP BRKBK \ Jump to BRKBK to set the standard BRKV handler for the \ game and return from the subroutine using a tail call