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BBC Micro Elite

Loader: scramble (Disc version)

Name: scramble [View in context] Type: Subroutine Category: Loader Summary: Decrypt the main docked code between &1300 and &5FFF and the main game loop
.scramble LDY #0 \ We're going to work our way through a large number of \ encrypted bytes, so we set Y to 0 to be the index of \ the current byte within its page in memory STY SC \ Set the low byte of SC(1 0) to 0 LDX #&13 \ Set X to &13 to be the page number of the current \ byte, so we start the decryption with the first byte \ of page &13 .scrl STX SCH \ Set the high byte of SC(1 0) to X, so SC(1 0) now \ points to the first byte of page X TYA \ Set A to Y, so A now contains the index of the current \ byte within its page EOR (SC),Y \ EOR the current byte with its index within the page EOR #&33 \ EOR the current byte with &33 STA (SC),Y \ Update the current byte \ The current byte is in page X at offset Y, and SC(1 0) \ points to the first byte of page X, so we just did \ this: \ \ (X Y) = (X Y) EOR Y EOR &33 DEY \ Decrement the index in Y to point to the next byte BNE scrl \ Loop back to scrl to decrypt the next byte until we \ have done the whole page INX \ Increment X to point to the next page in memory CPX #&60 \ Loop back to scrl to decrypt the next page until we BNE scrl \ reach the start of page &60 JMP BRKBK \ Call BRKBK to set BRKV to point to the BRBR routine \ and return from the subroutine using a tail call