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⌂ ∞ Elite on the BBC Micro and NES

Utility routines: DEEORS

[BBC Master version]

       Name: DEEORS                                                  [Show more]
       Type: Subroutine
   Category: Utility routines
    Summary: Decrypt a multi-page block of memory

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DEEOR calls DEEORS




 Arguments:

   FRIN(1 0)            The start address of the block to decrypt

   (A Y)                The end address of the block to decrypt

   X                    The decryption seed



.DEEORS

 STX T                  \ Store the decryption seed in T as our starting point

 STA SC+1               \ Set SC(1 0) = (A 0) to point to the start of page A,
 LDA #0                 \ so we can use SC(1 0) + Y as our pointer to the next
 STA SC                 \ byte to decrypt

.DEEORL

 LDA (SC),Y             \ Set A to the Y-th byte of SC(1 0)

 SEC                    \ Set A = A - T
 SBC T

 STA (SC),Y             \ Update the Y-th byte of SC to the new value in A

 STA T                  \ Update T with the new value in A

 TYA                    \ Set A to the current byte index in Y

 BNE P%+4               \ If A <> 0 then decrement the high byte of SC(1 0) to
 DEC SC+1               \ point to the previous page

 DEY                    \ Decrement the byte pointer

 CPY FRIN               \ Loop back to decrypt the next byte, until Y = the low
 BNE DEEORL             \ byte of FRIN(1 0), at which point we have decrypted a
                        \ whole page

 LDA SC+1               \ Check whether SC(1 0) matches FRIN(1 0) and loop back
 CMP FRIN+1             \ to decrypt the next byte until it does, at which point
 BNE DEEORL             \ we have decrypted the whole block

 RTS                    \ Return from the subroutine

 EQUB &B7, &AA          \ These bytes appear to be unused, though there is a
 EQUB &45, &23          \ comment in the original source that says "red
                        \ herring", so this would appear to be a red herring
                        \ aimed at confusing any crackers

.G%

                        \ The game code is scrambled from here to F% (or, as the
                        \ original source code puts it, "mutilated")