Loader: DEEOR

       Name: DEEOR                                                   [Show more]
       Type: Subroutine
   Category: Loader
    Summary: Decrypt the main docked code between &1300 and &5FFF and
             the main game loop
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DOBEGIN calls DEEOR
             * DOENTRY calls DEEOR
             * SCRAM calls DEEOR


.DEEOR

IF _STH_DISC OR _IB_DISC

 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

.DEEORL

 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 DEEORL             \ Loop back to DEEORL 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 DEEORL to decrypt the next page until we
 BNE DEEORL             \ 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

ELIF _SRAM_DISC

 NOP                    \ The sideways RAM variant is not encrypted, so the
 NOP                    \ decryption code is disabled and is replaced by NOPs
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP
 NOP

 JMP BRKBK              \ Call BRKBK to set BRKV to point to the BRBR routine
                        \ and return from the subroutine using a tail call

ENDIF