Cracker protection: SetSecretStash

       Name: SetSecretStash                                          [Show more]
       Type: Subroutine
   Category: Cracker protection
    Summary: Alter the secret code stash, as part of the anti-cracker code
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.SetSecretStash

                        \ This routine sets the value of stashOffset, which is
                        \ the offset into the secretCodeStash where we store a
                        \ set of generated values for later checking in the
                        \ GetRowVisibility routine
                        \
                        \ The offset is set to the middle byte from the
                        \ landscape seed linear feedback shift register (LFSR),
                        \ overwriting previous values until we get here for the
                        \ last time
                        \
                        \ As we only reach this routine when drawing flat tiles
                        \ in colour 0, so stashOffset is set to the middle byte
                        \ from the landscape seed linear feedback shift register
                        \ for the last flat colour 0 tile to be drawn
                        \
                        \ The value itself doesn't matter, it's just another way
                        \ of throwing crackers off the trail of working out how
                        \ to generate landscape codes, as it changes the address
                        \ of the secret code stash for each landscape

 LDA seedNumberLFSR+2-8,X   \ At this point, X = 8 from DrawFlatTile, so this
 STA stashOffset-8,X        \ sets the following:
                            \
                            \   stashOffset = seedNumberLFSR+2

                        \ seedNumberLFSR+2 is the middle byte of the LFSR at
                        \ seedNumberLFSR(4 3 2 1 0), so this sets the secret
                        \ code stash offset to a unique and consistent value for
                        \ each landscape

                        \ Fall through into DrawOneFaceTile to finish drawing
                        \ the flat tile