Cracker protection: JumpToPreview

       Name: JumpToPreview                                           [Show more]
       Type: Subroutine
   Category: Cracker protection
    Summary: An intentionally confusing jump point for controlling the main
             title loop flow when returning from the GenerateLandscape routine
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SmoothTileCorners (Part 2 of 4) calls JumpToPreview


.JumpToPreview

 EQUB &4C               \ This byte is never executed, as the stack modification
                        \ in the SmoothTileData routine sets the return address
                        \ on the stack to JumpToPreview, and the RTS instruction
                        \ will therefore jump to JumpToPreview+1 (as that's how
                        \ the RTS instruction works)
                        \
                        \ This byte is the opcode for a JMP instruction, so this
                        \ makes it look like there is a JumpToPreview routine
                        \ that contains the following:
                        \
                        \   &4C &30 &3F     JMP &3F30
                        \
                        \ as the BMI instruction below assembles into &30 &3F
                        \
                        \ This would jump to a valid instruction halfway through
                        \ the ConfigureMachine routine, so this byte, although
                        \ unused, is presumably a JMP opcode to confuse any
                        \ crackers who have reached this point in their analysis

 BMI PreviewLandscape   \ We only get here if the stack has been modified by the
                        \ SmoothTileData routine, which makes the RTS at the end
                        \ end of the GenerateLandscape routine jump here
                        \
                        \ The penultimate instruction in GenerateLandscape is a
                        \ call to the ProcessTileData routine, which happens to
                        \ set the N flag, so when the RTS instruction jumps here
                        \ using the modified return address, this BMI branch is
                        \ taken, so this instruction is effectively a JMP to the
                        \ PreviewLandscape routine