Cracker protection: SetPlayerIsOnTower

       Name: SetPlayerIsOnTower                                      [Show more]
       Type: Subroutine
   Category: Cracker protection
    Summary: Set up the playerIsOnTower value for checking the game is won, 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


.SetPlayerIsOnTower

                        \ We now work out whether the player just transferred
                        \ into a robot on the Sentinel's tower
                        \
                        \ We do this by starting at object #X, which is the
                        \ robot that the player just transferred into, and if
                        \ it's on top of another object or stack of objects, we
                        \ work our way down the stack, checking to see if the
                        \ stack contains the Sentinel's tower
                        \
                        \ The Sentinel's tower is always the first object to be
                        \ spawned, and object numbers are allocated from 63 and
                        \ down, so this means the tower is always object #63, or
                        \ %111111, so that's what we look for while working
                        \ through the object stack
                        \
                        \ This forms part of the code to protect the game from
                        \ crackers, as all we do with this information is to set
                        \ playerIsOnTower to the object type of the tower (which
                        \ is 6). This value is then used to generate the secret
                        \ code for the completed landscape, so if crackers have
                        \ jumped straight to the end of the level without
                        \ setting playerIsOnTower to 6, then the wrong code will
                        \ be generated in the SpawnSecretCode3D routine

.ptow1

 LDA objectFlags,X      \ Set A to the object flags for object #X

 CMP #%01000000         \ If both bits 6 and 7 of the object flags for object #X
 BCC ptow2              \ are clear then object #X is not stacked on top of
                        \ another object (so if we have looped back here from
                        \ below, we have reached the bottom of the stack), so
                        \ jump to ptow2 to stop looping through the object stack
                        \ and go back to the ProcessActionKeys routine

                        \ If we get here then object #X is stacked on top of
                        \ another object, and the number of the object below
                        \ object #X is in bits 0 to 5 of the object flags for
                        \ object #X, which is currently in A

 AND #%00111111         \ Extract bits 0 to 5 of the object flags into A and X,
 TAX                    \ so X now contains the object number of the next object
                        \ down in the stack

 EOR #%00111111         \ If the object number in A is not the Sentinel's tower
 BNE ptow1              \ (i.e. it is not 63, or %111111), then loop back to
                        \ check the next object down in the stack

                        \ If we get here then the player has just transferred
                        \ into a robot on the Sentinel's tower

 LDA objectTypes,X      \ Set playerIsOnTower = 6 (which is the object type of
 STA playerIsOnTower    \ the Sentinel's tower, whose object number is in X)

.ptow2

                        \ Fall through into part 2 of ProcessActionKeys to
                        \ continue with the robot transfer