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⌂ ∞ Lander on the Acorn Archimedes

Maths (Geometry): AddVectors

       Name: AddVectors                                              [Show more]
       Type: Subroutine
   Category: Maths (Geometry)
    Summary: An unused routine that adds two vectors and uses self-modifying
             code to store the results in a specified location
  Deep dive: Unused code in Lander

    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file




.vmod1

 ADD     R4, R11, #xVertexRotated   \ Modification code to store the result of
                                    \ the vector addition in xVertexRotated

.vmod2

 ADD     R4, R11, #xCoord           \ Modification code to restore the result of
                                    \ the vector addition to xCoord

.vmod3

 MOV     R4, R12                    \ Modification code to store the result of
                                    \ the vector addition in R12

.vmod4

 ADD     R4, R11, #xLandscapeCol    \ Modification code to store the result of
                                    \ the vector addition in xLandscapeCol

.vmod5

 ADD     R4, R11, #xLandscapeRow    \ Modification code to store the result of
                                    \ the vector addition in xLandscapeRow

.AddVectors

 STMFD   R13!, {R14}                \ Store the return address on the stack

                                    \ We now modify the following instruction in
                                    \ the AddVectorToVertices routine to change
                                    \ the way it works:
                                    \
                                    \   ADD R3, R11, #xCoord
                                    \
                                    \ Interestingly, though, none of the
                                    \ modifications above will work properly, as
                                    \ they all change R4 rather than R3, so
                                    \ presumably the modified subroutine was
                                    \ refactored and this unused routine wasn't
                                    \ updated
                                    \
                                    \ If this routine worked, it would use the
                                    \ modifications to change the address where
                                    \ we stored the result

 LDR     R1, vmod3                  \ Modify the eighth instruction in the
 STR     R1, AddVectorToVertices+28 \ AddVectorToVertices routine to the
                                    \ instruction in vmod3

 LDMIA   R0, {R0-R2}                \ Fetch the coordinate from R0 into
                                    \ (R0, R1, R2)

 MOV     R3, R12                    \ Set R3 = R12 to use as the address of the
                                    \ second vector to add

 BL      AddVectorToVertices+8      \ Add the vector (R0, R1, R2) to the vector
                                    \ at the address in R3 (by skipping the
                                    \ first two instructions in the modified
                                    \ routine

 LDR     R1, vmod2                  \ Revert the eighth instruction in the
 STR     R1, AddVectorToVertices+28 \ AddVectorToVertices routine to the
                                    \ instruction in vmod2 (though note that
                                    \ this doesn't actually revert the
                                    \ instruction, it corrupts it, for the
                                    \ reasons noted above)

 LDMFD   R13!, {PC}                 \ Return from the subroutine