Screen buffer: CheckDashData

       Name: CheckDashData                                           [Show more]
       Type: Subroutine
   Category: Screen buffer
    Summary: Check whether a dash data block index is pointing to dash data
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawGrassLeft calls CheckDashData
             * DrawGrassRight calls CheckDashData
             * DrawVergeByteLeft calls CheckDashData
             * DrawVergeByteRight calls CheckDashData


 This routine checks whether an index in Y, which is relative to the start of a
 dash data block, is pointing to dash data within the block.


 Arguments:

   UU                   Dash data block number (0 to 39)

   Y                    The index from the start of the dash data block


 Returns:

   C flag               The result, as follows:

                          * Clear if offset Y does not point to dash data

                          * Set if offset Y does point to dash data

   A                    A is unchanged

   X                    X is unchanged


.CheckDashData

 STA T                  \ Store A and X in T and U so we can retrieve them below
 STX U

 LDX UU                 \ Set X to the dash data block number

 TYA                    \ Set the C flag as follows:
 CMP dashDataOffset,X   \
                        \   * C flag clear if Y < dashDataOffset,X
                        \
                        \   * C flag set if Y >= dashDataOffset,X

 BNE cdas1              \ If Y <> the dashDataOffset of block X, skip the
                        \ following instruction

 CLC                    \ If we get here then Y = the dashDataOffset of block X,
                        \ so clear the C flag

                        \ So by this point we have:
                        \
                        \   * C flag clear if Y <= dashDataOffset,X
                        \
                        \   * C flag set if Y > dashDataOffset,X
                        \
                        \ As the dash data lives at the top of each dash data
                        \ block, this gives us the result we want (as Y is
                        \ pointing to data when Y > dashDataOffset,X)

.cdas1

 LDA T                  \ Restore the values of A and X that we stored above
 LDX U

 RTS                    \ Return from the subroutine