Drawing ships: LL9 (Part 11 of 12)

       Name: LL9 (Part 11 of 12)                                     [Show more]
       Type: Subroutine
   Category: Drawing ships
    Summary: Draw ship: Add all visible edges to the ship line heap
  Deep dive: Drawing ships
    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * SHPPT calls via LL81+2


 This part adds all the visible edges to the ship line heap, so we can draw
 them in part 12.

 Other entry points:

   LL81+2               Draw the contents of the ship line heap, used to draw
                        the ship as a dot from SHPPT


.LL80

 LDY U                  \ Fetch the ship line heap pointer, which points to the
                        \ next free byte on the heap, into Y

 LDA XX15               \ Add X1 to the end of the heap
 STA (XX19),Y

 INY                    \ Increment the heap pointer

 LDA XX15+1             \ Add Y1 to the end of the heap
 STA (XX19),Y

 INY                    \ Increment the heap pointer

 LDA XX15+2             \ Add X2 to the end of the heap
 STA (XX19),Y

 INY                    \ Increment the heap pointer

 LDA XX15+3             \ Add Y2 to the end of the heap
 STA (XX19),Y

 INY                    \ Increment the heap pointer

 STY U                  \ Store the updated ship line heap pointer in U

 CPY T1                 \ If Y >= T1 then we have reached the maximum number of
 BCS LL81               \ edge lines that we can store in the ship line heap, so
                        \ skip to LL81 so we don't loop back for the next edge

.LL78

 INC XX17               \ Increment the edge counter to point to the next edge

 LDY XX17               \ If Y >= XX20, which contains the number of edges in
 CPY XX20               \ the blueprint, jump to LL81 as we have processed all
 BCS LL81               \ the edges and don't need to loop back for the next one

 LDY #0                 \ Set Y to point to byte #0 again, ready for the next
                        \ edge

 LDA V                  \ Increment V by 4 so V(1 0) points to the data for the
 ADC #4                 \ next edge
 STA V

 BCC ll81               \ If the above addition didn't overflow, jump to ll81 to
                        \ skip the following instruction

 INC V+1                \ Otherwise increment the high byte of V(1 0), as we
                        \ just moved the V(1 0) pointer past a page boundary

.ll81

 JMP LL75               \ Loop back to LL75 to process the next edge

.LL81

                        \ We have finished adding lines to the ship line heap,
                        \ so now we need to set the first byte of the heap to
                        \ the number of bytes stored there

 LDA U                  \ Fetch the ship line heap pointer from U into A, which
                        \ points to the end of the heap, and therefore contains
                        \ the heap size

 LDY #0                 \ Store A as the first byte of the ship line heap, so
 STA (XX19),Y           \ the heap is now correctly set up