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⌂ ∞ ▽ Elite on the 6502

Drawing ships: LL9 (Part 11 of 12)

[NES version, Bank 1]

       Name: LL9 (Part 11 of 12)                                     [Show more]
       Type: Subroutine
   Category: Drawing ships
    Summary: Draw ship: Loop back for the next edge
  Deep dive: Drawing ships

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




.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

 LDA U                  ; This instruction is left over from the other versions
                        ; of Elite and has no effect
                        ;
                        ; It would fetch the ship line heap pointer from U, but
                        ; the NES version does not have a ship line heap as the
                        ; screen is redrawn for every frame