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BBC Micro Elite

Drawing ships: LL9 (Part 11 of 12) (6502SP version)

Name: LL9 (Part 11 of 12) [View in context] Type: Subroutine Category: Drawing ships Summary: Draw ship: Add all visible edges to the ship line heap Deep dive: Drawing ships
This part adds all the visible edges to the ship line heap, so we can draw them in part 12.
.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 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 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 STA (XX19) \ Store A as the first byte of the ship line heap, so \ the heap is now correctly set up