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Elite on the BBC Micro

Drawing ships: LL9 (Part 12 of 12) [Disc version, Docked]

Name: LL9 (Part 12 of 12) [Show more] Type: Subroutine Category: Drawing ships Summary: Draw ship: Draw all the visible edges from 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: No direct references to this subroutine in this source file

This part draws the lines in the ship line heap, which is used both to draw the ship, and to remove it from the screen.
.LL155 LDY #0 \ Fetch the first byte from the ship line heap into A, LDA (XX19),Y \ which contains the number of bytes in the heap STA XX20 \ Store the heap size in XX20 CMP #4 \ If the heap size is less than 4, there is nothing to BCC LL118-1 \ draw, so return from the subroutine (as LL118-1 \ contains an RTS) INY \ Set Y = 1, which we will use as an index into the ship \ line heap, starting at byte #1 (as byte #0 contains \ the heap size) .LL27 LDA (XX19),Y \ Fetch the X1 line coordinate from the heap and store STA XX15 \ it in XX15 INY \ Increment the heap pointer LDA (XX19),Y \ Fetch the Y1 line coordinate from the heap and store STA XX15+1 \ it in XX15+1 INY \ Increment the heap pointer LDA (XX19),Y \ Fetch the X2 line coordinate from the heap and store STA XX15+2 \ it in XX15+2 INY \ Increment the heap pointer LDA (XX19),Y \ Fetch the Y2 line coordinate from the heap and store STA XX15+3 \ it in XX15+3 JSR LL30 \ Draw a line from (X1, Y1) to (X2, Y2) INY \ Increment the heap pointer CPY XX20 \ If the heap counter is less than the size of the heap, BCC LL27 \ loop back to LL27 to draw the next line from the heap RTS \ Return from the subroutine