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Elite on the BBC Micro

Version analysis of LL9 (Part 11 of 12)

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

Name: LL9 (Part 11 of 12) Type: Subroutine Category: Drawing ships

Code variation 1 of 8A variation in the comments only

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Docked, 6502SP, Electron

Master

Summary: Draw ship: Add all visible edges to the ship line heap
Summary: Draw ship: Loop back for the next edge
Deep dive: Drawing ships

Code variation 2 of 8A variation in the comments only

This variation is blank in the Master version.

Cassette, Flight, Docked, 6502SP, Electron

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

Code variation 3 of 8Related to the Master version

The cassette, disc and 6502SP versions add all the lines in a ship to the heap and then draw them all in one go, whereas the Master version erases and draws lines as they are added to the ship line heap.

See below for more variations related to this code.

This variation is blank in the Master version.

Cassette, Flight, Docked, 6502SP, Electron

.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

Code variation 4 of 8Related to the Master version

See variation 3 above for details.

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Cassette, Flight, Docked, 6502SP, Electron

Master

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 XX14 \ If XX14 >= CNT, skip to LL81 so we don't loop back for CMP CNT \ the next edge (CNT was set to the maximum heap size BCS LL81 \ for this ship in part 10, so this checks whether we \ have just run out of space in the ship line heap, and \ stops drawing edges if we have)

Code variation 5 of 8Minor and very low-impact

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Cassette, Flight, Docked, 6502SP, Electron

Master

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

Code variation 6 of 8Related to the Master version

See variation 3 above for details.

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Cassette, Flight, Docked, 6502SP, Electron

Master

JMP LL75 \ Loop back to LL75 to process the next edge
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, loop back to LL75 to process the next BCC LL75 \ edge
.LL81

Code variation 7 of 8Related to the Master version

See variation 3 above for details.

Tap on a version to expand it, and tap it again to show to all variations.

Cassette, Flight, Docked, 6502SP, Electron

Master

\ 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
JMP LL155 \ Jump down to part 12 below to draw any remaining lines \ from the old ship that are still in the ship line heap

Code variation 8 of 8Minor and very low-impact

This variation is blank in the Master version.

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Cassette, Flight, Docked, Electron

6502SP

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