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Elite on the BBC Micro

Version analysis of SHPPT

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

Code variations between these versions are shown below.

Name: SHPPT Type: Subroutine Category: Drawing ships

Code variation 1 of 13A variation in the comments only

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Cassette, Flight, 6502SP, Master, Electron

Docked

Summary: Draw a distant ship as a point rather than a full wireframe
Summary: Draw a distant ship as a point in the middle of the screen
.SHPPT

Code variation 2 of 13Related to the Master version

When drawing a distant ship as a dot, the cassette, disc and 6502SP versions erase the entire on-screen ship before redrawing it, while the Master version erases and redraws each ship one line at a time.

See below for more variations related to this code.

This variation is blank in the Master version.

Cassette, Flight, Docked, 6502SP, Electron

JSR EE51 \ Call EE51 to remove the ship's wireframe from the \ screen, if there is one

Code variation 3 of 13Specific to an individual platform

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Cassette, Flight, 6502SP, Master, Electron

Docked

JSR PROJ \ Project the ship onto the screen, returning: \ \ * K3(1 0) = the screen x-coordinate \ * K4(1 0) = the screen y-coordinate \ * A = K4+1 ORA K3+1 \ If either of the high bytes of the screen coordinates BNE nono \ are non-zero, jump to nono as the ship is off-screen LDA K4 \ Set A = the y-coordinate of the dot
LDA #Y \ Set A = the y-coordinate of a dot halfway down the \ screen

Code variation 4 of 13A variation in the comments only

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Cassette, Flight, 6502SP, Master, Electron

Docked

CMP #Y*2-2 \ If the y-coordinate is bigger than the y-coordinate of BCS nono \ the bottom of the screen, jump to nono as the ship's \ dot is off the bottom of the space view
CMP #Y*2-2 \ If the y-coordinate is bigger than the y-coordinate of BCS nono \ the bottom of the screen, jump to nono as the ship's \ dot is off the bottom of the space view. This will \ never happen, but this code is copied from the flight \ code, where A can contain any y-coordinate

Code variation 5 of 13Related to the Master version

See variation 2 above for details.

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Cassette, Flight, Docked, 6502SP, Electron

Master

LDY #2 \ Call Shpt with Y = 2 to set up bytes 1-4 in the ship JSR Shpt \ lines space, aborting the call to LL9 if the dot is \ off the side of the screen. This call sets up the \ first row of the dot (i.e. a four-pixel dash) LDY #6 \ Set Y to 6 for the next call to Shpt
JSR Shpt \ Call Shpt to draw a horizontal 4-pixel dash for the \ first row of the dot (i.e. a four-pixel dash)

Code variation 6 of 13Specific to an individual platform

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Cassette, Flight, 6502SP, Electron

Docked

Master

LDA K4 \ Set A = y-coordinate of dot + 1 (so this is the second ADC #1 \ row of the two-pixel-high dot)
LDA #Y \ Set A = #Y + 1 (so this is the second row of the ADC #1 \ two-pixel-high dot halfway down the screen)
LDA K4 \ Set A = y-coordinate of dot + 1 (so this is the second CLC \ row of the two-pixel-high dot) ADC #1

Code variation 7 of 13A variation in the comments only

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Cassette, Flight, Docked, 6502SP, Electron

Master

JSR Shpt \ Call Shpt with Y = 6 to set up bytes 5-8 in the ship \ lines space, aborting the call to LL9 if the dot is \ off the side of the screen. This call sets up the \ second row of the dot (i.e. another four-pixel dash, \ on the row below the first one)
JSR Shpt \ Call Shpt to draw a horizontal 4-pixel dash for the \ first row of the dot (i.e. a four-pixel dash)
 LDA #%00001000         \ Set bit 3 of the ship's byte #31 to record that we
 ORA XX1+31             \ have now drawn something on-screen for this ship
 STA XX1+31

Code variation 8 of 13Related to an advanced feature

Ships in the advanced versions each have their own colour, including when they are shown as points.

This variation is blank in the Master version.

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Cassette, Flight, Docked, Electron

6502SP

LDA #8 \ Set A = 8 so when we call LL18+2 next, byte #0 of the \ heap gets set to 8, for the 8 bytes we just stuck on \ the heap
LDA #9 \ Set A = 9 so when we call LL18+2 next, byte #0 of the \ heap gets set to 9, to cover the 9 bytes we just stuck \ on the heap

Code variation 9 of 13Related to the Master version

See variation 2 above for details.

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Cassette, Flight, Docked, 6502SP, Electron

Master

JMP LL81+2 \ Call LL81+2 to draw the ship's dot, returning from the \ subroutine using a tail call PLA \ Pull the return address from the stack, so the RTS PLA \ below actually returns from the subroutine that called \ LL9 (as we called SHPPT from LL9 with a JMP)
JMP LL155 \ Jump to LL155 to draw any remaining lines that are \ still in the ship line heap and return from the \ subroutine using a tail call
.nono

 LDA #%11110111         \ Clear bit 3 of the ship's byte #31 to record that
 AND XX1+31             \ nothing is being drawn on-screen for this ship
 STA XX1+31

Code variation 10 of 13Related to the Master version

See variation 2 above for details.

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Cassette, Flight, Docked, 6502SP, Electron

Master

RTS \ Return from the subroutine
JMP LL155 \ Jump to LL155 to draw any remaining lines that are \ still in the ship line heap and return from the \ subroutine using a tail call
.Shpt

Code variation 11 of 13Related to the Master version

See variation 2 above for details.

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Cassette, Flight, Docked, 6502SP, Electron

Master

\ This routine sets up four bytes in the ship line heap, \ from byte Y-1 to byte Y+2. If the ship's screen point \ turns out to be off-screen, then this routine aborts \ the entire call to LL9, exiting via nono. The four \ bytes define a horizontal 4-pixel dash, for either the \ top or the bottom of the ship's dot STA (XX19),Y \ Store A in byte Y of the ship line heap INY \ Store A in byte Y+2 of the ship line heap INY STA (XX19),Y
\ This routine draws a horizontal 4-pixel dash, for \ either the top or the bottom of the ship's dot STA Y1 \ Store A in both y-coordinates, as this is a horizontal STA Y2 \ dash at y-coordinate A

Code variation 12 of 13Specific to an individual platform

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Cassette, Flight, 6502SP, Master, Electron

Docked

LDA K3 \ Set A = screen x-coordinate of the ship dot
LDA #X \ Set A = x-coordinate of the middle of the screen

Code variation 13 of 13Related to the Master version

The Master implements flicker-free ship drawing using the LLX30 routine, which is used both for drawing wireframes and for drawing distant ships as dots.

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Cassette, Flight, Docked, 6502SP, Electron

Master

DEY \ Store A in byte Y+1 of the ship line heap STA (XX19),Y ADC #3 \ Set A = screen x-coordinate of the ship dot + 3 BCS nono-2 \ If the addition pushed the dot off the right side of \ the screen, jump to nono-2 to return from the parent \ subroutine early (i.e. LL9). This works because we \ called Shpt from above with a JSR, so nono-2 removes \ that return address from the stack, leaving the next \ return address exposed. LL9 called SHPPT with a JMP. \ so the next return address is the one that was put on \ the stack by the original call to LL9. So the RTS in \ nono will actually return us from the original call \ to LL9, thus aborting the entire drawing process DEY \ Store A in byte Y-1 of the ship line heap DEY STA (XX19),Y RTS \ Return from the subroutine
STA X1 \ Store the x-coordinate of the ship dot in X1, as this \ is where the dash starts CLC \ Set A = screen x-coordinate of the ship dot + 3 ADC #3 BCC P%+4 \ If the addition overflowed, set A = 255, the LDA #255 \ x-coordinate of the right edge of the screen STA X2 \ Store the x-coordinate of the ship dot in X1, as this \ is where the dash starts JMP LLX30 \ Draw this edge using flicker-free animation, by first \ drawing the ship's new line and then erasing the \ corresponding old line from the screen, and return \ from the subroutine using a tail call
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