Version analysis of PLANET - Elite on the BBC Micro and NES

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

Cassette, Disc (flight), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: PLANET
       Type: Subroutine
   Category: Drawing planets

Code variation 1 of 8A variation in the comments only

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Cassette, Flight, 6502SP, Master
Electron
    Summary: Draw the planet or sun
    Summary: Draw the planet



 Arguments:

Code variation 2 of 8A variation in the comments only

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Cassette, Flight, 6502SP, Master
Electron
   INWK                 The planet or sun's ship data block
   INWK                 The planet's ship data block



.PLANET

Code variation 3 of 8Related to the Electron version

In the Electron version, the PLANET routine only draws planets and will terminate if asked to draw a sun.

This variation is blank in the Cassette, Disc (flight), 6502 Second Processor and Master versions.

Electron
 LDA TYPE               \ If bit 0 of the ship type is set, then this is 129,
 LSR A                  \ which is the placeholder used to denote there is no
 BCS PL2-1              \ space station, so return from the subroutine (as PL2-1
                        \ contains an RTS)

Code variation 4 of 8Related to the screen mode

This variation is blank in the Cassette, Disc (flight) and Electron versions.

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Master
6502SP
 LDA #GREEN             \ Switch to stripe 3-1-3-1, which is cyan/yellow in the
 STA COL                \ space view
 LDA #GREEN             \ Send a #SETCOL GREEN command to the I/O processor to
 JSR DOCOL              \ switch to stripe 3-1-3-1, which is cyan/yellow in the
                        \ space view

Code variation 5 of 8A variation in the comments only

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Cassette, Flight, 6502SP, Master
Electron
 LDA INWK+8             \ Set A = z_sign (the highest byte in the planet/sun's
                        \ coordinates)
 LDA INWK+8             \ Set A = z_sign (the highest byte in the planet's
                        \ coordinates)

Code variation 6 of 8Other (e.g. bug fix, optimisation)

In the cassette version, in part 14 of the main flight loop, we didn't remove the planet/sun if it was behind us, so we do it here instead.

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Cassette, Flight
6502SP, Master
Electron
 BMI PL2                \ If A is negative then the planet/sun is behind us, so
                        \ jump to PL2 to remove it from the screen, returning
                        \ from the subroutine using a tail call
\BMI PL2                \ This instruction is commented out in the original
                        \ source. It would remove the planet from the screen
                        \ when it's behind us
 BMI PL2                \ If A is negative then the planet is behind us, so
                        \ jump to PL2 to remove it from the screen, returning
                        \ from the subroutine using a tail call

Code variation 7 of 8A variation in the comments only

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Cassette, Flight, 6502SP, Master
Electron
 CMP #48                \ If A >= 48 then the planet/sun is too far away to be
 BCS PL2                \ seen, so jump to PL2 to remove it from the screen,
                        \ returning from the subroutine using a tail call

 ORA INWK+7             \ Set A to 0 if both z_sign and z_hi are 0

 BEQ PL2                \ If both z_sign and z_hi are 0, then the planet/sun is
                        \ too close to be shown, so jump to PL2 to remove it
                        \ from the screen, returning from the subroutine using a
                        \ tail call

 JSR PROJ               \ Project the planet/sun onto the screen, returning the
                        \ centre's coordinates in K3(1 0) and K4(1 0)

 BCS PL2                \ If the C flag is set by PROJ then the planet/sun is
                        \ not visible on-screen, so jump to PL2 to remove it
                        \ from the screen, returning from the subroutine using
                        \ a tail call

 LDA #96                \ Set (A P+1 P) = (0 96 0) = 24576
 STA P+1                \
 LDA #0                 \ This represents the planet/sun's radius at a distance
 STA P                  \ of z = 1

 JSR DVID3B2            \ Call DVID3B2 to calculate:
                        \
                        \   K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo)
                        \              = (0 96 0) / z
                        \              = 24576 / z
                        \
                        \ so K now contains the planet/sun's radius, reduced by
                        \ the actual distance to the planet/sun. We know that
                        \ K+3 and K+2 will be 0, as the number we are dividing,
                        \ (0 96 0), fits into the two bottom bytes, so the
                        \ result is actually in K(1 0)
 CMP #48                \ If A >= 48 then the planet is too far away to be
 BCS PL2                \ seen, so jump to PL2 to remove it from the screen,
                        \ returning from the subroutine using a tail call

 ORA INWK+7             \ Set A to 0 if both z_sign and z_hi are 0

 BEQ PL2                \ If both z_sign and z_hi are 0, then the planet/sun is
                        \ too close to be shown, so jump to PL2 to remove it
                        \ from the screen, returning from the subroutine using a
                        \ tail call

 JSR PROJ               \ Project the planet onto the screen, returning the
                        \ centre's coordinates in K3(1 0) and K4(1 0)

 BCS PL2                \ If the C flag is set by PROJ then the planet is
                        \ not visible on-screen, so jump to PL2 to remove it
                        \ from the screen, returning from the subroutine using
                        \ a tail call

 LDA #96                \ Set (A P+1 P) = (0 96 0) = 24576
 STA P+1                \
 LDA #0                 \ This represents the planet's radius at a distance
 STA P                  \ of z = 1

 JSR DVID3B2            \ Call DVID3B2 to calculate:
                        \
                        \   K(3 2 1 0) = (A P+1 P) / (z_sign z_hi z_lo)
                        \              = (0 96 0) / z
                        \              = 24576 / z
                        \
                        \ so K now contains the planet's radius, reduced by
                        \ the actual distance to the planet. We know that
                        \ K+3 and K+2 will be 0, as the number we are dividing,
                        \ (0 96 0), fits into the two bottom bytes, so the
                        \ result is actually in K(1 0)

 LDA K+1                \ If the high byte of the reduced radius is zero, jump
 BEQ PL82               \ to PL82, as K contains the radius on its own

 LDA #248               \ Otherwise set K = 248, to round up the radius in
 STA K                  \ K(1 0) to the nearest integer (if we consider the low
                        \ byte to be the fractional part)

.PL82

Code variation 8 of 8Related to the Electron version

Planets in the Electron version are simple circles, so the code to draw meridians and craters is omitted, and the simple CIRCLE routine is used instead.

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Cassette, Flight, 6502SP, Master
Electron
 LDA TYPE               \ If the planet/sun's type has bit 0 clear, then it's
 LSR A                  \ either 128 or 130, which is a planet (the sun has type
 BCC PL9                \ 129, which has bit 0 set). So jump to PL9 to draw the
                        \ planet with radius K, returning from the subroutine
                        \ using a tail call

 JMP SUN                \ Otherwise jump to SUN to draw the sun with radius K,
                        \ returning from the subroutine using a tail call
 JSR WPLS2              \ Call WPLS2 to remove the planet from the screen

 JMP CIRCLE             \ Jump to CIRCLE to draw the planet (which is just a
                        \ simple circle) and return from the subroutine using
                        \ a tail call