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Elite on the BBC Micro

Version analysis of WARP

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

Code variations between these versions are shown below.

Name: WARP Type: Subroutine Category: Flight Summary: Perform an in-system jump
This is called when we press "J" during flight. The following checks are performed: * Make sure we don't have any ships or space stations in the vicinity * Make sure we are not in witchspace * If we are facing the planet, make sure we aren't too close

Code variation 1 of 9A variation in the comments only

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Cassette, Flight, 6502SP, Master

Electron

* If we are facing the sun, make sure we aren't too close If the above checks are passed, then we perform an in-system jump by moving the sun and planet in the opposite direction to travel, so we appear to jump in space. This means that any asteroids, cargo canisters or escape pods get dragged along for the ride.
If the above checks are passed, then we perform an in-system jump by moving the planet in the opposite direction to travel, so we appear to jump in space. This means that any asteroids, cargo canisters or escape pods get dragged along for the ride.
.WARP

Code variation 2 of 9Specific to an individual platform

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Cassette

Flight, 6502SP, Master

Electron

IF _SOURCE_DISC LDA MANY+AST \ Set X to the total number of asteroids, escape pods CLC \ and cargo canisters in the vicinity ADC MANY+ESC CLC \ The second CLC instruction has no effect, as there is ADC MANY+OIL \ is no way that adding the number of asteroids and the TAX \ number escape pods will cause a carry ELIF _TEXT_SOURCES LDA MANY+AST \ Set X to the total number of asteroids, escape pods CLC \ and cargo canisters in the vicinity ADC MANY+ESC \ ADC MANY+OIL \ This code saves one byte of memory over the code in TAX \ the source disc version. The second CLC is not needed \ as there is no way that adding the number of asteroids \ and the number of escape pods will cause a carry ENDIF
LDX JUNK \ Set X to the total number of junk items in the \ vicinity (e.g. asteroids, escape pods, cargo \ canisters, Shuttles, Transporters and so pn)
LDA MANY+AST \ Set X to the total number of asteroids, escape pods CLC \ and cargo canisters in the vicinity ADC MANY+ESC CLC \ The second CLC instruction has no effect, as there is ADC MANY+OIL \ is no way that adding the number of asteroids and the TAX \ number escape pods will cause a carry
 LDA FRIN+2,X           \ If the slot at FRIN+2+X is non-zero, then we have
                        \ something else in the vicinity besides asteroids,
                        \ escape pods and cargo canisters, so to check whether
                        \ we can jump, we first grab the slot contents into A

 ORA SSPR               \ If there is a space station nearby, then SSPR will
                        \ be non-zero, so OR'ing with SSPR will produce a
                        \ a non-zero result if either A or SSPR are non-zero

Code variation 3 of 9Related to the Electron version

The Electron version doesn't have witchspace, so there's no need to disable in-system jumping there.

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Cassette, Flight, 6502SP, Master

Electron

ORA MJ \ If we are in witchspace, then MJ will be non-zero, so \ OR'ing with MJ will produce a non-zero result if \ either A or SSPR or MJ are non-zero BNE WA1 \ A is non-zero if we have either a ship or a space \ station in the vicinity, or we are in witchspace, in \ which case jump to WA1 to make a low beep to show that \ we can't do an in-system jump
BNE WA1 \ A is non-zero if we have either a ship or a space \ station in the vicinity, in which case jump to WA1 to \ make a low beep to show that we can't do an in-system \ jump
 LDY K%+8               \ Otherwise we can do an in-system jump, so now we fetch
                        \ the byte at K%+8, which contains the z_sign for the
                        \ first ship slot, i.e. the distance of the planet

 BMI WA3                \ If the planet's z_sign is negative, then the planet
                        \ is behind us, so jump to WA3 to skip the following

 TAY                    \ Set A = Y = 0 (as we didn't BNE above) so the call
                        \ to MAS2 measures the distance to the planet

 JSR MAS2               \ Call MAS2 to set A to the largest distance to the
                        \ planet in any of the three axes (we could also call
                        \ routine m to do the same thing, as A = 0)

Code variation 4 of 9Minor and very low-impact

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Cassette

Flight

6502SP, Master, Electron

IF _SOURCE_DISC CMP #2 \ If A < 2 then jump to WA1 to abort the in-system jump BCC WA1 \ with a low beep, as we are facing the planet and are \ too close to jump in that direction ELIF _TEXT_SOURCES LSR A \ If A < 2 then jump to WA1 to abort the in-system jump BEQ WA1 \ with a low beep, as we are facing the planet and are \ too close to jump in that direction \ \ These instructions between them save one byte of \ memory over the CMP-based code in the source disc \ version, as LSR A is a one-byte opcode, while CMP #2 \ takes up two bytes (though the code does exactly the \ same thing) ENDIF
LSR A \ If A < 2 then jump to WA1 to abort the in-system jump BEQ WA1 \ with a low beep, as we are facing the planet and are \ too close to jump in that direction
CMP #2 \ If A < 2 then jump to WA1 to abort the in-system jump BCC WA1 \ with a low beep, as we are facing the planet and are \ too close to jump in that direction
.WA3

Code variation 5 of 9A variation in the comments only

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Cassette, Flight, 6502SP, Master

Electron

LDY K%+NI%+8 \ Fetch the z_sign (byte #8) of the second ship in the \ ship data workspace at K%, which is reserved for the \ sun or the space station (in this case it's the \ former, as we already confirmed there isn't a space \ station in the vicinity) BMI WA2 \ If the sun's z_sign is negative, then the sun is \ behind us, so jump to WA2 to skip the following LDY #NI% \ Set Y to point to the offset of the ship data block \ for the sun, which is NI% (as each block is NI% bytes \ long, and the sun is the second block) JSR m \ Call m to set A to the largest distance to the sun \ in any of the three axes
LDY K%+NI%+8 \ Fetch the z_sign (byte #8) of the second ship in the \ ship data workspace at K%, which is reserved for the \ space station BMI WA2 \ If the station's z_sign is negative, then it is \ behind us, so jump to WA2 to skip the following LDY #NI% \ Set Y to point to the offset of the ship data block \ for the station, which is NI% (as each block is NI% \ bytes long, and the station is the second block) JSR m \ Call m to set A to the largest distance to the station \ in any of the three axes

Code variation 6 of 9Minor and very low-impact

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Cassette

Flight

6502SP, Master

Electron

IF _SOURCE_DISC CMP #2 \ If A < 2 then jump to WA1 to abort the in-system jump BCC WA1 \ with a low beep, as we are facing the planet and are \ too close to jump in that direction ELIF _TEXT_SOURCES LSR A \ If A < 2 then jump to WA1 to abort the in-system jump BEQ WA1 \ with a low beep, as we are facing the planet and are \ too close to jump in that direction \ \ These instructions between them save one byte of \ memory over the CMP-based code in the source disc \ version, as LSR A is a one-byte opcode, while CMP #2 \ takes up two bytes (though the code does exactly the \ same thing) ENDIF
LSR A \ If A < 2 then jump to WA1 to abort the in-system jump BEQ WA1 \ with a low beep, as we are facing the sun and are too \ close to jump in that direction
CMP #2 \ If A < 2 then jump to WA1 to abort the in-system jump BCC WA1 \ with a low beep, as we are facing the sun and are too \ close to jump in that direction
CMP #2 \ If A < 2 then jump to WA1 to abort the in-system jump BCC WA1 \ with a low beep, as we are facing the station and are \ too close to jump in that direction
.WA2

Code variation 7 of 9A variation in the comments only

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Cassette, Flight, 6502SP, Master

Electron

\ If we get here, then we can do an in-system jump, as \ we don't have any ships or space stations in the \ vicinity, we are not in witchspace, and if we are \ facing the planet or the sun, we aren't too close to \ jump towards it \ \ We do an in-system jump by moving the sun and planet, \ rather than moving our own local bubble (this is why \ in-system jumps drag asteroids, cargo canisters and \ escape pods along for the ride). Specifically, we move \ them in the z-axis by a fixed amount in the opposite \ direction to travel, thus performing a jump towards \ our destination
\ If we get here, then we can do an in-system jump, as \ we don't have any ships or space stations in the \ vicinity, we are not in witchspace, and if we are \ facing the planet, we aren't too close to jump \ towards it \ \ We do an in-system jump by moving the planet, rather \ than moving our own local bubble (this is why \ in-system jumps drag asteroids, cargo canisters and \ escape pods along for the ride). Specifically, we move \ them in the z-axis by a fixed amount in the opposite \ direction to travel, thus performing a jump towards \ our destination
 LDA #&81               \ Set R = R = P = &81
 STA S
 STA R
 STA P

 LDA K%+8               \ Set A = z_sign for the planet

 JSR ADD                \ Set (A X) = (A P) + (S R)
                        \           = (z_sign &81) + &8181
                        \           = (z_sign &81) - &0181
                        \
                        \ This moves the planet against the direction of travel
                        \ by reducing z_sign by 1, as the above maths is:
                        \
                        \         z_sign 00000000
                        \   +   00000000 10000001
                        \   -   00000001 10000001
                        \
                        \ or:
                        \
                        \         z_sign 00000000
                        \   +   00000000 00000000
                        \   -   00000001 00000000
                        \
                        \ i.e. the high byte is z_sign - 1, making sure the sign
                        \ is preserved

 STA K%+8               \ Set the planet's z_sign to the high byte of the result

Code variation 8 of 9A variation in the comments only

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Cassette, Flight, 6502SP, Master

Electron

LDA K%+NI%+8 \ Set A = z_sign for the sun JSR ADD \ Set (A X) = (A P) + (S R) \ = (z_sign &81) + &8181 \ = (z_sign &81) - &0181 \ \ which moves the sun against the direction of travel \ by reducing z_sign by 1
LDA K%+NI%+8 \ Set A = z_sign for the station JSR ADD \ Set (A X) = (A P) + (S R) \ = (z_sign &81) + &8181 \ = (z_sign &81) - &0181 \ \ which moves the station against the direction of \ travel by reducing z_sign by 1
 STA K%+NI%+8           \ Set the planet's z_sign to the high byte of the result

 LDA #1                 \ These instructions have no effect, as the call to
 STA QQ11               \ LOOK1 below starts by setting QQ11 to 0; instead they
                        \ just set the current view type in QQ11 to 1 for the
                        \ duration of the next three instructions

 STA MCNT               \ Set the main loop counter to 1, so the next iteration
                        \ through the main loop will potentially spawn ships
                        \ (see part 2 of the main game loop at me3)

 LSR A                  \ Set EV, the extra vessels spawning counter, to 0
 STA EV                 \ (the LSR produces a 0 as A was previously 1)

 LDX VIEW               \ Set X to the current view (front, rear, left or right)
 JMP LOOK1              \ and jump to LOOK1 to initialise that view, returning
                        \ from the subroutine using a tail call

.WA1

Code variation 9 of 9Minor and very low-impact

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Cassette, Electron

Flight, 6502SP

Master

LDA #40 \ If we get here then we can't do an in-system jump, so JMP NOISE \ call the NOISE routine with A = 40 to make a long, low \ beep and return from the subroutine using a tail call
LDA #40 \ If we get here then we can't do an in-system jump, so BNE NOISE \ call the NOISE routine with A = 40 to make a long, low \ beep and return from the subroutine using a tail call \ (the BNE is effectively a JMP as A is never zero)
JMP LOWBEEP \ Call the LOWBEEP routine to make a long, low beep, and \ return from the subroutine using a tail call RTS \ This instruction has no effect as we already returned \ from the subroutine