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Elite on the BBC Micro

Version analysis of TACTICS (Part 7 of 7)

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

Code variations between these versions are shown below.

Name: TACTICS (Part 7 of 7) Type: Subroutine Category: Tactics Summary: Apply tactics: Set pitch, roll, and acceleration Deep dive: Program flow of the tactics routine
This section looks at manoeuvring the ship. Specifically:

Code variation 1 of 12A variation in the comments only

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Cassette, Electron

Flight, 6502SP, Master

* Work out which direction the ship should be moving, depending on whether it's an escape pod, where it is, which direction it is pointing, and how aggressive it is
* Work out which direction the ship should be moving, depending on the type of ship, where it is, which direction it is pointing, and how aggressive it is
* Set the pitch and roll counters to head in that direction * Speed up or slow down, depending on where the ship is in relation to us

Code variation 2 of 12A variation in the comments only

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master

Other entry points: TA151 Make the ship head towards the planet
.TA4 LDA INWK+7 \ If z_hi >= 3 then the ship is quite far away, so jump CMP #3 \ down to TA5 BCS TA5 LDA INWK+1 \ Otherwise set A = x_hi OR y_hi and extract bits 1-7 ORA INWK+4 AND #%11111110 BEQ TA15 \ If A = 0 then the ship is pretty close to us, so jump \ to TA15 so it heads away from us .TA5 \ If we get here then the ship is quite far away JSR DORND \ Set A and X to random numbers ORA #%10000000 \ Set bit 7 of A CMP INWK+32 \ If A >= byte #32 (the ship's AI flag) then jump down BCS TA15 \ to TA15 so it heads away from us \ We get here if A < byte #32, and the chances of this \ being true are greater with high values of byte #32. \ In other words, higher byte #32 values increase the \ chances of a ship changing direction to head towards \ us - or, to put it another way, ships with higher \ byte #32 values are spoiling for a fight. Thargoids \ have byte #32 set to 255, which explains an awful lot .TA20 \ If this is a missile we will have jumped straight \ here, but we also get here if the ship is either far \ away and aggressive, or not too close

Code variation 3 of 12Minor and very low-impact

This code is in the TAS6 routine in the enhanced versions.

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Cassette, Electron

Flight, 6502SP, Master

LDA XX15 \ Reverse the signs of XX15 and the dot product in CNT, EOR #%10000000 \ starting with the x-coordinate STA XX15 LDA XX15+1 \ Then reverse the sign of the y-coordinate EOR #%10000000 STA XX15+1 LDA XX15+2 \ And then the z-coordinate, so now the XX15 vector goes EOR #%10000000 \ from the enemy ship to our ship (it was previously the STA XX15+2 \ other way round) LDA CNT \ And finally change the sign of the dot product in CNT, EOR #%10000000 \ so now it's positive if the ships are facing each STA CNT \ other, and negative if they are facing the same way
JSR TAS6 \ Call TAS6 to negate the vector in XX15 so it points in \ the opposite direction LDA CNT \ Change the sign of the dot product in CNT, so now it's EOR #%10000000 \ positive if the ships are facing each other, and \ negative if they are facing the same way .TA152 STA CNT \ Update CNT with the new value in A
.TA15

                        \ If we get here, then one of the following is true:
                        \

Code variation 4 of 12A variation in the comments only

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Cassette, Electron

Flight, 6502SP, Master

\ * This is an escape pod and XX15 is pointing towards \ the planet
\ * This is a trader and XX15 is pointing towards the \ planet
                        \
                        \   * The ship is pretty close to us, or it's just not
                        \     very aggressive (though there is a random factor
                        \     at play here too). XX15 is still pointing from our
                        \     ship towards the enemy ship
                        \
                        \   * The ship is aggressive (though again, there's an
                        \     element of randomness here). XX15 is pointing from
                        \     the enemy ship towards our ship
                        \
                        \   * This is a missile heading for a target. XX15 is
                        \     pointing from the missile towards the target
                        \
                        \ We now want to move the ship in the direction of XX15,
                        \ which will make aggressive ships head towards us, and

Code variation 5 of 12A variation in the comments only

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Cassette, Electron

Flight, 6502SP, Master

\ ships that are too close turn away. Escape pods,
\ ships that are too close turn away. Peaceful traders,
                        \ meanwhile, head off towards the planet in search of a
                        \ space station, and missiles home in on their targets

 LDY #16                \ Set (A X) = roofv . XX15
 JSR TAS3               \
                        \ This will be positive if XX15 is pointing in the same
                        \ direction as an arrow out of the top of the ship, in
                        \ other words if the ship should pull up to head in the
                        \ direction of XX15

Code variation 6 of 12Related to an enhanced feature

The cassette version has fairly basic pitch and roll control over ships when applying AI tactics, whereas the enhanced versions have finer control using the RAT, RAT2 and CNT2 variables, which is particularly useful when the docking computer is in control.

See below for more variations related to this code.

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Cassette, Electron

Flight

6502SP, Master

EOR #%10000000 \ Set the ship's pitch counter to 3, with the opposite AND #%10000000 \ sign to the dot product result, which gently pitches ORA #%00000011 \ the ship towards the direction of the XX15 vector STA INWK+30
TAX \ Copy A into X so we can retrieve it below JSR nroll \ Call nroll to calculate the value of the ship's pitch \ counter STA INWK+30 \ Store the result in the ship's pitch counter
TAX \ Copy A into X so we can retrieve it below EOR #%10000000 \ Give the ship's pitch counter the opposite sign to the AND #%10000000 \ dot product result, with a value of 0 STA INWK+30 TXA \ Retrieve the original value of A from X ASL A \ Shift A left to double it and drop the sign bit CMP RAT2 \ If A < RAT2, skip to TA11 (so if RAT2 = 0, we always BCC TA11 \ set the pitch counter to RAT) LDA RAT \ Set the magnitude of the ship's pitch counter to RAT ORA INWK+30 \ (we already set the sign above) STA INWK+30 .TA11

Code variation 7 of 12Related to an enhanced feature

See variation 6 above for details.

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Cassette, Electron

Flight, 6502SP, Master

LDA INWK+29 \ Fetch the roll counter from byte #29 into A and clear AND #%01111111 \ the sign bit CMP #16 \ If A >= 16 then jump to TA6, as the ship is already BCS TA6 \ in the process of rolling
LDA INWK+29 \ Fetch the roll counter from byte #29 into A ASL A \ Shift A left to double it and drop the sign bit CMP #32 \ If A >= 32 then jump to TA6, as the ship is already BCS TA6 \ in the process of rolling
 LDY #22                \ Set (A X) = sidev . XX15
 JSR TAS3               \
                        \ This will be positive if XX15 is pointing in the same
                        \ direction as an arrow out of the right side of the
                        \ ship, in other words if the ship should roll right to
                        \ head in the direction of XX15

Code variation 8 of 12Related to an enhanced feature

See variation 6 above for details.

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Cassette, Electron

Flight

6502SP, Master

EOR INWK+30 \ Set the ship's roll counter to 5, with the sign set to AND #%10000000 \ positive if the pitch counter and dot product have EOR #%10000101 \ different signs, negative if they have the same sign STA INWK+29
TAX \ Copy A into X so we can retrieve it below EOR INWK+30 \ Give A the correct sign of the dot product * the \ current pitch direction (i.e. the sign is negative if \ the pitch counter and dot product have different \ signs, positive if they have the same sign) JSR nroll \ Call nroll to calculate the value of the ship's pitch \ counter STA INWK+29 \ Store the result in the ship's roll counter .TA12
TAX \ Copy A into X so we can retrieve it below EOR INWK+30 \ Give the ship's roll counter a positive sign if the AND #%10000000 \ pitch counter and dot product have different signs, EOR #%10000000 \ negative if they have the same sign, with a value of 0 STA INWK+29 TXA \ Retrieve the original value of A from X ASL A \ Shift A left to double it and drop the sign bit CMP RAT2 \ If A < RAT2, skip to TA12 (so if RAT2 = 0, we always BCC TA12 \ set the roll counter to RAT) LDA RAT \ Set the magnitude of the ship's roll counter to RAT ORA INWK+29 \ (we already set the sign above) STA INWK+29 .TA12
.TA6

 LDA CNT                \ Fetch the dot product, and if it's negative jump to
 BMI TA9                \ TA9, as the ships are facing away from each other and
                        \ the ship might want to slow down to take another shot

Code variation 9 of 12Minor and very low-impact

CNT2 is set to 22 in the enhanced versions.

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Cassette, Electron

Flight, 6502SP, Master

CMP #22 \ The dot product is positive, so the ships are facing BCC TA9 \ each other. If A < 22 then the ships are not heading \ directly towards each other, so jump to TA9 to slow \ down
CMP CNT2 \ The dot product is positive, so the ships are facing BCC TA9 \ each other. If A < CNT2 then the ships are not heading \ directly towards each other, so jump to TA9 to slow \ down .PH10E
 LDA #3                 \ Otherwise set the acceleration in byte #28 to 3
 STA INWK+28

 RTS                    \ Return from the subroutine

.TA9

 AND #%01111111         \ Clear the sign bit of the dot product in A

 CMP #18                \ If A < 18 then the ship is way off the XX15 vector, so
 BCC TA10               \ return from the subroutine (TA10 contains an RTS)
                        \ without slowing down, as it still has quite a bit of
                        \ turning to do to get on course

 LDA #&FF               \ Otherwise set A = -1

 LDX TYPE               \ If this is not a missile then skip the ASL instruction
 CPX #MSL
 BNE P%+3

 ASL A                  \ This is a missile, so set A = -2, as missiles are more
                        \ nimble and can brake more quickly

 STA INWK+28            \ Set the ship's acceleration to A

.TA10

 RTS                    \ Return from the subroutine

Code variation 10 of 12Related to an enhanced feature

The tactics routines to point the ship at the planet are shared with the docking computer.

This variation is blank in the Cassette and Electron versions.

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Flight

6502SP, Master

.TA151 \ This is called from part 3 with the vector to the \ planet in XX15, when we want the ship to turn towards \ the planet. It does the same dot product calculation \ as part 3, but it can also change the value of RAT2 \ so that roll and pitch is always applied JSR TAS3-2 \ Set (A X) = nosev . XX15 \ \ The bigger the value of the dot product, the more \ aligned the two vectors are, with a maximum magnitude \ in A of 36 (96 * 96 >> 8). If A is positive, the \ vectors are facing in a similar direction, if it's \ negative they are facing in opposite directions
.TA151 \ This is called from part 3 with the vector to the \ planet in XX15, when we want the ship to turn towards \ the planet. It does the same dot product calculation \ as part 3, but it can also change the value of RAT2 \ so that roll and pitch is always applied LDY #10 \ Set (A X) = nosev . XX15 JSR TAS3 \ \ The bigger the value of the dot product, the more \ aligned the two vectors are, with a maximum magnitude \ in A of 36 (96 * 96 >> 8). If A is positive, the \ vectors are facing in a similar direction, if it's \ negative they are facing in opposite directions

Code variation 11 of 12Related to an enhanced feature

See variation 6 above for details.

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master

CMP #&98 \ If A is positive or A <= -24, jump to ttt BCC ttt LDX #0 \ A > -24, which means the vectors are facing in STX RAT2 \ opposite directions but are quite aligned, so set \ RAT2 = 0 instead of the default value of 4, so we \ always apply roll and pitch when we turn the ship \ towards the planet .ttt JMP TA152 \ Jump to TA152 to store A in CNT and move the ship in \ the direction of XX15

Code variation 12 of 12Related to an enhanced feature

The tactics routines to pitch and roll the ship are shared with the docking computer.

This variation is blank in the Cassette, 6502 Second Processor, Master and Electron versions.

Flight

.nroll EOR #%10000000 \ Give the ship's pitch counter the opposite sign to the AND #%10000000 \ dot product result, with a value of 0, and store it in STA T \ T TXA \ Retrieve the original value of A from X ASL A \ Shift A left to double it and drop the sign bit CMP RAT2 \ If A < RAT2, skip to nroll2 (so if RAT2 = 0, we always BCC nroll2 \ set the pitch counter to RAT) LDA RAT \ Set the magnitude of the ship's pitch counter to RAT ORA T \ (we already set the sign above and stored it in T) RTS \ Return from the subroutine .nroll2 LDA T \ Set A to the value we stored in T above, which has a \ value of 0 and the opposite sign to the dot product \ result RTS \ Return from the subroutine
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