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Elite on the BBC Micro

Version analysis of TACTICS (Part 2 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 2 of 7) Type: Subroutine Category: Tactics

Code variation 1 of 16A variation in the comments only

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Cassette, Flight, 6502SP, Master

Electron

Summary: Apply tactics: Escape pod, station, lone Thargon, safe-zone pirate
Summary: Apply tactics: Escape pod, station, safe-zone pirate
Deep dive: Program flow of the tactics routine
This section contains the main entry point at TACTICS, which is called from part 2 of MVEIT for ships that have the AI flag set (i.e. bit 7 of byte #32). This part does the following: * If this is a missile, jump up to the missile code in part 1

Code variation 2 of 16A variation in the comments only

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Cassette

Flight, 6502SP, Master

Electron

* If this is an escape pod, point it at the planet and jump to the manoeuvring code in part 7 * If this is the space station and it is hostile, consider spawning a cop (45% chance, up to a maximum of four) and we're done * If this is a lone Thargon without a mothership, set it adrift aimlessly and we're done * If this is a pirate and we are within the space station safe zone, stop the pirate from attacking by removing all its aggression
* If this is the space station and it is hostile, consider spawning a cop (6.2% chance, up to a maximum of seven) and we're done * If this is the space station and it is not hostile, consider spawning (0.8% chance if there are no Transporters around) a Transporter or Shuttle (equal odds of each type) and we're done
* If this is an escape pod, point it at the planet and jump to the manoeuvring code in part 7 * If this is the space station and it is hostile, consider spawning a cop (45% chance, up to a maximum of four) and we're done * If this is a pirate and we are within the space station safe zone, stop the pirate from attacking by removing all its aggression

Code variation 3 of 16A variation in the comments only

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

6502SP, Master

* If this is a rock hermit, consider spawning (22% chance) a highly aggressive and hostile Sidewinder, Mamba, Krait, Adder or Gecko (equal odds of each type) and we're done
* Recharge the ship's energy banks by 1 Arguments: X The ship type
.TACTICS

Code variation 4 of 16Related to an enhanced feature

The docking computer in the enhanced versions uses its own turning circle configuration, which is different to the turning circle used by the tactics routine, so the latter switches to its own configuration when it starts (as they share configuration variables).

See below for more variations related to this code.

This variation is blank in the Cassette and Electron versions.

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Flight

6502SP, Master

LDY #3 \ Set RAT = 3, which is the magnitude we set the pitch STY RAT \ or roll counter to in part 7 when turning a ship \ towards a vector (a higher value giving a longer \ turn). This value is not changed in the TACTICS \ routine, but it is set to different values by the \ DOCKIT routine INY \ Set RAT2 = 4, which is the threshold below which we STY RAT2 \ don't apply pitch and roll to the ship (so a lower \ value means we apply pitch and roll more often, and a \ value of 0 means we always apply them). The value is \ compared with double the high byte of sidev . XX15, \ where XX15 is the vector from the ship to the enemy \ or planet. This value is set to different values by \ both the TACTICS and DOCKIT routines
LDA #3 \ Set RAT = 3, which is the magnitude we set the pitch STA RAT \ or roll counter to in part 7 when turning a ship \ towards a vector (a higher value giving a longer \ turn). This value is not changed in the TACTICS \ routine, but it is set to different values by the \ DOCKIT routine LDA #4 \ Set RAT2 = 4, which is the threshold below which we STA RAT2 \ don't apply pitch and roll to the ship (so a lower \ value means we apply pitch and roll more often, and a \ value of 0 means we always apply them). The value is \ compared with double the high byte of sidev . XX15, \ where XX15 is the vector from the ship to the enemy \ or planet. This value is set to different values by \ both the TACTICS and DOCKIT routines

Code variation 5 of 16Related to an enhanced feature

See variation 4 above for details.

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master

LDA #22 \ Set CNT2 = 22, which is the maximum angle beyond which STA CNT2 \ a ship will slow down to start turning towards its \ prey (a lower value means a ship will start to slow \ down even if its angle with the enemy ship is large, \ which gives a tighter turn). This value is not changed \ in the TACTICS routine, but it is set to different \ values by the DOCKIT routine
 CPX #MSL               \ If this is a missile, jump up to TA18 to implement
 BEQ TA18               \ missile tactics

Code variation 6 of 16Related to an enhanced feature

The enhanced versions let the NEWB flags determine whether ships should be heading for the planet (which is applied to traders, ships who are docking, escape pods and so on). The cassette version is a lot simpler and only sends escape pods in the direction of the planet.

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

Cassette, Electron

CPX #ESC \ If this is not an escape pod, skip the following two BNE P%+8 \ instructions JSR SPS1 \ This is an escape pod, so call SPS1 to calculate the \ vector to the planet and store it in XX15 JMP TA15 \ Jump down to TA15
 CPX #SST               \ If this is not the space station, jump down to TA13
 BNE TA13

Code variation 7 of 16Related to an enhanced feature

Space stations in the enhanced versions regularly spawn Transporters and Shuttles that ply their trade between the station and the planet.

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master

LDA NEWB \ This is the space station, so check whether bit 2 of AND #%00000100 \ the ship's NEWB flags is set, and if it is (i.e. the BNE TN5 \ station is hostile), jump to TN5 to spawn some cops LDA MANY+SHU+1 \ The station is not hostile, so check how many BNE TA1 \ Transporters there are in the vicinity, and if we \ already have one, return from the subroutine (as TA1 \ contains an RTS) \ If we get here then the station is not hostile, so we \ can consider spawning a Transporter or Shuttle JSR DORND \ Set A and X to random numbers CMP #253 \ If A < 253 (99.2% chance), return from the subroutine BCC TA1 \ (as TA1 contains an RTS) AND #1 \ Set A = a random number that's either 0 or 1 ADC #SHU-1 \ The C flag is set (as we didn't take the BCC above), TAX \ so this sets X to a value of either #SHU or #SHU + 1, \ which is the ship type for a Shuttle or a Transporter BNE TN6 \ Jump to TN6 to spawn this ship type and return from \ the subroutine using a tail call (this BNE is \ effectively a JMP as A is never zero) .TN5
                        \ We only call the tactics routine for the space station
                        \ when it is hostile, so if we get here then this is the
                        \ station, and we already know it's hostile, so we need
                        \ to spawn some cops

 JSR DORND              \ Set A and X to random numbers

Code variation 8 of 16Related to a standard feature

In the cassette version there is a 45% chance that an angry station will spawn a cop, while in the enhanced versions there is only a 6.2% chance.

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Cassette

Flight, 6502SP, Master

Electron

CMP #140 \ If A < 140 (55% chance) then return from the BCC TA14-1 \ subroutine (as TA14-1 contains an RTS)
CMP #240 \ If A < 240 (93.8% chance), return from the subroutine BCC TA1 \ (as TA1 contains an RTS)
CMP #140 \ If A < 140 (55% chance) then return from the BCC TA1 \ subroutine (as TA1 contains an RTS)

Code variation 9 of 16Related to a standard feature

In the Electron version there can be up to three cops in the vicinity, in the cassette and disc versions there can be up to four, in the Master version there can be up to six, and in the 6502SP there can be up to seven.

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Cassette

Flight

Master

6502SP

Electron

LDA MANY+COPS \ We only call the tactics routine for the space station CMP #4 \ when it is hostile, so first check the number of cops BCS TA14-1 \ in the vicinity, and if we already have 4 or more, we \ don't need to spawn any more, so return from the \ subroutine (as TA14-1 contains an RTS)
LDA MANY+COPS \ Check how many cops there are in the vicinity already, CMP #4 \ and if there are 4 or more, return from the subroutine BCS TA22 \ (as TA22 contains an RTS)
LDA MANY+COPS \ Check how many cops there are in the vicinity already, CMP #6 \ and if there are 6 or more, return from the subroutine BCS TA22 \ (as TA22 contains an RTS)
LDA MANY+COPS \ Check how many cops there are in the vicinity already, CMP #7 \ and if there are 7 or more, return from the subroutine BCS TA22 \ (as TA22 contains an RTS)
LDA MANY+COPS \ We only call the tactics routine for the space station CMP #3 \ when it is hostile, so first check the number of cops BCS TA1 \ in the vicinity, and if we already have 3 or more, we \ don't need to spawn any more, so return from the \ subroutine (as TA1 contains an RTS)
 LDX #COPS              \ Set X to the ship type for a cop

Code variation 10 of 16A variation in the labels only

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master

.TN6

Code variation 11 of 16Related to the Electron version

The cops that the space station spawns to defend itself are slightly less aggressive in the Electron version than in the other versions.

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Cassette, Flight, 6502SP, Master

Electron

LDA #%11110001 \ Set the AI flag to give the ship E.C.M., enable AI and \ make it very aggressive (60 out of 63)
LDA #%11100001 \ Set the AI flag to give the ship E.C.M., enable AI and \ make it pretty aggressive (56 out of 63)
 JMP SFS1               \ Jump to SFS1 to spawn the ship, returning from the
                        \ subroutine using a tail call

.TA13

Code variation 12 of 16Specific to an individual platform

This logic is in part 3 for the other versions.

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

Cassette

CPX #TGL \ If this is not a Thargon, jump down to TA14 BNE TA14 LDA MANY+THG \ If there is at least one Thargoid in the vicinity, BNE TA14 \ jump down to TA14 LSR INWK+32 \ This is a Thargon but there is no Thargoid mothership, ASL INWK+32 \ so clear bit 0 of the AI flag to disable its E.C.M. LSR INWK+27 \ And halve the Thargon's speed RTS \ Return from the subroutine .TA14

Code variation 13 of 16Specific to an individual platform

Without the NEWB flags, the cassette version logic is much simpler (traders always fly a Cobra Mk III).

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

Cassette, Electron

CPX #CYL \ If A >= #CYL, i.e. this is a Cobra Mk III trader (as BCS TA62 \ asteroids and cargo canisters never have AI), jump \ down to TA62 CPX #COPS \ If this is a cop, jump down to TA62 BEQ TA62 LDA SSPR \ If we aren't within range of the space station, jump BEQ TA62 \ down to TA62 LDA INWK+32 \ This is a pirate or bounty hunter, but we are inside AND #%10000001 \ the space station's safe zone, so clear bits 1-6 of STA INWK+32 \ the AI flag to stop it being hostile, because even \ pirates aren't crazy enough to breach the station's \ no-fire zone .TA62

Code variation 14 of 16Related to an advanced feature

Rock hermits have a 22% chance of spawning a ship.

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

6502SP, Master

CPX #HER \ If this is not a rock hermit, jump down to TA17 BNE TA17 JSR DORND \ Set A and X to random numbers CMP #200 \ If A < 200 (78% chance), return from the subroutine BCC TA22 \ (as TA22 contains an RTS) LDX #0 \ Set byte #32 to %00000000 to disable AI, aggression STX INWK+32 \ and E.C.M.

Code variation 15 of 16Related to the Master version

In the Master version, rock hermits spawn ships that are hostile and innocent bystanders.

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

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Master

6502SP

LDX #%00100100 \ Set the ship's NEWB flags to %00100100 so the ship we STX NEWB \ spawn below will inherit the default values from E% as \ well as having bit 2 (hostile) and bit 5 (innocent \ bystander) set
STX NEWB \ Set the ship's NEWB flags to %00000000 so the ship we \ spawn below will inherit the default values from E%

Code variation 16 of 16Related to an advanced feature

Rock hermits can spawn a Sidewinder, Mamba, Krait, Adder or Gecko.

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

6502SP, Master

AND #3 \ Set A = a random number that's in the range 0-3 ADC #SH3 \ The C flag is set (as we didn't take the BCC above), TAX \ so this sets X to a random value between #SH3 + 1 and \ #SH3 + 4, so that's a Sidewinder, Mamba, Krait, Adder \ or Gecko JSR TN6 \ Call TN6 to spawn this ship with E.C.M., AI and a high \ aggression (56 out of 63) LDA #0 \ Set byte #32 to %00000000 to disable AI, aggression STA INWK+32 \ and E.C.M. (for the rock hermit) RTS \ Return from the subroutine .TA17
 LDY #14                \ If the ship's energy is greater or equal to the
 LDA INWK+35            \ maximum value from the ship's blueprint pointed to by
 CMP (XX0),Y            \ XX0, then skip the next instruction
 BCS TA21

 INC INWK+35            \ The ship's energy is not at maximum, so recharge the
                        \ energy banks by 1

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