Version analysis of TACTICS (Part 4 of 7) - 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: TACTICS (Part 4 of 7)
       Type: Subroutine
   Category: Tactics
    Summary: Apply tactics: Check energy levels, maybe launch escape pod if low
  Deep dive: Program flow of the tactics routine


 This section works out what kind of condition the ship is in. Specifically:

Code variation 1 of 7A variation in the comments only

This variation is blank in the Cassette and Electron versions.

Tap on a block to expand it, and tap it again to revert.

Flight
6502SP, Master
   * If this is an Anaconda, consider spawning (22% chance) a Worm
   * If this is an Anaconda, consider spawning (22% chance) a Worm (61% of the
     time) or a Sidewinder (39% of the time)

   * Rarely (2.5% chance) roll the ship by a noticeable amount

   * If the ship has at least half its energy banks full, jump to part 6 to
     consider firing the lasers

   * If the ship is not into the last 1/8th of its energy, jump to part 5 to
     consider firing a missile

Code variation 2 of 7A variation in the comments only

Tap on a block to expand it, and tap it again to revert.

Cassette, Electron
Flight, 6502SP, Master
   * If the ship is into the last 1/8th of its energy, then rarely (10% chance)
     the ship launches an escape pod and is left drifting in space
   * If the ship is into the last 1/8th of its energy, and this ship type has
     an escape pod fitted, then rarely (10% chance) the ship launches an escape
     pod and is left drifting in space



 LDA TYPE               \ If this is not a missile, skip the following
 CMP #MSL               \ instruction
 BNE P%+5

 JMP TA20               \ This is a missile, so jump down to TA20 to get
                        \ straight into some aggressive manoeuvring

Code variation 3 of 7Related to an enhanced feature

In the enhanced versions, Anacondas can spawn other ships.

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master
 CMP #ANA               \ If this is not an Anaconda, jump down to TN7 to skip
 BNE TN7                \ the following

 JSR DORND              \ Set A and X to random numbers

 CMP #200               \ If A < 200 (78% chance), jump down to TN7 to skip the
 BCC TN7                \ following

Code variation 4 of 7Related to an advanced feature

In the disc version, Anacondas can only spawn Worms, while in the advanced versions they can also spawn Sidewinders.

This variation is blank in the Cassette and Electron versions.

Tap on a block to expand it, and tap it again to revert.

Flight
6502SP, Master
 LDX #WRM               \ Set X to the ship type for a Worm

 JMP TN6                \ Jump to TN6 to spawn the Worm and return from
                        \ the subroutine using a tail call

.TN7
 JSR DORND              \ Set A and X to random numbers

 LDX #WRM               \ Set X to the ship type for a Worm

 CMP #100               \ If A >= 100 (61% chance), skip the following
 BCS P%+4               \ instruction

 LDX #SH3               \ Set X to the ship type for a Sidewinder

 JMP TN6                \ Jump to TN6 to spawn the Worm or Sidewinder and return
                        \ from the subroutine using a tail call

.TN7

 JSR DORND              \ Set A and X to random numbers

 CMP #250               \ If A < 250 (97.5% chance), jump down to TA7 to skip
 BCC TA7                \ the following

 JSR DORND              \ Set A and X to random numbers

 ORA #104               \ Bump A up to at least 104 and store in the roll
 STA INWK+29            \ counter, to gives the ship a noticeable roll

.TA7

 LDY #14                \ Set A = the ship's maximum energy / 2
 LDA (XX0),Y
 LSR A

 CMP INWK+35            \ If the ship's current energy in byte #35 > A, i.e. the
 BCC TA3                \ ship has at least half of its energy banks charged,
                        \ jump down to TA3

 LSR A                  \ If the ship's current energy in byte #35 > A / 4, i.e.
 LSR A                  \ the ship is not into the last 1/8th of its energy,
 CMP INWK+35            \ jump down to ta3 to consider firing a missile
 BCC ta3

 JSR DORND              \ Set A and X to random numbers

 CMP #230               \ If A < 230 (90% chance), jump down to ta3 to consider
 BCC ta3                \ firing a missile

Code variation 5 of 7Related to a standard feature

The cassette version has Thargoids but no NEWB flags, so we have to manually cater for Thargoids launching Thargons rather than using the same code as escape pods.

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

Cassette
 LDA TYPE               \ If this is a Thargoid, jump down to ta3 to consider
 CMP #THG               \ launching a Thargon
 BEQ ta3

Code variation 6 of 7Related to an enhanced feature

In the enhanced versions, the NEWB flags determine whether or not a ship has an escape pod it can launch when things go south, while in the cassette and Electron versions, every ship has an escape pod fitted.

This variation is blank in the Cassette and Electron versions.

Flight, 6502SP, Master
 LDX TYPE               \ Fetch the ship blueprint's default NEWB flags from the
 LDA E%-1,X             \ table at E%, and if bit 7 is clear (i.e. this ship
 BPL ta3                \ does not have an escape pod), jump to ta3 to skip the
                        \ spawning of an escape pod

                        \ By this point, the ship has run out of both energy and
                        \ luck, so it's time to bail

Code variation 7 of 7Related to the Master version

In the Master version, escape pods that other ships drop do not inherit the parent ship's characteristics (i.e. trader, bounty hunter, hostile, pirate).

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

Master
 LDA NEWB               \ Clear bits 0-3 of the NEWB flags, so the ship is no
 AND #%11110000         \ longer a trader, a bounty hunter, hostile or a pirate
 STA NEWB               \ and the escape pod we are about to spawn won't inherit
                        \ any of these traits

 LDY #36                \ Update the NEWB flags in the ship's data block
 STA (INF),Y

 LDA #0                 \ Set the AI flag to 0 to disable AI, hostility and
 STA INWK+32            \ E.C.M., so the ship's a sitting duck

 JMP SESCP              \ Jump to SESCP to spawn an escape pod from the ship,
                        \ returning from the subroutine using a tail call