Version analysis of TACTICS (Part 1 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 1 of 7)
       Type: Subroutine
   Category: Tactics
    Summary: Apply tactics: Process missiles, both enemy missiles and our own
  Deep dive: Program flow of the tactics routine


 This section implements missile tactics and is entered at TA18 from the main
 entry point below, if the current ship is a missile. Specifically:

   * If E.C.M. is active, destroy the missile

   * If the missile is hostile towards us, then check how close it is. If it
     hasn't reached us, jump to part 3 so it can streak towards us, otherwise
     we've been hit, so process a large amount of damage to our ship

   * Otherwise see how close the missile is to its target. If it has not yet
     reached its target, give the target a chance to activate its E.C.M. if it
     has one, otherwise jump to TA19 with K3 set to the vector from the target
     to the missile

   * If it has reached its target and the target is the space station, destroy
     the missile, potentially damaging us if we are nearby

   * If it has reached its target and the target is a ship, destroy the missile
     and the ship, potentially damaging us if we are nearby

Code variation 1 of 10Related to the Master version

In the Master version, destroying a missile using E.C.M. gives us the same number of fractional kill points as killing an alloy plate, while the other versions always award one point, whatever is killed.

See below for more variations related to this code.

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

Master
.TA352

                        \ If we get here, the missile has been destroyed by
                        \ E.C.M. or by the space station

 LDA INWK               \ Set A = x_lo OR y_lo OR z_lo of the missile
 ORA INWK+3
 ORA INWK+6

 BNE TA872              \ If A is non-zero then the missile is not near our
                        \ ship, so skip the next two instructions to avoid
                        \ damaging our ship

 LDA #80                \ Otherwise the missile just got destroyed near us, so
 JSR OOPS               \ call OOPS to damage the ship by 80, which is nowhere
                        \ near as bad as the 250 damage from a missile slamming
                        \ straight into us, but it's still pretty nasty

.TA872

 LDX #PLT               \ Set X to the ship type for plate alloys, so we get
                        \ awarded the kill points for the missile scraps in TA87

 BNE TA353              \ Jump to TA353 to process the missile kill tally and
                        \ make an explosion sound

.TA34

                        \ If we get here, the missile is hostile

 LDA #0                 \ Set A to x_hi OR y_hi OR z_hi
 JSR MAS4

 BEQ P%+5               \ If A = 0 then the missile is very close to our ship,
                        \ so skip the following instruction

Code variation 2 of 10Related to the Master version

In the Master version, the tactics routine is slightly more efficient as it skips the NEWB logic checks for missiles, which aren't relevant (the disc and 6502SP versions still perform the checks, though this has no effect).

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

Cassette, Flight, 6502SP, Electron
Master
 JMP TA21               \ Jump down to part 3 to set up the vectors and skip
                        \ straight to aggressive manoeuvring

 JSR TA87+3             \ The missile has hit our ship, so call TA87+3 to set
                        \ bit 7 of the missile's byte #31, which marks the
                        \ missile as being killed
 JMP TN4                \ Jump down to part 3 to set up the vectors and skip
                        \ straight to aggressive manoeuvring

 JSR TA873              \ The missile has hit our ship, so call TA873 to set
                        \ bit 7 of the missile's byte #31, which marks the
                        \ missile as being killed

 JSR EXNO3              \ Make the sound of the missile exploding

 LDA #250               \ Call OOPS to damage the ship by 250, which is a pretty
 JMP OOPS               \ big hit, and return from the subroutine using a tail
                        \ call

.TA18

                        \ This is the entry point for missile tactics and is
                        \ called from the main TACTICS routine below

Code variation 3 of 10Related to the Master version

See variation 1 above for details.

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

Cassette, Flight, 6502SP, Electron
Master
 LDA ECMA               \ If an E.C.M. is currently active (either ours or an
 BNE TA35               \ opponent's), jump to TA35 to destroy this missile
 LDA ECMA               \ If an E.C.M. is currently active (either ours or an
 BNE TA352              \ opponent's), jump to TA352 to destroy this missile

 LDA INWK+32            \ Fetch the AI flag from byte #32 and if bit 6 is set
 ASL A                  \ (i.e. missile is hostile), jump up to TA34 to check
 BMI TA34               \ whether the missile has hit us

 LSR A                  \ Otherwise shift A right again. We know bits 6 and 7
                        \ are now clear, so this leaves bits 0-5. Bits 1-5
                        \ contain the target's slot number, and bit 0 is cleared
                        \ in FRMIS when a missile is launched, so A contains
                        \ the slot number shifted left by 1 (i.e. doubled) so we
                        \ can use it as an index for the two-byte address table
                        \ at UNIV

Code variation 4 of 10Minor and very low-impact

This does exactly the same, but some code has moved into the VCSUB routine.

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

Cassette, Electron
Flight, 6502SP, Master
 TAX                    \ Copy the address of the target ship's data block from
 LDA UNIV,X             \ UNIV(X+1 X) to V(1 0)
 STA V
 LDA UNIV+1,X
 STA V+1

 LDY #2                 \ K3(2 1 0) = (x_sign x_hi x_lo) - x-coordinate of
 JSR TAS1               \ target ship

 LDY #5                 \ K3(5 4 3) = (y_sign y_hi z_lo) - y-coordinate of
 JSR TAS1               \ target ship

 LDY #8                 \ K3(8 7 6) = (z_sign z_hi z_lo) - z-coordinate of
 JSR TAS1               \ target ship
 TAX                    \ Copy the address of the target ship's data block from
 LDA UNIV,X             \ UNIV(X+1 X) to (A V)
 STA V
 LDA UNIV+1,X

 JSR VCSUB              \ Calculate vector K3 as follows:
                        \
                        \ K3(2 1 0) = (x_sign x_hi x_lo) - x-coordinate of
                        \ target ship
                        \
                        \ K3(5 4 3) = (y_sign y_hi z_lo) - y-coordinate of
                        \ target ship
                        \
                        \ K3(8 7 6) = (z_sign z_hi z_lo) - z-coordinate of
                        \ target ship

                        \ So K3 now contains the vector from the target ship to
                        \ the missile

 LDA K3+2               \ Set A = OR of all the sign and high bytes of the
 ORA K3+5               \ above, clearing bit 7 (i.e. ignore the signs)
 ORA K3+8
 AND #%01111111
 ORA K3+1
 ORA K3+4
 ORA K3+7

 BNE TA64               \ If the result is non-zero, then the missile is some
                        \ distance from the target, so jump down to TA64 see if
                        \ the target activates its E.C.M.

Code variation 5 of 10A variation in the labels only

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

Cassette, Flight, 6502SP, Electron
Master
 LDA INWK+32            \ Fetch the AI flag from byte #32 and if only bits 7 and
 CMP #%10000010         \ 1 are set (AI is enabled and the target is slot 1, the
 BEQ TA35               \ space station), jump to TA35 to destroy this missile,
                        \ as the space station ain't kidding around
 LDA INWK+32            \ Fetch the AI flag from byte #32 and if only bits 7 and
 CMP #%10000010         \ 1 are set (AI is enabled and the target is slot 1, the
 BEQ TA352              \ space station), jump to TA352 to destroy this missile,
                        \ as the space station ain't kidding around

 LDY #31                \ Fetch byte #31 (the exploding flag) of the target ship
 LDA (V),Y              \ into A

 BIT M32+1              \ M32 contains an LDY #32 instruction, so M32+1 contains
                        \ 32, so this instruction tests A with %00100000, which
                        \ checks bit 5 of A (the "already exploding?" bit)

 BNE TA35               \ If the target ship is already exploding, jump to TA35
                        \ to destroy this missile

 ORA #%10000000         \ Otherwise set bit 7 of the target's byte #31 to mark
 STA (V),Y              \ the ship as having been killed, so it explodes

.TA35

 LDA INWK               \ Set A = x_lo OR y_lo OR z_lo of the missile
 ORA INWK+3
 ORA INWK+6

Code variation 6 of 10A variation in the labels only

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

Cassette, Flight, 6502SP, Electron
Master
 BNE TA87               \ If A is non-zero then the missile is not near our
                        \ ship, so jump to TA87 to skip damaging our ship
 BNE P%+7               \ If A is non-zero then the missile is not near our
                        \ ship, so skip the next two instructions to avoid
                        \ damaging our ship

 LDA #80                \ Otherwise the missile just got destroyed near us, so
 JSR OOPS               \ call OOPS to damage the ship by 80, which is nowhere
                        \ near as bad as the 250 damage from a missile slamming
                        \ straight into us, but it's still pretty nasty

.TA87

Code variation 7 of 10Related to the Master version

In the Master version, destroying a missile (not using E.C.M.) gives us a number of kill points that depends on the missile's target slot number, and therefore is fairly random.

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

Master
 LDA INWK+32            \ Set X to bits 1-6 of the missile's AI flag in ship
 AND #%01111111         \ byte #32, so bits 0-4 of X are the target's slot
 LSR A                  \ number, and bit 5 is set (as the missile is hostile)
 TAX                    \ so X is fairly random and in the range 32-43 (as the
                        \ maximum slot number is 11)
                        \
                        \ The value of X is used to determine the number of kill
                        \ points awarded for the destruction of the missile

.TA353

 JSR EXNO2              \ Call EXNO2 to process the fact that we have killed a
                        \ missile (so increase the kill tally, make an explosion
                        \ sound and so on)

Code variation 8 of 10A variation in the labels only

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

Master
.TA873

 ASL INWK+31            \ Set bit 7 of the missile's byte #31 flag to mark it as
 SEC                    \ having been killed, so it explodes
 ROR INWK+31

.TA1

 RTS                    \ Return from the subroutine

.TA64

                        \ If we get here then the missile has not reached the
                        \ target

 JSR DORND              \ Set A and X to random numbers

Code variation 9 of 10Minor and very low-impact

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

Cassette, Electron
Flight, 6502SP, Master
 CMP #16                \ If A >= 16 (94% chance), jump down to TA19 with the
 BCS TA19               \ vector from the target to the missile in K3
 CMP #16                \ If A >= 16 (94% chance), jump down to TA19S with the
 BCS TA19S              \ vector from the target to the missile in K3

.M32

 LDY #32                \ Fetch byte #32 for the target and shift bit 0 (E.C.M.)
 LDA (V),Y              \ into the C flag
 LSR A

Code variation 10 of 10Minor and very low-impact

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

Cassette, Electron
Flight, 6502SP, Master
 BCC TA19               \ If the C flag is clear then the target does not have
                        \ E.C.M. fitted, so jump down to TA19 with the vector
                        \ from the target to the missile in K3
 BCS P%+5               \ If the C flag is set then the target has E.C.M.
                        \ fitted, so skip the next instruction

.TA19S

 JMP TA19               \ The target does not have E.C.M. fitted, so jump down
                        \ to TA19 with the vector from the target to the missile
                        \ in K3

 JMP ECBLB2             \ The target has E.C.M., so jump to ECBLB2 to set it
                        \ off, returning from the subroutine using a tail call