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BBC Micro Elite

Tactics: TACTICS (Part 6 of 7) (Cassette version)

Name: TACTICS (Part 6 of 7) [View in context] Type: Subroutine Category: Tactics Summary: Apply tactics: Consider firing a laser at us, if aim is true Deep dive: Program flow of the tactics routine
This section looks at potentially firing the ship's laser at us. Specifically: * If the ship is not pointing at us, skip to the next part * If the ship is pointing at us but not accurately, fire its laser at us and skip to the next part * If we are in the ship's crosshairs, register some damage to our ship, slow down the attacking ship, make the noise of us being hit by laser fire, and we're done
.TA3 \ If we get here then the ship either has plenty of \ energy, or levels are low but it couldn't manage to \ launch a missile, so maybe we can fire the laser? LDA #0 \ Set A to x_hi OR y_hi OR z_hi JSR MAS4 AND #%11100000 \ If any of the hi bytes have any of bits 5-7 set, then BNE TA4 \ jump to TA4 to skip the laser checks, as the ship is \ too far away from us to hit us with a laser LDX CNT \ Set X = the dot product set above in CNT. If this is \ positive, this ship and our ship are facing in similar \ directions, but if it's negative then we are facing \ each other, so for us to be in the enemy ship's line \ of fire, X needs to be negative. The value in X can \ have a maximum magnitude of 36, which would mean we \ were facing each other square on, so in the following \ code we check X like this: \ \ X = 0 to -31, we are not in the enemy ship's line \ of fire, so they can't shoot at us \ \ X = -32 to -34, we are in the enemy ship's line \ of fire, so they can shoot at us, but they can't \ hit us as we're not dead in their crosshairs \ \ X = -35 to -36, we are bang in the middle of the \ enemy ship's crosshairs, so they can not only \ shoot us, they can hit us CPX #160 \ If X < 160, i.e. X > -32, then we are not in the enemy BCC TA4 \ ship's line of fire, so jump to TA4 to skip the laser \ checks LDA INWK+31 \ Set bit 6 in byte #31 to denote that the ship is ORA #%01000000 \ firing its laser at us STA INWK+31 CPX #163 \ If X < 163, i.e. X > -35, then we are not in the enemy BCC TA4 \ ship's crosshairs, so jump to TA4 to skip the laser .HIT LDY #19 \ We are being hit by enemy laser fire, so fetch the LDA (XX0),Y \ enemy ship's byte #19 from their ship's blueprint \ into A LSR A \ Halve the enemy ship's byte #19 (which contains both \ the laser power and number of missiles) to get the \ amount of damage we should take JSR OOPS \ Call OOPS to take some damage, which could do anything \ from reducing the shields and energy, all the way to \ losing cargo or dying (if the latter, we don't come \ back from this subroutine) DEC INWK+28 \ Halve the attacking ship's acceleration in byte #28 LDA ECMA \ If an E.C.M. is currently active (either our's or an BNE TA10 \ opponent's), return from the subroutine without making \ the laser-strike sound (as TA10 contains an RTS) LDA #8 \ Call the NOISE routine with A = 8 to make the sound JMP NOISE \ of us being hit by lasers, returning from the \ subroutine using a tail call