Routines
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63a3: Update Jet Fighter.
Input
IX Alien object for one of the Jet Fighters.
63a3 ld hl,$5dcb Increment current alien number
63a6 inc (hl)
63a7 call $6cb0 Update Alien direction
63aa ld a,(ix+$04) Alien "moving" field
63ad ld hl,$0000
63b0 bit 1,a Move if bit-1 is set
63b2 jr nz,$63ec
63b4 call $63d1 Check and update speed
63b7 ld a,($5d02) A=Jetman Y position
63ba sub $0c
63bc cp (ix+$02) Update Alien speed if Jetman Y position-12 equals Alien Y position
63bf call z,$63d7
63c2 ld a,($5dcc) A=LSB of game timer
63c5 and $40
63c7 jr z,$63cd Move vertically if bit-6 is reset
63c9 inc h
63ca inc h
63cb inc h
63cc inc h
63cd dec h
63ce dec h
63cf jr $6406 Move vertically +2 or -2
Decide if speed should be updated.
63d1 ld a,($5dce) A=random number
63d4 and $1f
63d6 ret nz Return if not zero
Update speed.
63d7 set 1,(ix+$04) Alien "moving" field
63db ld c,a C=Jetman Y position-12 ?
63dc ld a,($5dcc) A=LSB of game timer
63df add a,c
63e0 and $7f
63e2 or $20
63e4 ld (ix+$05),a Set new X speed value
63e7 ld (ix+$03),$47 Set "colour" to White
63eb ret
Move Jet Fighter horizontally.
63ec dec (ix+$05) Decrement alien X speed
63ef jr z,$644a Jet Fighter killed if X speed is zero
63f1 ld a,$04 A=default speed
63f3 bit 6,(ix+$00) Check bit-6 of Alien "direction"
63f7 jr z,$63fb
63f9 neg
63fb ld l,a L=$FC or $04?
Target the player and move towards them.
63fc ld a,($5d02) Move upwards if Jetman Y position higher than Alien Y position
63ff cp (ix+$02)
6402 jr c,$63cd
6404 jr $63c9 else, move down
Move Jet Fighter vertically and horizontally.
6406 ld a,(ix+$00) Alien "direction"
6409 and $c0 Reset FLY and WALK bits
640b or $03
640d ld (ix+$00),a Update "direction" value
6410 ld a,(ix+$01) Update Alien X position (L is probably 3 or 4)
6413 add a,l
6414 ld (ix+$01),a
6417 ld a,(ix+$02) Update Alien Y position
641a add a,h
641b ld (ix+$02),a
641e call $7232 Update Actor Position X and draw
6421 call $7197 Colourize the sprite
6424 ld a,(ix+$02) Kill Fighter if Y position < $28
6427 cp $28
6429 jr c,$644a
642b call $6e1b Fire laser beam - returns C
642e bit 0,c Kill if a laser hit the Alien
6430 jr nz,$644a
6432 call $75e8 Platform collision - returns E
6435 bit 2,e Kill if Alien hit a platform
6437 jr nz,$644a
6439 call $6de9 Alien collision - returns DE
643c dec e
643d jr z,$6456 Alien killed by collision
643f ld a,(ix+$00) Update Alien "direction"
6442 and $c0
6444 or $07
6446 ld (ix+$00),a
6449 ret
Jet Fighter is dead. Added score and make exploding sound.
644a ld bc,$0055 55 points for a dead jet fighter (decimal value)
644d call $70c2 Add points to score
6450 call $67b6 Exploding jet fighter SFX - actually Thruster SFX!
6453 jp $685d Update current alien state
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