Routines
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6a35: Update Squidgy Alien.
Input
IX Alien object.
6a35 ld hl,$5dcb Increment current alien number
6a38 inc (hl)
6a39 call $6cb0 Update actor direction
6a3c call $6e1b Fire laser beam (returns C)
6a3f dec c
6a40 jp z,$6aaf Add the points for a kill if alien is dead
6a43 call $6de9 Alien collision check (returns E)
6a46 dec e
6a47 jp z,$6456 Alien killed by collision if E is zero
6a4a xor a
6a4b ld ($5dc9),a Reset alien new direction flag
Check which direction alien is travelling after hitting a platform.
6a4e call $75e8 Platform collision (returns E)
6a51 bit 2,e Update alien X position if bit-2 is reset
6a53 jr z,$6a6a
6a55 bit 7,e Set alien direction to up if bit-7 set
6a57 jr nz,$6a93
6a59 bit 4,e Set alien direction to down if bit-4 set
6a5b jr nz,$6a99
Alien has hit the end of a platform: change direction.
6a5d ld a,e Update alien "moving" value
6a5e and $40
6a60 ld e,a
6a61 ld a,(ix+$04)
6a64 and $bf
6a66 or e
6a67 ld (ix+$04),a
Update the alien X position.
6a6a bit 6,(ix+$04) Check alien moving left/right bit
6a6e ld a,(ix+$01) A=alien X position (fetch before the jump)
6a71 jr z,$6a9f If moving left, subtract 2 from direction (via jump)
6a73 add a,$02 otherwise, we add 2 to move right
6a75 ld (ix+$01),a Set new X position +/- 2
Update the alien Y position, first checking if within the upper/lower bounds of the screen.
6a78 bit 7,(ix+$04) Check alien moving up/down bit
6a7c ld a,(ix+$02) A=alien Y position (fetch before the jump)
6a7f jr z,$6aa3 If moving up, subtract 2 from direction (via jump)
6a81 add a,$02 otherwise, we add 2 to move down
6a83 ld (ix+$02),a Set new alien Y position +/- 2
Draw the alien if new direction is set, otherwise repeat collision check again.
6a86 ld a,($5dc9) Draw the alien if the new direction flag is set, otherwise, increment the value
6a89 and a
6a8a jp nz,$6d43
6a8d inc a
6a8e ld ($5dc9),a
6a91 jr $6a4e Jump back and repeat collision check
Change alien moving direction to up.
6a93 res 7,(ix+$04)
6a97 jr $6a6a
Change alien moving direction to down.
6a99 set 7,(ix+$04)
6a9d jr $6a6a
Subtract 2 from the X position (move left).
6a9f sub $02
6aa1 jr $6a75
Subtract 2 from the Y position (move up), unless it has reached top of screen.
6aa3 sub $02
6aa5 cp $24
6aa7 jr nc,$6a83 Return if not at top of screen
6aa9 set 7,(ix+$04) Otherwise, set alien to moving down.
6aad jr $6a83
Add score to player if alien is dead.
6aaf ld bc,$0080 80 points (decimal value)
6ab2 call $70c2 Add points to score
6ab5 jp $6bf1 Kill Alien SFX #1
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