jetpac: Routine at 6bf8

6bf8: Update Sphere alien.

Input
IX	Alien object.

6bf8

ld hl,$5dcb

Increment current alien number

6bfb

inc (hl)

6bfc

call $6cb0

Update actor direction

6bff

xor a

6c00

ld ($5dc9),a

Reset alien new direction flag

6c03

call $6e1b

Fire laser beam (returns C)

6c06

dec c

6c07

ld bc,$0040

40 points (decimal value)

6c0a

jp z,$6d5c

Kill Alien SFX #2 if alien is dead

6c0d

call $6de9

Alien collision check (returns E)

6c10

dec e

6c11

jp z,$6456

Alien killed by collision if E is zero

Check which direction alien is travelling after hitting a platform.

6c14

call $75e8

Platform collision (returns E)

6c17

bit 2,e

Update alien Y position if bit-2 is reset

6c19

jr z,$6c30

6c1b

bit 7,e

Set alien direction to up if bit-7 set

6c1d

jr nz,$6c94

6c1f

bit 4,e

Set alien direction to down if bit-4 set

6c21

jr nz,$6c9a

Alien has hit the end of a platform: change direction.

6c23

ld a,e

Update alien "moving" value

6c24

and $40

6c26

ld e,a

6c27

ld a,(ix+$04)

6c2a

and $bf

6c2c

or e

6c2d

ld (ix+$04),a

Update the alien Vertical position.

6c30

bit 0,(ix+$04)

Check alien moving direction

6c34

jr nz,$6c5b

Update Y position if vertical movement

6c36

ld a,($5dce)

E=random number

6c39

ld e,a

6c3a

and $0f

6c3c

jr nz,$6c5b

Jump if != 0

6c3e

set 0,(ix+$04)

else set "moving" bit-0

6c42

ld a,($5dcc)

Use Game timer to Y speed

6c45

and $1f

6c47

add a,$10

6c49

ld (ix+$06),a

6c4c

ld a,e

A=original random number

6c4d

and $80

6c4f

ld e,a

E = 0 or 128

6c50

ld a,(ix+$04)

Update alien "moving" to either 127 or 255

6c53

and $7f

6c55

or e

6c56

ld (ix+$04),a

6c59

jr $6c14

Jump back and repeat collision check

Update the alien Horizontal position.

6c5b

bit 0,(ix+$04)

Check alien "moving"

6c5f

jr z,$6c78

Update X position if horizontal movement

6c61

ld a,(ix+$02)

else update Y position

Update the alien Y position, first checking if within the upper/lower bounds of the screen.

6c64

bit 7,(ix+$04)

Check moving up/down bit

6c68

jr z,$6ca0

If moving up, subtract 2 from direction (via jump)

6c6a

add a,$02

otherwise, we add 2 to move down

6c6c

ld (ix+$02),a

Set new Y position +/- 2

6c6f

dec (ix+$06)

Decrement Y speed

6c72

jr nz,$6c78

6c74

res 0,(ix+$04)

Reset bit-0 of alien "moving" if speed is zero

Update the alien X position.

6c78

ld a,(ix+$01)

A=X position (fetch before the jump)

6c7b

bit 6,(ix+$04)

Check moving left/right bit

6c7f

jr z,$6cac

If moving left, subtract 2 from direction (via jump)

6c81

add a,$02

otherwise, we add 2 to move right

6c83

ld (ix+$01),a

Set new X position +/- 2

Draw the alien if a new direction is set, otherwise repeat platform collision check again.

6c86

ld a,($5dc9)

Draw the alien if the new direction flag is set, otherwise, increment the value

6c89

and a

6c8a

jp nz,$6d43

6c8d

inc a

6c8e

ld ($5dc9),a

6c91

jp $6c14

Jump back and repeat collision check

Change alien moving direction to up.

6c94

res 7,(ix+$04)

6c98

jr $6c30

Change alien moving direction to down.

6c9a

set 7,(ix+$04)

6c9e

jr $6c30

Subtract 2 from the Y position (move up), unless it has reached top of screen.

6ca0

sub $02

6ca2

cp $28

6ca4

jr nc,$6c6c

Return if not at top of screen

6ca6

set 7,(ix+$04)

Otherwise, set to moving down

6caa

jr $6c6c

Subtract 2 from horizontal direction.

6cac

sub $02

6cae

jr $6c83