jetpac: Routine at 6ab8

6ab8: Update UFO alien -- this alien is a chaser!

TODO: annotations are really messy/wrong here, needs lots more work to understand how the movement is calculated.

Input
IX	Alien object.

6ab8

ld hl,$5dcb

Increment current alien number

6abb

inc (hl)

6abc

call $6cb0

Update actor direction

6abf

call $6e1b

Fire laser beam (returns C)

6ac2

dec c

6ac3

jp z,$6beb

Add the points for a kill if alien is dead

6ac6

call $6de9

Alien collision check (returns E)

6ac9

dec e

6aca

jp z,$6456

Alien killed by collision if E is zero

6acd

xor a

6ace

ld ($5dc9),a

Reset alien new direction flag

Check which direction alien is travelling after hitting a platform.

6ad1

call $75e8

Platform collision (returns E)

6ad4

bit 2,e

Update alien X position if bit-2 is reset

6ad6

jr z,$6af1

6ad8

bit 7,e

Set alien direction to up if bit-7 set

6ada

jp nz,$6bcd

6add

bit 4,e

Set alien direction to down if bit-4 set

6adf

jp nz,$6bd4

Move the flying saucer in a slightly more erratic way (the XOR), rather than just heading directly for the Jetman.

6ae2

ld a,e

Update alien "moving" value

6ae3

and $40

6ae5

xor $40

6ae7

ld e,a

6ae8

ld a,(ix+$04)

6aeb

and $bf

6aed

or e

6aee

ld (ix+$04),a

Calculate new horizontal position and speed.

6af1

ld a,(ix+$05)

Alien X speed

6af4

ld b,a

Calculate new speed?

6af5

and $0f

6af7

ld c,a

6af8

ld a,b

6af9

and $f0

6afb

ld b,a

6afc

ld a,($5d01)

Jetman X position

6aff

sub (ix+$01)

Subtract alien X position

6b02

jp p,$6b74

If Jetman X > alien X, then jump

6b05

bit 6,(ix+$04)

If alien is moving left, then jump

6b09

jp z,$6b96

6b0c

ld a,c

If current speed is less than max, then increment

6b0d

cp $0f

6b0f

jr nc,$6b12

6b11

inc a

Calculate new X position.

6b12

ld c,a

C=current speed

6b13

ld h,(ix+$01)

H=X position

6b16

call $6bdb

Calculates and returns DE, new position offset

6b19

and a

Clear the carry flag

6b1a

sbc hl,de

Update the X position and speed

6b1c

ld (ix+$01),h

Update X position

6b1f

ld a,l

Calculate and update X speed

6b20

and $f0

6b22

or c

6b23

ld (ix+$05),a

Calculate new Y position and speed.

6b26

ld a,(ix+$06)

Alien Y speed

6b29

ld b,a

6b2a

and $0f

6b2c

ld c,a

6b2d

ld a,b

6b2e

and $f0

6b30

ld b,a

6b31

ld a,($5d02)

Jetman Y position

6b34

sub (ix+$02)

Subtract alien Y position

6b37

jp p,$6ba2

If Jetman Y > alien Y, then jump

6b3a

bit 7,(ix+$04)

If alien is moving up then increment Y position (if not at top of screen) and jump to 6b49

6b3e

jp z,$6bc3

6b41

ld a,c

Calculate new down Y position if C > 0 (presumably above the ground)

6b42

dec a

6b43

jr nz,$6bae

6b45

res 7,(ix+$04)

else moving direction must be set to up...

6b49

ld c,a

..and C will have been decremented here

6b4a

ld h,(ix+$02)

H=Y position

6b4d

call $6bdb

Calculates and returns DE, new position offset

6b50

and a

Clear the carry flag

6b51

sbc hl,de

6b53

ld a,h

Set moving direction as down if at top of screen

6b54

cp $28

6b56

jr nc,$6b5c

6b58

set 7,(ix+$04)

6b5c

ld (ix+$02),a

Update Y position

6b5f

ld a,l

Update Y speed

6b60

and $f0

6b62

or c

6b63

ld (ix+$06),a

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

6b66

ld a,($5dc9)

Draw the alien if the new direction flag is set,

6b69

and a

otherwise, increment it

6b6a

jp nz,$6d43

6b6d

inc a

6b6e

ld ($5dc9),a

}

6b71

jp $6ad1

Jump back and repeat collision check

Check moving direction and update.

6b74

bit 6,(ix+$04)

Check moving left/right bit

6b78

jr z,$6b8e

If moving left, increment speed (via jump)

6b7a

ld a,c

6b7b

dec a

6b7c

jp nz,$6b12

If not zero, calculate X direction/speed

6b7f

res 6,(ix+$04)

else set X moving to "right"

6b83

ld c,a

A (speed) was decremented from 0, so will now be FF

6b84

ld h,(ix+$01)

H=X position

6b87

call $6bdb

Calculates and returns DE, new position offset

6b8a

add hl,de

6b8b

jp $6b1c

Update the X position and speed

6b8e

ld a,c

If current speed is less than max, then increment

6b8f

cp $0f

6b91

jr nc,$6b83

6b93

inc a

6b94

jr $6b83

Update position

6b96

ld a,c

A=speed

6b97

dec a

6b98

jp nz,$6b83

Update X position and speed unless zero

6b9b

set 6,(ix+$04)

Set moving direction to "right"

6b9f

jp $6b12

Update X direction/speed

6ba2

bit 7,(ix+$04)

Check Jetman moving direction

6ba6

jr z,$6bb9

Jump if top bit not set

6ba8

ld a,c

Jump if C >= 15

6ba9

cp $0f

6bab

jr nc,$6bae

6bad

inc a

else increment it

6bae

ld c,a

(will be C = C - 1)

6baf

ld h,(ix+$02)

6bb2

call $6bdb

Calculates and returns DE, new position offset

6bb5

add hl,de

6bb6

jp $6b53

Update Y position and speed

6bb9

ld a,c

A=speed

6bba

dec a

6bbb

jr nz,$6b49

If speed > 0 move upwards

6bbd

set 7,(ix+$04)

else set moving direction to "down"

6bc1

jr $6bae

...and update Y position and speed

6bc3

ld a,c

If current speed is less than max, then increment it, and update position

6bc4

cp $0f

6bc6

jp nc,$6b49

6bc9

inc a

6bca

jp $6b49

Change alien moving direction to up.

6bcd

res 7,(ix+$04)

6bd1

jp $6af1

Change alien moving direction to down.

6bd4

set 7,(ix+$04)

6bd8

jp $6af1

Returns DE (new position) as calculated from the speed (C).

6bdb

ld l,b

6bdc

ld a,c

C should be <= 15. Example, if value is $0F:

6bdd

rla

0 00011110

6bde

rla

0 00111100

6bdf

rla

0 01111000

6be0

rla

0 11110000

6be1

and $f0

Clear lower 4 bits...just in case!

6be3

ld e,a

6be4

ld d,$00

6be6

sla e

1 11100000

6be8

rl d

If E >= 8, then bit-0 of D is set

6bea

ret