Used by the routine at GenerateRandomNumber.

Mothership level.

Used by the routine at Handler_Pheenix_01.

The number of alien enemies "in-play" on the screen (not counting the mothership/ mothership alien which is handled separately).

Set and handled by the routine at Shield_ActivateShield.

Byte	Meaning
0	No Collision
1	Collision Detected
2	Explosion Complete

Used by the routine at Handler_ShipExplosion.

A counter that cycles from 0 to 7 controlling ship movement animation timing. Movement is skipped when the counter is 3. Also used as a timer in shield drawing routines.

Holds the mothership alien state which is used only in phase 4 to indicate if the phase has been completed or not (and then shows the bonus):

Byte	State
0	Alive
1	Dead

Used by the routine at Handler_Mothership.

Indicates whether an extra life should be awarded or not.

Maybe not directly the phase number, but when it's 4 it draws the mothership.

Indicates the start state while the title screen is showing.

Byte	Method
0	Keyboard
1	AGF joystick
2	Kempston joystick

See GameIntro.