Routines
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993C: Handler: Bouncing Wheel
Steer the Bouncing Wheel toward the scroll position and bounce it at the play-area edges.
Input
IX Actor slot
Handler_BouncingWheel 993C LD H,(IX+$03) Fetch the high byte of the world X.
993F LD L,(IX+$02) Fetch the low byte of the world X.
9942 LD DE,($607A) Fetch Jetman_Position.
9946 AND A Subtract the scroll position.
9947 SBC HL,DE
9949 LD C,$00 C=00.
Leave the speed unchanged if the wheel is on this screen or just off its left edge.
994B LD A,H Fetch the high byte of the gap.
994C AND %00011111 Keep only bits 0-4.
994E JR Z,Handler_BouncingWheel_AddY Jump to Handler_BouncingWheel_AddY if the alien is on this screen.
9950 OR %11100000 Set bits 5-7.
9952 CPL Flip the bits.
9953 AND A Jump to Handler_BouncingWheel_AddY if the result is zero.
9954 JR Z,Handler_BouncingWheel_AddY
9956 BIT 4,H Test bit 4 of the gap.
30 is a signed +30 rightward speed; D0 is signed -30 leftward.
9958 LD A,$30 A=30.
995A JR NZ,Handler_BouncingWheel_WriteSpeed Jump to Handler_BouncingWheel_WriteSpeed if the alien is left of the scroll.
995C LD A,$D0 A=D0.
Handler_BouncingWheel_WriteSpeed 995E LD (IX+$05),A Write the speed to *IX+05.
Move the wheel vertically, bouncing it off the play-area edges.
Handler_BouncingWheel_AddY 9961 LD A,(IX+$04) Fetch the alien's Y position.
9964 ADD A,(IX+$06) Add the vertical thrust.
9967 CP $38 Jump to Handler_BouncingWheel_ThrustDown if the alien is above the play area.
9969 JR C,Handler_BouncingWheel_ThrustDown
996B CP $B0 Jump to Handler_BouncingWheel_ThrustUp if the alien has hit the ground line.
996D JR NC,Handler_BouncingWheel_ThrustUp
996F LD (IX+$04),A Write the Y position back.
Step the animation timer and pick the animation frame.
Handler_BouncingWheel_TickAnimation 9972 INC (IX+$11) Increase the animation timer.
9975 LD A,(IX+$11) Fetch the animation timer.
9978 CP $06 Jump to Handler_BouncingWheel_WriteFrame if it is still below 06.
997A JR C,Handler_BouncingWheel_WriteFrame
997C XOR A Write 00 to the animation timer.
997D LD (IX+$11),A
Sprite IDs begin at A8 ("Bouncing Wheel: Frame 1").
Handler_BouncingWheel_WriteFrame 9980 ADD A,$A8 Add A8 for these frames.
9982 LD (IX+$00),A Write it to the actor type.
9985 JP PlaceSpawnedAlien_Age Jump to PlaceSpawnedAlien_Age.
Bounce off the top by thrusting down, or off the ground by thrusting up.
Handler_BouncingWheel_ThrustDown 9988 LD (IX+$06),$03 Write 03 to the vertical thrust.
998C JR Handler_BouncingWheel_TickAnimation Jump to Handler_BouncingWheel_TickAnimation.
Handler_BouncingWheel_ThrustUp 998E LD (IX+$06),$FD Write FD to the vertical thrust.
9992 JR Handler_BouncingWheel_TickAnimation Jump to Handler_BouncingWheel_TickAnimation.
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