Alien 8: Routine at C069

C069: Nudge Player North South

Sets the adjustment to the speed of the player along the x axis, to bring the player a step closer to the middle line of an arch that goes north and south. Nothing is done unless the sprite of the player is flipped.

Input
IX	The address of the arch object
IY	The address of the player

Skip if not flipped or already in the middle.

NudgePlayerNorthSouth

C069

BIT 6,(IY+$07)

Jump to NudgePlayerNorthSouth_Done if the player is not flipped.

C06D

JR Z,NudgePlayerNorthSouth_Done

C06F

LD A,(IX+$09)

Fetch the x of the doorway middle.

C072

CP (IY+$01)

Jump to NudgePlayerNorthSouth_Done if the player is in the middle.

C075

JR Z,NudgePlayerNorthSouth_Done

Step toward the middle along x.

C077

LD A,$01

Set A to 01.

C079

JR NC,NudgePlayerNorthSouth_Set

Jump to NudgePlayerNorthSouth_Set if carry is not set.

C07B

NEG

Negate A.

NudgePlayerNorthSouth_Set

C07D

LD (IY+$0E),A

Set the x-speed adjustment of the player.

This entry point is used by the routine at NudgePlayerEastWest.

Finish the nudge.

NudgePlayerNorthSouth_Done

C080

POP BC

Restore BC from the stack.

C081

RET

Return.