Knight Lore: Routine at B9E0

B9E0: Guard Heading West

Keeps a guard heading west, at a speed of 02, until it hits something. It then turns north.

Input
IX	Address of the guard

Output
H	Speed in Y
L	Speed in X

GuardHeadingWest

B9E0

LD HL,$00FE

Set the speed in X to FE and the speed in Y to 00.

B9E3

BIT 0,(IX+$0C)

Test whether the guard has hit something along the X axis.

B9E7

RET Z

Return if it has not.

B9E8

LD HL,$0200

Set the speed in X to 00 and the speed in Y to 02.

This entry point is used by the routines at GuardHeadingNorth, GuardHeadingEast and GuardHeadingSouth.

Turn the guard, to follow the wall.

TurnGuard

B9EB

LD A,(IX+$0D)

Fetch the direction flags of the guard.

B9EE

LD C,A

Stash them in C.

B9EF

INC A

Move to the next direction.

B9F0

AND %00000011

Keep the direction.

B9F2

LD B,A

Stash it in B.

B9F3

LD A,C

Fetch the direction flags.

B9F4

AND %11111100

Keep the other flags.

B9F6

OR B

Combine them with the new direction.

B9F7

LD (IX+$0D),A

Store the flags.

B9FA

RET

Return.