Rex (Side A): Routine at B4EA

B4EA: Computes offset into the room graphic data based on projectile y-delta

Used by the routines at B456, B4D3 and BE75.

Input
A	Projectile y-delta

B4EA

LD (IY+$05),A

Set y-delta

B4ED

LD E,A

Set offset into HL based on y-delta

B4EE

LD L,E

B4EF

RL E

B4F1

SBC A,A

B4F2

LD H,A

B4F3

ADD HL,HL

B4F4

ADD HL,HL

B4F5

ADD HL,HL

B4F6

ADD HL,HL

B4F7

ADD HL,HL

B4F8

LD E,(IY+$07)

Set DE to the base offset $FFFF left $0001 right

B4FB

LD D,(IY+$08)

B4FE

ADD HL,DE

Adds base offset to HL

B4FF

LD (IY+$07),L

Set computed offset into enemy configuration

B502

LD (IY+$08),H

B505

RET