Through the Trap Door: Routine at 54539

54539: Load B with index of room to left of an entity's current room

Used by the routines at 48641, 52098, 54283 and 54348.

Input
IX	Address of complex state data for an entity
D	(entry at 54542 only) Index of base room for check

Output
B	Index of room found if there is one, undefined otherwise
F	Zero flag set if there is no room to the left, reset otherwise

54539

LD D,(IX+0)

Load current room of entity pointed to by IX into D

This entry point is used by the routine at 53194.

54542

LD IY,(34250)

Load start address of current level's horizontal room connectivity data into IY

54546

INC IY

Advance by one byte to second byte (right room index) of first entry

54548

LD A,(IY+0)

Load second byte (right room index) of current entry into A

54551

CP 255

If byte is 255 (i.e. the end marker)...

54553

RET Z

...then return

54554

CP D

If byte is not the same as the room index in D...

54555

JR NZ,54563

...then skip ahead to 54563

54557

LD B,(IY-1)

Load first byte (left room index) of current entry into B

54560

CP 255

Reset zero flag (as current room cannot be 255)

54562

RET

Return

54563

INC IY

Advance by two bytes to the next entry...

54565

INC IY

...

54567

JR 54548

Loop back to 54548