Sabre Wulf: Routine at B9A9

B9A9: Display Entity

Used by the routines at HandlerOrchid, DisplayActor and GamePlay_Start.

Moves an entity on the screen, by erasing the sprite at its old position (saved by StoreEntity) and drawing it at its new one.

Both sprites are drawn with XOR, a row at a time, so that the rows are alternated between the two sprites and the screen is never without the sprite for long. If the sprites are offset vertically, the rows of the sprite that has no matching row in the other one are drawn first.

Before	After DisplayEntity and ColourEntity

Input
IX	Actor record

DisplayEntity

B9A9

Prepare the registers for the sprite at its new position.

B9AC

EXX

Switch to the registers of the old sprite.

B9AD

CALL PrepDraw23

Prepare the registers for the sprite at its old position.

B9B0

LD A,($96A1)

Fetch the old Y position from Previous_Y_Position.

B9B3

SUB (IX+$04)

Subtract the new Y position of the entity.

B9B6

JP Z,XORSpriteLoop_Reload

Jump to XORSpriteLoop_Reload if the Y position has not changed.

B9B9

JP M,DisplayEntity_NewLower

Jump to DisplayEntity_NewLower if the sprite has moved down the screen.

The old sprite is lower than the new one, so draw the bottom rows of the old sprite on their own first.

B9BC

LD C,A

Copy the number of rows that stand alone to C.

B9BD

LD A,($96A3)

Fetch the number of rows of the old sprite from Old_Sprite_Row_Count.

B9C0

CP C

Compare it with the number of rows that stand alone.

B9C1

JP C,XORSpriteLoop_Reload

Jump to XORSpriteLoop_Reload if the old sprite has no more rows than that, so that the sprites do not overlap and are each drawn in full.

B9C4

SUB C

Subtract the number of rows that stand alone.

B9C5

LD ($96A3),A

Write the number of rows that remain to Old_Sprite_Row_Count.

B9C8

JP XORSpriteLoop

Jump to XORSpriteLoop to draw those rows of the old sprite on their own.

View the equivalent code in;