Martianoids: Routine at A9C3

A9C3: Move Entity

Used by the routines at TryMove and MoveDestroyer.

Moves an entity in the direction that it is heading, by up to the number of steps that is given. The entity stops at the start of a cell if the next cell is not empty. The routine keeps a copy of the position, so that a move can be undone.

Input
B	Number of steps to take
DE	X position
HL	Y position
IX	Entity

Output
A	01 if the entity has been blocked
B	Number of steps that are left
DE	New X position
HL	New Y position
F	The zero flag is reset if the entity has been blocked

MoveEntity

A9C3

LD ($9653),DE

Keep the X position...

A9C7

LD ($9655),HL

...and the Y position, so that the move can be undone.

A9CA

LD A,(IX+$07)

Fetch the direction of the entity...

A9CD

AND $03

...and jump to move in direction zero (X decreasing)...

A9CF

JP Z,MoveLeft

A9D2

DEC A

...or direction one (Y increasing)...

A9D3

JP Z,MoveUp

A9D6

DEC A

...or direction two (X increasing)...

A9D7

JP Z,MoveRight

A9DA

JP MoveDown

...or otherwise direction three (Y decreasing).