Nightshade: Routine at DFFA

DFFA: Test Cell Moving Up

Used by the routine at TestMoveUp.

Tests an object that is moving up the screen (along the positive Y axis) against the walls in one cell. If the object hits a wall then its velocity along the Y axis is reduced by twice the depth that it has gone into the wall, and the flag for a collision is set.

Input
DE	X position to test
HL	Y position to test
IX	Object

Output
F	The carry flag is set if there was a collision

Check the walls, then push the object back if it is touching one.

TestCellMovingUp

DFFA

CALL CheckWallsHorizontalFirst

Check whether the object hits a wall in the cell.

DFFD

RET NC

Return if it did not.

This entry point is used by the routine at TestCellMovingDown.

Reduce the velocity along the Y axis.

ReduceVelocityY

DFFE

ADD A,A

Double the depth.

DFFF

ADD A,(IX+$0B)

Add it to the velocity along the Y axis, which stops the object at the wall.

E002

LD (IX+$0B),A

This entry point is used by the routine at TestCellMovingRight.

Set the flag for a collision.

SetCollisionFlag

E005

SET 0,(IX+$07)

Set the flag for a collision.

E009

SCF

Set the carry flag and return.

E00A

RET