Routines
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DFFA: Cut a step in +V short at a box of a cell
Used by the routine at CLIP_PLUS_V.
Called by the +V routine (CLIP_PLUS_V) for each cell its object's leading edge covers. If the object's box, moved by this turn's step, overlaps one of the cell's boxes (HIT_BOXES_V), the step in V (+B) is cut by the overlap, so that the move ends with the object against the box's face; bit 0 of the flags (+7) is set to say it was stopped, and carry is returned.
The entry point CUT_STEP_V adds twice A, the overlap in half units, to the step; MARK_BLOCKED sets the flag. CUT_STEP_MINUS_V, CUT_STEP_PLUS_U and CUT_STEP_MINUS_U are the same for -V, +U and -U.
Input
D The column of the cell to test (the high byte of a U in it)
H Its row (the high byte of a V in it)
IX The object
Output
F Carry set if the step was cut
CUT_STEP_PLUS_V DFFA CALL HIT_BOXES_V Test the cell's boxes; nothing in the way returns with carry clear
DFFD RET NC
This entry point is used by the routine at CUT_STEP_MINUS_V.
CUT_STEP_V DFFE ADD A,A CUT_STEP_V: A is minus the overlap, in half units; twice it added to a positive step ends the move at the box
DFFF ADD A,(IX+$0B)
E002 LD (IX+$0B),A
This entry point is used by the routine at CUT_STEP_PLUS_U.
MARK_BLOCKED E005 SET 0,(IX+$07) MARK_BLOCKED: bit 0 of the flags says the object was stopped; return with carry set
E009 SCF
E00A RET
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