Routines
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B1FA: Make a two-part creature one box for its move
Used by the routines at WANDERER and PACER.
Lowers the upper half's Z by 12 to where its lower half stands and doubles its height, so that the move and the collision code treat the two as one object; the lower half, in the next record, is taken out of the collision tests (bit 1 of its flags) meanwhile. PART_HALVES undoes it.
Input
IX the upper half
JOIN_HALVES B1FA LD A,(IX+$03) down to the lower half's Z
B1FD SUB $0C
B1FF LD (IX+$03),A
B202 SLA (IX+$06) twice the height
B206 SET 1,(IX+$27) the lower half out of collisions
B20A RET
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