Routines
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6e87: Flip byte pair of an alien sprite so it faces opposite direction.
Used by the routine at 6ea5.
Input
B Loop counter.
HL Pointer to current byte of sprite.
Output
B Same value as on entry.
DE The flipped byte pair.
HL Pointer to next byte of sprite.
6e87 ld a,b Backup B value
6e88 ex af,af'
6e89 ld c,$00
6e8b ld a,(hl) Get byte from sprite
6e8c call $6e7c A=reversed byte
6e8f ld e,a
6e90 inc hl Get next byte from sprite
6e91 ld a,(hl)
6e92 call $6e7c A=reversed byte
6e95 ld d,a
6e96 inc hl Point HL to next byte
6e97 ld a,b Return if all bytes have been processed
6e98 and a
6e99 ret z
6e9a sla e Shift sprite 4-pixels to the right?
6e9c rl d
6e9e rl c
6ea0 djnz $6e9a
6ea2 ex af,af' Restore B to the original value
6ea3 ld b,a
6ea4 ret
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