Routines
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B9EB: Walking guard: next heading
Used by the routines at guard_N, guard_E and guard_S.
Adds 1 to bits 0 and 1 of +13, wrapping round, and leaves the other bits alone.
next_guard_dir B9EB LD A,(IX+$0D) heading + 1, modulo 4
B9EE LD C,A
B9EF INC A
B9F0 AND $03
B9F2 LD B,A
B9F3 LD A,C
B9F4 AND $FC
B9F6 OR B
B9F7 LD (IX+$0D),A
B9FA RET
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