Writes the value from (HL) to the (C) port, then decrements B and HL.
OUTD
C is preserved, N is set, H, S, and P/V are undefined. Z is set only if B becomes zero after decrement, otherwise it is reset.
16 t-states
IN,IND,INDR,INI,INIR,OUT,OTDR,OUTI,OTIR