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It relies on OUTI
opcode to set some unexpected flags which are left over from the Z80 bus. Sean Young's "The Undocumented Z80 documented" explains how the flags are set in chapter 4.3:
OUTI
/OTIR
/OUTD
/OTDR
instructions. At every iteration, a byte is read from memory and sent to an I/O port. Take that value (call it n), and add register L
to it, after the HL
register is increased (OUTI
and OTIR
) or decreased (OUTD
and OTDR
). If that calculation makes a carry (ie. it's larger than 255), both CF
and HF
are set. Otherwise CF
and HF
are reset.