Input and Output // OUTI (Out and Increment), OTIR (Out and Increment Repeated), OUTD (Out and Decrement), OTDR (Out and Decrement Repeated)


The contents of the HL register pair are placed on the address bus to select a location in memory. The byte contained in this memory location is temporarily stored in the CPU. Then, after the byte counter (B) is decremented, the contents of Register C are placed on the bottom half (A0 through A7) of the address bus to select the I/O device at one of 256 possible ports. Register B can be used as a byte counter, and its decremented value is placed on the top half (A8 through A15) of the address bus. The byte to be output is placed on the data bus and written to a selected peripheral device. Finally, the register pair HL is incremented.


If Register C contains 07h, Register B contains 10h, the HL register pair contains 100014 and memory address 1000h contains 5914, then upon the execution of an OUTI instruction, Register B contains 0Fh, the HL register pair contains 1001h, and byte 59h is written to the peripheral device mapped to I/O port address 07h.