(C) <- r
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. The contents of Register r are placed on the top half (A8 through A15) of the address bus at this time. Then the byte contained in register r is placed on the data bus and written to the selected peripheral device. Register r identifies any of the CPU registers shown in the following table, which also shows the corresponding three-bit r field for each that appears in the assembled object code.
If Register C contains $01 and the D Register contains $5A, then upon the execution of
byte $5A is written to the peripheral device mapped to I/O port address $01.