8255 Control and Status (Basic Mode Definition – MSFLAG=1)

DIO Base+03h (8255-1), DIO Base+07h (8255-2) - (Read/Write)

PCLDIR	Port C (lower) direction. Sets the direction of the port C I/O signals 0-3. 0 = output 1 = input ◄ (Reset value)
PBDIR	Port B direction. Sets the direction of the port B I/O signals. 0 = output 1 = input ◄ (Reset value)
MSELB	Group B mode selection. Sets the mode of operation for the group B signals. 0 = mode 0 ◄ (Reset value) 1 = mode 1 NOTE: 1. Only mode 0 is currently implemented. 2. All output registers are reset when the mode is changed.
PCUDIR	Port C (upper) direction. Sets the direction of the port C I/O signals 4-7. 0 = output 1 = input ◄ (Reset value)
PADIR	Port A direction Sets the direction of the port A I/O signals. 0 = output 1 = input ◄ (Reset value)
MSELA	Group A mode selection. Sets the mode of operation for the group B signals. 00h = mode 0 ◄ (Reset value) 01h = mode 1 1xh = mode NOTE: 1. Only mode 0 is currently implemented. 2. All output registers are reset when the mode is changed.
MSFLAG	Mode set flag. Selects the port configuration mode. 0 = Bit set/reset control register mode When MSFLAG is reset, this register is used to set/reset individual Port C bits. 1 = Basic mode definition control register mode ◄ (Reset value) When MSFLAG is set, this register is used for direction and mode selection. NOTE: When the control word is read, the value of MSFLAG is always 1, implying basic control word information is being read.