PIO

The programmed input-output (PIO) is a module that serves as an interface to connect generic devices to the CPU. It is based on Intel's PPI 8255 in "mode 0," but with some modifications to simplify its operation.

It has two programmable 8-bit ports (A and B). The available registers are:

• PA (address 30h in the I/O memory),
• PB (address 31h in the I/O memory),
• CA (address 32h in the I/O memory),
• and CB (address 33h in the I/O memory).

The value of port A is stored in the PA register, and its configuration is stored in the CA register. The CA register is also 8 bits and tells the PIO the mode of each bit: a 0 if it's an output, and a 1 if it's an input. For example, with CA = 00001111b, the four most significant bits are outputs, and the four least significant bits are inputs. Port B works in the same way.

The PIO can be connected to switches and LEDs or to a printer.