The following paragraphs summarise the main features of the 8255, which should be enough for most applications. More complex applications require the full data sheet.
The 8255 has three 8-bit ports (A, B and C) which may be used in three modes. Modes 2 and 3 use certain port C lines for handshaking. Mode 0 is easy to use for most applications.
The 8255 appears as four registers
| Address | Offset | Function |
| B000 | +00 | Port A |
| +01 | Port B | |
| +02 | Port C | |
| +03 | Control |
In mode 0, ports A and B may be set up as 8
input lines or 8 output lines.
Port C is divided into upper and lower 4-bit sections, each of
which may be set up for input or output.
| B003 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | Bit function |
| 0 | Port C lower = output | ||||||||
| 1 | Port C lower = input | ||||||||
| 0 | Port B = output | ||||||||
| 1 | Port B = input | ||||||||
| 0 | Mode 0 | ||||||||
| 1 | Mode 1 | ||||||||
| 0 | Port C upper = output | ||||||||
| 1 | Port C upper = input | ||||||||
| 0 | Port A = output | ||||||||
| 1 | Port A = input | ||||||||
| 0 | 0 | Mode 0: Basic input/output | |||||||
| 0 | 1 | Mode 1: Strobed Input/output | |||||||
| 1 | x | Mode 2: Bi-directional Bus | |||||||
| 0 | Mode select flag = inactive | ||||||||
| 1 | Mode select flag = active |
Thus to set port A for output, port B for input, port C lower for output and port C upper for input (the normal conditions in the Atom), we have to write 10001010 binary (8A hex) to B003. We can then read the data on port B at B001.
Port C lines can also be changed individually (if set for output) by writing to the control register a value equal to 2*BitNumber +Q where Q is the value of the output.
When set for output, the 24 I/O lines can each drive one TTL load, sinking 1.7 mA at 0.4V, sourcing 200 mA at 2.4V. As inputs, they present a high impedance to the driving source, recognising voltages below 0.8V as '0' and voltages between 2.0V and 5V as '1'