Reading mouse packets

A basic PS/2 packet has three bytes:

Packet byte	Contains
byte 0	buttons, sign bits, overflow bits, synchronization bit
byte 1	X movement low byte
byte 2	Y movement low byte

Byte 0 includes a bit that should always be set for a normal packet. Use it to recover when parsing starts mid-stream, like a start marker in a serial protocol.

Signed movement

X and Y movement are signed. The sign bits are in byte 0; the low movement bytes are bytes 1 and 2. Assemble the value deliberately.

Do not treat movement bytes as unsigned coordinates. A packet usually reports relative movement since the previous packet, not an absolute position. The application can add deltas to a cursor, clamp to the screen, or control a camera.

Packet state

Track how many bytes have been collected, whether byte 0 is synchronized, whether a complete packet is ready, and whether overflow flags invalidate or only report movement. The parser receives one byte at a time and exposes a packet only after all three bytes arrive.

This state-machine shape returns in UART framing, sprite parsing, and replay scripts: partial input becomes meaningful only after enough bytes or tokens arrive.

TIP

Keep packet assembly separate from coordinate updates. The library parses packets; a drawing program uses the decoded deltas later.