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

The first byte includes a bit that should always be set for a normal packet. Use that bit to recover if parsing starts in the middle of a stream. This is the same general idea as finding a start marker in a serial protocol: the parser needs a way to regain trust after it loses its place.

Signed Movement

X and Y movement are signed values. The sign bits are in byte 0, while the low movement bytes are byte 1 and byte 2. This split forces you to assemble the value deliberately.

Do not treat the movement bytes as ordinary unsigned coordinates. A mouse packet usually reports relative movement since the previous packet, not an absolute screen position. The application can later add those deltas to a cursor position, clamp the result to the screen, or use the movement to control a game 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 should invalidate or merely report movement. The parser should be able to receive one byte at a time and only expose a packet after all three bytes are available.

This state-machine shape will look familiar by the end of the course. UART framing, sprite parsing, and replay scripts all involve partial input that only becomes meaningful after enough bytes or tokens arrive.

TIP

Keep packet assembly separate from coordinate updates. A drawing program can use decoded deltas later; the lab library should only parse packets.