The EM320 is a very small assembly about 45mm x 48mm with all connections brought out to 0.1 spaced pins around it's edges. Connection to the main board is through pin socket strip so that the unit can be unplugged for use with another mouse.
The Dallas 80C320 is a fast 80C31/80C32 compatible microcontroller which can process instructions at 4 clocks/machine cycles instead of the normal 12.
The computer also contains a 82C55, but I only use the 2 I/O ports directly available from microcontroller.