BBC Micro expansion unit

A BBC Micro expansion unit, for the BBC Micro is one of a number of peripherals in a box with the same profile and styling as the main computer.

Second processors

The second processors are connected to Acorn's proprietary and trademarked Tube interface. Only one second processor could be connected at a time. The terminology of second processor was slightly misleading, since connected and switched on, the system worked on a "master-slave" model, with the processor in the external unit taking control while the 6502 in the "main" computer simply took on responsibility for I/O.

6502 Second Processor

The 6502 Second Processor (using a 65C02) was clocked at 3 MHz, a full 50% faster than the 6502 inside a BBC Model B, and also had 64 kB of RAM, of which typically 30–44 kB was free for application data (compared to as little as 8½ kB on an unexpanded Model B in graphics mode, or even less with the disc interface). A version of BBC BASIC, called Hi-BASIC was produced on ROM for this processor. (A version of BASIC II)