With a second processor connected, the CPU in the BBC micro became the 'host' and handled just the I/O (screen, keyboard, disk, etc.) while the second processor, the 'parasite', actually ran the main program. The BBC micro had a specialised interface for second processors known as the 'Tube' interface but the main Tube hardware (an ULA) was in the second processor unit, lowering the cost of the BBC Micro somewhat. Unique for microcomputers of the time, a number of different second processor units could be connected to the BBC microcomputer as to increase its performance so that programs ran faster, could handle more text, make better use of the high resolution graphics and so on.
