The OS has calls to handle reading and writing to all I/O (ports and screen memory) and programmers are strongly advised to use these by the Acorn documentation. The reason for this is that when a second processor is installed, user software is run from the separate memory map on the far side of the Tube processor bus, and direct access to memory-mapped I/O registers and video memory is impossible. However, for the sake of performance, many apps including many games, write directly to main address space for I/O, and hence crash or give a blank screen if a 6502 second processor is attached. One such performance-critical area is sprite support: BBC Micro hardware does not support sprites, and games must implement sprites in software. In practice, the widespread use of direct access in place of the OS calls very rarely caused problems. Second processor units were expensive and very little software was written to make use of them, so few people bought them, and those who did have them could simply switch them off or unplug the cable if a problem arose.