Not all ROMs offered star commands (ROMs containing data files, for instance), but any ROM could claim certain vectors in order to extend the system's functionality. This feature was exploited in 1985 when Micro Power designed and marketed a Basic Extension Rom. This introduced new statements such as WHILE, ENDWHILE, CASE, WHEN, OTHERWISE, and ENDCASE, as well as many new direct mode commands including VERIFY. There were also DFS ROMs providing access to files on disc, and the Acorn GXR (Graphics eXtension ROM) added functionality to the VDU processing queue to allow flood fills and geometric shapes. Connecting an external EPROM programmer, one could write extensive programs, burn to PROM or EPROM, then run them without taxing user memory.
[[Acorn]] strongly discouraged programmers from directly accessing the system variables and hardware, favouring official system calls. This was ostensibly to make sure programs kept working when run on the Tube coprocessor, but it also made [https://www.romspedia.com/roms/bbc-micro BBC Micro] software more portable across the Acorn range. Whereas untrappable PEEKs and POKEs were commonly used on other computers to reach the system elements, both BBC BASIC and assembly language programs would instead pass parameters to an operating system routine. In this way the MOS could translate the request for the local machine's devices and memory layout or send it across the Tube interface, as direct access was impossible from the coprocessor. Published programs largely stuck to the API but games routinely made direct accesses for increased speed, and thus required a particular Acorn model or MOS version.
As the early '''BBC Micros''' had ample I/O allowing machines to be interconnected, and as many schools and universities employed the machines in Econet networks, numerous networked multiplayer games were created. With the exception of a tank game, ''Bolo'', few rose to popularity; in no small measure due to the limited number of machines aggregated in one place. A relatively late but well documented example can be found in a dissertation based on a ringed RS-423 interconnect.
[[Category:Non CPC Computers]]