Acorn MOS - User Interface

User Interface

The original MOS versions – 0 to 2 – did not have a user interface per se: applications were expected to forward operating system command lines to the OS on its behalf, and the BBC BASIC ROM supplied with the BBC Micro is the default application used for this purpose. The BBC Micro would halt with a "Language?" error if no ROM is present that advertises to the OS an ability to provide a user interface (so-called "language ROMs"). MOS version 3 onwards did feature a simple command-line interface, normally only seen when the CMOS did not contain a setting for the default language ROM.

Application programs on ROM, and some cassette and disc-based software also, typically provide a command line, useful for working with file storage such as browsing the currently inserted disc. The OS provides the line entry facility and obeys the commands entered, but the application itself oversees running the command prompt.

Cassette and disc based software typically relies on BBC BASIC's own user interface in order to be loaded, although it is possible to configure a floppy disc to boot up without needing to have BASIC commands executed; in practice, this was rarely performed.

In BBC BASIC, OS commands are preceded with an asterisk or passed via the OSCLI keyword, to instruct BASIC to forward that command directly to the OS. This led to the asterisk being the prompt symbol for any software providing an OS command line; MOS version 3 onwards officially uses the asterisk as the command prompt symbol. When referring to an OS command, they generally include the asterisk as part of the name, for example *RUN, *CAT, *SPOOL etc., although only the part after the asterisk is the command itself.

Unrecognised commands are offered to any "service" (extension) ROMs; filing system ROMs will often check to see if a file on disc matches that name, the same most other command-line interfaces do. The operating system call OSWORD with accumulator = 0 does however offer programs single line input (with ctrl-U for clear line and the cursor copying keys enabled) with basic character filtering and line length limit.

The MOS command line interpreter features a rather unusual idea: abbreviation of commands. To save typing a dot could be used after the first few characters, such as *L. for *LOAD and *SA. for *SAVE. *RUN was abbreviated to */ alone. *CAT, the command to catalogue (list) a cassette or disc, can be abbreviated right down to *.