The original MOS versions, from 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 programming language BBC BASIC ROM, with 6502 assembler built in, supplied with the BBC Micro is the default application used for this purpose. The BBC Micro would halt with a codice_1 error if no ROM is present that advertises to the OS an ability to provide a user interface (called "language ROMs"). MOS version 3 onwards did feature a simple command-line interface, normally only seen when the CMOS memory did not contain a setting for the default language ROM.