The BBC Model A was the entry-level machine in the range and represented an unexpanded Model B. The Model A could be upgraded or partly upgraded to a Model B at any time. This would involve the addition of, amongst other things, the later operating system contained in a single ROM, an extra 16K RAM, the analogue to digital converter and analogue port, the user versatile interface adapter and all the connectors visible from the underside of the machine.
Essentially the Model A was a tape-only 16K machine. The limited memory also limited the number of graphics modes available and therefore the amount of software which the Model A could support.