MOS 2.00 (~June 1985) is for the BBC Model B+, the immediate successor to the Model B. This is a quick stop-gap machine between the Model B and the Master, which ships just 9 months later. Acorn were clearly already working on what was to become the Master.

The B+ has more ROM sockets than its predecessor, and each ROM socket can hold up to 32K.

The B+ ships with either 64K or 128K of RAM.

The extra RAM is used to support 20K of 'shadow RAM' (allowing screen memory to be stored in a separate area away from main memory) and 12K (or more in the 128K machine) of sideways RAM. Acorn stated this sideways RAM should not be used in 'applications that need to be compatible with future Acorn products' (Indeed this 12K is reserved for other purposes on the Master). Options for configuring this extra memory are more limited here than in the Master OS versions.

The CPU is a 6512, which improves Tube performance (via better clock circuitry) but is otherwise fully compatible with the 6502. It has an identical instruction set and speed. A new disk interface (WD 1770) is also introduced, which provides compatibility problems for some games.

It identifies on boot as 'Acorn OS 64K', no longer the 'BBC Computer'. Although the MOS always reports '64K', a sideways ROM can count the RAM and overwrite this. It seems that this is usually handled by SRAM Utils in DFS 2.xx ROM, where it looks for installed RAM in a particular set of RAM banks.

Changes from MOS 1.20 to MOS 2.00: