So what is a Master 512? What's it for? How is it different from a Master 128, and what does the 512 refer to?
Looking at it from a hardware level, a Master 512 is a BBC Master 128 with an Acorn 80186 co-processor board fitted, and a Master 512 mouse attached.
On the software side, the Master 512 provides a 80186 environment in place of the familiar 6502-based system we are used to. The 80186 cannot run 6502-based software, so the normal language ROMs do not work. Instead, the DOS Plus operating system is provided on disc. As the 80x86 series of processors are commonly found in PCs, what we end up with is a type of early Personal Computer running DOS Plus, which is an alternative to MS-DOS.The 512 refers to the amount of RAM on the co-processor board. That's half a megabyte, you know, and the same as the earliest Archimedes!
In this picture, we can see the 80186 co-processor board in more detail.
At its heart is the Intel 80186 CPU, surrounded by some logic chips and rather a lot of RAM chips. In addition, we have the two Master 512 boot ROMs on either side of the Acorn Computers marque, and to the right is the Tube ULA chip, a feature of all second processors.