Design
The Master featured several improvements on its predecessor. The systems had 128 kB RAM as standard, alleviating the shortage of available RAM which had amongst other things discouraged use of the best graphics modes in the original design, and had two cartridge slots mounted above the new numerical keypad. These were physically identical to those used by the Acorn Electron 'Plus 1' interface, but with enhanced electrical characteristics for some of the cartridge connector pins. Rather than the MOS Technology 6502 microprocessor used by the Model B it ran on the slightly improved 65SC12: the cost of this CPU compatibility with the Model B was that the address bus was still only 16 bits, meaning that only 64 kB could be directly addressed at any one time and the remaining memory had to be paged in as required.
This paging occurred via 3 separate pages, each with a Code-name, per previous BBC Microcomputer architecture tradition:
| Codename | Page | Description |
|---|---|---|
| LYNNE | 0x3000 - 0x8000 | CRT frame-switch region (Allows All screen modes to be used without using main memory) |
| HAZEL | 0xC000 - 0xDFFF | ROM/RAM switching |
| ANDY | 0x8000 - 0x8FFF | ROM/RAM switching |
However the 65SC12's extra instructions allowed a little more to be shoehorned into the OS and BBC BASIC ROMs, limited by the memory architecture to 16kB each. The improved version of BBC Basic was named Basic4.
Although the Master was intended to be compatible with "legally written" software for the older models, there were some problems running older programs, particularly games. Conversely, although few programs were ever targeted specifically at Master series machines (except the Master 512), many later BBC games (and Master versions of earlier classics such as Elite) included enhanced features which took advantage of the extra memory.
Read more about this topic: BBC Master
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