Shadow RAM (Acorn)

Shadow RAM, on the Acorn BBC Micro, Master-series and Acorn Electron microcomputers is the name given to a special framebuffer implementation to free up main memory for use by program code and data. Some implementations of shadow RAM also permit double-buffered graphics.

Background

The BBC Micro, Master-series and Electron machines use the 8-bit 6502 and 65C102 processors with a 16-bit address space. This address space is split into 32 KB RAM (0x0000 to 0x7FFF), 16 KB sideways "paged" address space (0x8000 to 0xBFFF) and 16 KB operating system space (0xC000 to 0xFFFF). Video or screen memory is typically allocated from 0x7FFF downwards as necessary, occupying as little as 1 KB for Teletext mode 7 (and thus the region from 0x7C00 to 0x7FFF), or as much as 20 KB for modes 0-2 (and thus the region from 0x3000 to 0x7FFF). Thus, screen memory can therefore occupy a considerable amount of the available directly-addressed 32 KB RAM.

Overview

Shadow RAM is a block of RAM that can be considered to reside in parallel to the normal memory map and is accessed by the system only under certain conditions. When shadow RAM is enabled, the memory region normally used for screen memory becomes available for BASIC program use and for applications employing officially documented operating system interfaces. Given the maximum requirement of 20 KB for screen memory with the systems concerned, the amount of shadow RAM provided is typically 20 KB.

Shadow RAM was fitted as standard on the BBC Micro Model B+ and on the BBC Master series, but was an optional feature provided by third-party expansions on earlier BBC Micro systems and the Acorn Electron. The Aries-B20 product, initially sold by Cambridge Computer Consultants,[1] offered 20 KB shadow RAM for the BBC Model B, transparently diverting non-framebuffer accesses to the shadow RAM for addresses in the 20 KB video memory region.[2]

BBC Master Implementation

Unlike the expansion boards for earlier systems, the BBC Master implementation of shadow RAM permits the selection of shadow memory instead of main memory for use as screen memory, this being done via the Access Control Register.[3] By switching between main and shadow memory on alternate frames, double-buffered video could be used. Acorn provided a demonstration program in BASIC showing scrolling cloud animation with and without double buffering. The video game Firetrack would also use double buffering if shadow RAM was present.

On the BBC Master (and also the BBC Model B+[4]), shadow RAM is activated by setting the most significant bit of the memory mode number. For example, to use mode 1 with shadow RAM enabled, mode 129 (128 combined with 1) is selected.

Another significant difference between the Master implementation of shadow RAM and previous implementations also offering 32 KB of shadow RAM is the allocation of the extra 12 KB beyond the 20 KB shadowing the screen memory. Instead of this memory occupying a single region from 0x8000 to 0xAFFF in sideways RAM space, as it does with the Aries-B32 product and the BBC Model B+, it instead occupies two regions in the Master as "private RAM": a 4 KB region from 0x8000 to 0x8FFF holding function key definitions, workspace for the operating system, and character and font definitions; an 8 KB region from 0xC000 to 0xDFFF holding paged (sideways) ROM and operating system workspace. Thus, the Master was able to support character set redefinition and to allocate memory to filing systems without the amount of available user RAM being reduced.[5]

Further Refinements

Subsequent products augmented the shadow RAM with additional RAM that could be used for other purposes. For instance, the Aries-B32 product permitted shadow/sideways RAM combinations of 20 KB/12 KB and 16KB/16KB, or the use of the 32 KB RAM as two sideways RAM banks.[6] The Slogger Master RAM Board offered a 32K RAM solution for the Acorn Electron alongside a "turbo mode" enhancement.[7]

Patent Dispute

A dispute arose between the designers of the Aries-B20 shadow RAM board (Aries Computers Limited) and two other companies offering similar products, Raven Micro Products and Watford Electronics, over the alleged infringement of patent GB2137382A describing techniques employed in the design of the Aries-B20 board.[8] The products involved were the Raven Micro Products (Raven-20) and Watford Electronics (32K RAM Expansion Board). Ultimately, in 1986, Watford Electronics acquired Aries Computers in a "five figure deal" including the patents involved, with Watford subsequently selling Aries' products alongside the company's own.[9]

References
  1. "52k BBC micro". Acorn User. January 1984. p. 10. Retrieved 10 October 2020.
  2. "A 20K RAM boost for the 'B' with a unique paging system". Acorn User. April 1984. p. 162. Retrieved 29 August 2020.
  3. The BBC Microcomputer System Master Series Reference Manual Part 1 (PDF). Acorn Computers Limited. 1986. pp. F.2-3. Retrieved 30 August 2020.
  4. "BBC B+ revealed". Acorn User. June 1985. pp. 7, 11. Retrieved 30 August 2020.
  5. Smith, Bruce (March 1986). "Master Work". Acorn User. pp. 73–77. Retrieved 19 October 2020.
  6. Aries-B32 32K RAM expansion for the BBC Microcomputer Model B (PDF). Aries Computers. p. 6. Retrieved 29 August 2020.
  7. Emblem, Bernard (April 1987). "Slogging it out". Acorn User. pp. 146–147. Retrieved 29 August 2020.
  8. "Patent clash over boards". Acorn User. December 1984. p. 16. Retrieved 29 August 2020.
  9. "Aries acquired". Acorn User. July 1986. p. 9. Retrieved 3 September 2020.
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