paul@33 | 1 | Potential Design Improvements for the Acorn Electron
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paul@33 | 2 | ====================================================
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paul@33 | 3 |
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paul@33 | 4 | The Acorn Electron was designed to be a variant of the BBC Microcomputer that
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paul@33 | 5 | was intended to be simpler, easier and cheaper to produce whilst retaining a
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paul@33 | 6 | degree of compatibility and offering many of the same features, principally
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paul@33 | 7 | the wide range of graphics modes, BBC BASIC, and extensible hardware and
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paul@33 | 8 | software capabilities. Upon its introduction in late 1981, the BBC Micro
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paul@33 | 9 | competed favourably against its immediate contemporaries, such as the ZX81 and
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paul@36 | 10 | VIC-20, as well as machines introduced slightly later, such as the ZX Spectrum
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paul@33 | 11 | and Commodore 64. By producing a less expensive machine that retained certain
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paul@33 | 12 | key features, the motivation was to bring BBC Micro technology to bear on the
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paul@33 | 13 | lower end of the home computer market, albeit approximately two years after
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paul@33 | 14 | its initial introduction.
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paul@33 | 15 |
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paul@33 | 16 | Unfortunately, various features were omitted from the Acorn Electron that made
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paul@33 | 17 | it less competitive than it could have been against a steadily improving range
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paul@33 | 18 | of competitors: multi-channel sound support, MODE 7 teletext, support for
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paul@33 | 19 | relatively smooth horizontal hardware scrolling (and other display control
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paul@33 | 20 | features), and the double-speed bus with interleaved CPU and video access.
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paul@33 | 21 | More RAM would also have been beneficial, although costly at the prices of the
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paul@33 | 22 | day. Such deficiencies outweighed the significant benefits of substantial
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paul@33 | 23 | software compatibility, and some of them effectively curtailed that
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paul@33 | 24 | compatibility by making even reasonably well-written software titles
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paul@33 | 25 | effectively unusable, particularly games relying on the BBC Micro's hardware
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paul@33 | 26 | scrolling capabilities, including "official" Acornsoft titles.
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paul@33 | 27 |
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paul@33 | 28 | In hindsight, numerous features could be suggested that would make the
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paul@33 | 29 | Electron more competitive, but many of these features would incur a
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paul@33 | 30 | substantial cost. For example, giving the Electron 64K of RAM would have
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paul@33 | 31 | increased the price substantially. Introducing the double-speed bus and faster
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paul@33 | 32 | memory may also have increased the price in a prohibitive fashion. Thus, it
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paul@33 | 33 | becomes worthwhile to consider minimal alterations to the machine's
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paul@33 | 34 | specification that offer the greatest benefits for the least additional cost.
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paul@33 | 35 |
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paul@33 | 36 | Improving System Performance
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paul@33 | 37 | ----------------------------
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paul@33 | 38 |
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paul@33 | 39 | Although RAM is accessed by the CPU at 1MHz, ROM is accessed at 2MHz. Thus,
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paul@33 | 40 | deploying software that runs from ROM can potentially provide significant
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paul@33 | 41 | performance benefits. Since the unexpanded Electron provides no convenient
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paul@33 | 42 | means of installing ROM-based software - the Plus 1 and other expansion units
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paul@33 | 43 | offered ROM cartridge slots, and various expansions provided ROM sockets - the
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paul@36 | 44 | improved Electron would ideally need to offer a ROM cartridge slot as part of
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paul@36 | 45 | the unexpanded machine. A side-benefit of adding this feature to the base
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paul@36 | 46 | machine would arguably be an increased demand for cartridge-based software,
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paul@36 | 47 | potentially at a slightly higher price and also offering additional hardware
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paul@36 | 48 | features if necessary, thus making any cost incurred in the manufacture of the
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paul@36 | 49 | base unit more bearable.
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paul@33 | 50 |
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paul@33 | 51 | The Slogger/Elektuur turbo board modified the system to permit the CPU to
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paul@33 | 52 | access the bottom 8K of RAM without interruption by the ULA. This feature,
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paul@33 | 53 | already known at Acorn during the Electron's design period, permitted
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paul@33 | 54 | substantial improvements to performance and could also be incorporated into an
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paul@36 | 55 | improved Electron, although it presumably needs motherboard-level changes.
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paul@36 | 56 | Such turbo boards may have employed an additional RAM chip to avoid
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paul@36 | 57 | complicated changes to the memory access logic, since the ULA appears to
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paul@36 | 58 | access four memory chips at once to provide each byte, and it is therefore not
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paul@36 | 59 | possible to just "borrow" one of the chips in order to isolate 8K of RAM for
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paul@36 | 60 | direct access by the CPU.
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paul@47 | 61 |
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paul@47 | 62 | Improving Display Capabilities
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paul@47 | 63 | ------------------------------
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paul@47 | 64 |
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paul@47 | 65 | Perhaps the simplest improvement to the display capabilities would be to
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paul@47 | 66 | permit the RGB output levels to "float" between the current TTL high and low
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paul@47 | 67 | states, presumably enforced by various circuits. This would permit the choice
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paul@47 | 68 | of colours beyond the primary and secondary colour selection at a cost of some
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paul@47 | 69 | extra palette bits in the ULA and an adjustment to the board circuitry.
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