1 Potential Design Improvements for the Acorn Electron
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3
4 The Acorn Electron was designed to be a variant of the BBC Microcomputer that
5 was intended to be simpler, easier and cheaper to produce whilst retaining a
6 degree of compatibility and offering many of the same features, principally
7 the wide range of graphics modes, BBC BASIC, and extensible hardware and
8 software capabilities. Upon its introduction in late 1981, the BBC Micro
9 competed favourably against its immediate contemporaries, such as the ZX81 and
10 VIC-20, as well as machines introduced slightly later, such as the ZX Spectrum
11 and Commodore 64. By producing a less expensive machine that retained certain
12 key features, the motivation was to bring BBC Micro technology to bear on the
13 lower end of the home computer market, albeit approximately two years after
14 its initial introduction.
15
16 Unfortunately, various features were omitted from the Acorn Electron that made
17 it less competitive than it could have been against a steadily improving range
18 of competitors: multi-channel sound support, MODE 7 teletext, support for
19 relatively smooth horizontal hardware scrolling (and other display control
20 features), and the double-speed bus with interleaved CPU and video access.
21 More RAM would also have been beneficial, although costly at the prices of the
22 day. Such deficiencies outweighed the significant benefits of substantial
23 software compatibility, and some of them effectively curtailed that
24 compatibility by making even reasonably well-written software titles
25 effectively unusable, particularly games relying on the BBC Micro's hardware
26 scrolling capabilities, including "official" Acornsoft titles.
27
28 In hindsight, numerous features could be suggested that would make the
29 Electron more competitive, but many of these features would incur a
30 substantial cost. For example, giving the Electron 64K of RAM would have
31 increased the price substantially. Introducing the double-speed bus and faster
32 memory may also have increased the price in a prohibitive fashion. Thus, it
33 becomes worthwhile to consider minimal alterations to the machine's
34 specification that offer the greatest benefits for the least additional cost.
35
36 Improving System Performance
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38
39 Although RAM is accessed by the CPU at 1MHz, ROM is accessed at 2MHz. Thus,
40 deploying software that runs from ROM can potentially provide significant
41 performance benefits. Since the unexpanded Electron provides no convenient
42 means of installing ROM-based software - the Plus 1 and other expansion units
43 offered ROM cartridge slots, and various expansions provided ROM sockets - the
44 improved Electron would ideally need to offer a ROM cartridge slot as part of
45 the unexpanded machine.
46
47 A side-benefit of adding this feature to the base machine would arguably be an
48 increased demand for cartridge-based software, potentially at a slightly
49 higher price and also offering additional hardware features if necessary, thus
50 making any cost incurred in the manufacture of the base unit more bearable.
51 And in school environments where unexpanded BBC Microcomputers were often used
52 with tapes, the use of ROM cartridges for instant and reliable loading of
53 software would have given the Electron a practical advantage: it would be a
54 cheaper machine that could later be expanded with disk drives (the other main
55 way of providing fast and reliable storage) while still offering a substantial
56 saving over the BBC Micro.
57
58 The Slogger/Elektuur turbo board modified the system to permit the CPU to
59 access the bottom 8K of RAM without interruption by the ULA. This feature,
60 already known at Acorn during the Electron's design period, permitted
61 substantial improvements to performance and could also be incorporated into an
62 improved Electron, although it presumably needs motherboard-level changes.
63 Such turbo boards may have employed an additional RAM chip to avoid
64 complicated changes to the memory access logic, since the ULA appears to
65 access four memory chips at once to provide each byte, and it is therefore not
66 possible to just "borrow" one of the chips in order to isolate 8K of RAM for
67 direct access by the CPU.
68
69 Being able to disable the ULA's access to RAM for a period of time while also
70 disabling the video signal, effectively achieving the same as blanking the
71 palette, would be a very simple but useful enhancement that would speed up
72 programs needing to render large amounts of non-real-time content to the
73 screen.
74
75 Improving Display Capabilities
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77
78 Perhaps the simplest improvement to the display capabilities would be to
79 permit the RGB output levels to hold intermediate values between the current
80 high and low states, presumably enforced by various circuits. This would
81 permit the choice of colours beyond the primary and secondary colour selection
82 at a cost of some extra palette bits in the ULA and an adjustment to the board
83 circuitry and would only benefit UHF and colour composite video displays, but
84 the latter limitation might not be a significant issue for the majority of the
85 intended audience.
86
87 Supporting Local Area Networking
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89
90 The Electron was mostly aimed at the home market, but a cheaper computer would
91 have been very attractive for schools, especially those wanting to purchase a
92 number of machines for networking. Having an option available in the standard
93 Electron would have given such customers a cheap Econet terminal, albeit
94 without the MODE 7 capabilities of the BBC, whilst ignoring the requirement of
95 reliable, fast storage that standalone machines need to have in such
96 environments. With content available on demand over the network, the need for
97 low-memory screen mode usage - in order to retain as much content in memory as
98 possible - would also be diminished.
99
100 About this Document
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102
103 The most recent version of this document and accompanying distribution should
104 be available from the following location:
105
106 http://hgweb.boddie.org.uk/ULA
107
108 Copyright and licence information can be found in the docs directory of this
109 distribution - see docs/COPYING.txt for more information.