# HG changeset patch # User Paul Boddie # Date 1326065643 -3600 # Node ID fdc7521435842c5e76c3020a09d78ab84adbe7ca # Parent 579ebc9db48b41705f59e547a8fa081dcd124430 Added some timing information and some general notes on system improvements. diff -r 579ebc9db48b -r fdc752143584 Electron.txt --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Electron.txt Mon Jan 09 00:34:03 2012 +0100 @@ -0,0 +1,54 @@ +Potential Design Improvements for the Acorn Electron +==================================================== + +The Acorn Electron was designed to be a variant of the BBC Microcomputer that +was intended to be simpler, easier and cheaper to produce whilst retaining a +degree of compatibility and offering many of the same features, principally +the wide range of graphics modes, BBC BASIC, and extensible hardware and +software capabilities. Upon its introduction in late 1981, the BBC Micro +competed favourably against its immediate contemporaries, such as the ZX81 and +VIC-20, as well as machine introduced slightly later, such as the ZX Spectrum +and Commodore 64. By producing a less expensive machine that retained certain +key features, the motivation was to bring BBC Micro technology to bear on the +lower end of the home computer market, albeit approximately two years after +its initial introduction. + +Unfortunately, various features were omitted from the Acorn Electron that made +it less competitive than it could have been against a steadily improving range +of competitors: multi-channel sound support, MODE 7 teletext, support for +relatively smooth horizontal hardware scrolling (and other display control +features), and the double-speed bus with interleaved CPU and video access. +More RAM would also have been beneficial, although costly at the prices of the +day. Such deficiencies outweighed the significant benefits of substantial +software compatibility, and some of them effectively curtailed that +compatibility by making even reasonably well-written software titles +effectively unusable, particularly games relying on the BBC Micro's hardware +scrolling capabilities, including "official" Acornsoft titles. + +In hindsight, numerous features could be suggested that would make the +Electron more competitive, but many of these features would incur a +substantial cost. For example, giving the Electron 64K of RAM would have +increased the price substantially. Introducing the double-speed bus and faster +memory may also have increased the price in a prohibitive fashion. Thus, it +becomes worthwhile to consider minimal alterations to the machine's +specification that offer the greatest benefits for the least additional cost. + +Improving System Performance +---------------------------- + +Although RAM is accessed by the CPU at 1MHz, ROM is accessed at 2MHz. Thus, +deploying software that runs from ROM can potentially provide significant +performance benefits. Since the unexpanded Electron provides no convenient +means of installing ROM-based software - the Plus 1 and other expansion units +offered ROM cartridge slots, and various expansions provided ROM sockets - the +improved Electron would ideally need to have a ROM cartridge slot. A +side-benefit of adding this feature would arguably be an increased demand for +cartridge-based software, potentially at a slightly higher price and also +offering additional hardware features if necessary, thus making any cost +incurred in the manufacture of the base unit more bearable. + +The Slogger/Elektuur turbo board modified the system to permit the CPU to +access the bottom 8K of RAM without interruption by the ULA. This feature, +already known at Acorn during the Electron's design period, permitted +substantial improvements to performance and could also be incorporated into an +improved Electron. diff -r 579ebc9db48b -r fdc752143584 ULA.txt --- a/ULA.txt Tue Dec 20 00:56:33 2011 +0100 +++ b/ULA.txt Mon Jan 09 00:34:03 2012 +0100 @@ -11,6 +11,12 @@ See: Acorn Electron Advanced User Guide See: http://mdfs.net/Docs/Comp/Electron/Techinfo.htm +Access to RAM involves accessing four 64Kb dynamic RAM devices (IC4 to IC7, +each providing two bits of each byte) using two cycles within the 500ns period +of the 2MHz clock to complete each access operation. + +See: Acorn Electron Service Manual + Hardware Scrolling ------------------