# HG changeset patch # User Paul Boddie # Date 1327870213 -3600 # Node ID d3cc0494f825319d9285062e52581bb83b6c483f # Parent d357bdd67748cc1e63ed5371e57955ca656e34da Added more timing notes to the ULA document. Tidied up the wording in the discussion of the Electron, adding some remarks about turbo board hardware modifications. diff -r d357bdd67748 -r d3cc0494f825 Electron.txt --- a/Electron.txt Sun Jan 29 20:40:59 2012 +0100 +++ b/Electron.txt Sun Jan 29 21:50:13 2012 +0100 @@ -7,7 +7,7 @@ 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 +VIC-20, as well as machines 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 @@ -41,14 +41,20 @@ 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. +improved Electron would ideally need to offer a ROM cartridge slot as part of +the unexpanded machine. A side-benefit of adding this feature to the base +machine 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. +improved Electron, although it presumably needs motherboard-level changes. +Such turbo boards may have employed an additional RAM chip to avoid +complicated changes to the memory access logic, since the ULA appears to +access four memory chips at once to provide each byte, and it is therefore not +possible to just "borrow" one of the chips in order to isolate 8K of RAM for +direct access by the CPU. diff -r d357bdd67748 -r d3cc0494f825 ULA.txt --- a/ULA.txt Sun Jan 29 20:40:59 2012 +0100 +++ b/ULA.txt Sun Jan 29 21:50:13 2012 +0100 @@ -13,7 +13,11 @@ 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. +of the 2MHz clock to complete each access operation. Since the CPU and ULA +have to take turns in accessing the RAM in MODE 4, 5 and 6, the CPU must +effectively run at 1MHz (since every other 500ns period involves the ULA +accessing RAM). The CPU is driven by an external clock (IC8) whose 16MHz +frequency is divided by the ULA (IC1) depending on the screen mode in use. See: Acorn Electron Service Manual