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<!doctype html public "-//W3C//DTD HTML 3.2//EN"><html><head><title>Processor Types</title><meta http-equiv="content-type" content="text/html; charset=ISO-8859-1" /><meta http-equiv="content-language" content="en" /><meta name="resource-type" content="document"><meta name="copyright" content="This document copyright 2002 by Richard Murray. Use for non-profit and education purposes explicitly granted."><meta name="author" content="Richard Murray"><meta name="rating" content="general"></head><!-- /assembler/history.html --><!-- --><!-- (C) Copyright 2002 Richard Murray --><!-- Designed by Richard Murray --><!-- rmurray@heyrick.co.uk --><!-- --><body bgcolor="#f0f0f0" text="#000000" link="#0022dd" vlink="#002288"><table border = "0" width="100%"> <tr> <td align=center width=100> <img src="arm3.gif" width=79 height=78 align = middle> </td> <td> <h1 align="center"><font color="#800080">Processor Types</font></h1> </td> <td align=center width=100> <img src="arm3.gif" width=79 height=78 align = middle> </td></table><p> <p><h2>Where did it come from?</h2>Anybody reading this in the UK will no doubt be familiar with Acorn's BBC micro. Some, sadly,seem to feel that the company never made it beyond that "odd thing with the black and redkeys", while others can cast their mind back to the moment they booted RISC OS 4 on theirnew Kinetic, and gloat.<p>Either way, Acorn made use of the 6502 processor in the Atom, some kits, and some rackmountmachines in the late seventies. As 1980 rolled in, the BBC went looking for a computer to fit aseries of programmes they wanted to produce. Rather than these days, when the programmes aremuch more likely to fit the computer; the BBC had in mind the sort of specification it waslooking for. A number of companies well known at the time tendered their designs. Acorn revampedthe Atom design, throwing into it as much as possible, and building an entire working machinefrom the ground up in a matter of days. That's the stuff legends are made of, and that seems tobe the stuff Acorn was good at, like "Hey, guys, let's pull an all-nighter and write anoperating system".<br>The BBC loved the machine, and the rather naffly named "The Micro Program" was releasedin 1982 alongside the BBC microcomputer. It filled school computer rooms. Many were sold. Notmany in American terms, but staggering in European terms.<br>The BBC micro, like earlier Acorn machines, was based around the 6502 processor - as were otherpopular computers such as the Apple II.<br>From the outset, you could have colour graphics and text on-screen. Not to be outdone, the BBCmicro offered seven screen 'modes' of varying types - ranging from high resolution monochrome toeight colour (plus eight flashing colours) and an eight colour 'teletext' mode that only required1K of memory per screen; a cassette interface for cheap and cheerful use, on-board provision fora floppy disc interface (you needed to add a couple of ICs like the 1772 disc controller, that'sall), serial, four channel analogue, eight channel digital I/O, tube for co-processors, a 1MHzsystem bus for serious fiddling and for harddiscs... and by adding a couple of extra components,you had built-in networking.<br>Econet might have been slow and simple, but it was a revolution in those days, when it wasstated that Bill Gates, among other notable gaffs, asked "what's a network?" - thoughthis may well be urban legend. In any case, running multiple processor systems, and networkingall sorts of machines was something that Acorn users were au fait with long before the PCmarketplace kicked off, never mind implementing such things for itself.<p>However, Acorn had their sights set on the future, and between 1983 and 1985 the ARM processordesign was designed by Steve Furber and Sophie Wilson (or, Roger Wilson, back then). This was aleap of faith and optimism, when only a year previous they had released a 32K 8 bit machine,they were then designing a 32 bit machine that could cope with up to 16Mb RAM, and some ROM aswell.<br>Why?<p>Acorn continued to produce the BBC micro and variants. Indeed, the production of their mostsuccessful version of the BBC micro - the Master - only finished in May 1993. However, back adecade in 1983 it was quite clear to the innovators inside Acorn that the next generation ofmachine should provide something far better than rehashing old ideas over and over. In this, laythe problem. Which processor to use? There was nothing that stood out from the crowd. Acorn hadproduced a machine with the 16 bit 6502-alike, the 65C816, but this wasn't up to the vision thatAcorn had. They tried all of the 16 and 32 bit processors available by building secondprocessor units for the BBC micro to aid in their evaluation.<p>So there was one idea left. To <em>make</em> the processor that they were looking for. Somethingthat kept the ideals of the 6502, but provided raw power. Something small, cheap - both toproduce and to power, and something fairly simple both internally and to program. The importantearly design decisions were to use a fixed instruction length (which makes it possible toaccurately disassemble any random memory address simply by looking to see what is there - everyinstruction is word aligned), and to use a load/store model.<p>In that day, companies were talking about extending their CISC processors. The 8088 became the80186 (briefly), the 80286, and so on to the processor it is today. RISC processors existed, butthe majority of them were designed in-house as embedded controllers. Acorn took their ideas andrequirements and wrote a BASIC program that emulated the ARM 1 instruction set. The designers ofthe processor were new to processor design, some of the tools used were not exactly cutting edge.This prevented the processor design from being large and complex, which in it's way was the bestthing, and is now being spun as a 'plus' for the ARM processor, as indeed it is.<br>While Acorn had very clear ideas of what they wanted the processor to do, they also wanted goodall-round performance, rather than something so tailored to the end design that it obsoletesitself.<p>So. For the processor, Acorn rolled their own.<p>Please, take a moment to consider this.<br>Not only did Acorn create an entire powerful and innovative operating system with a tinycrew (MicroSoft probably employs more people to clean their toilets than Acorn employed intotal); but <i>they also designed their own chipset</i>.<br>So basically these guys designed an entire computer from the ground up, on a tiny budget andwith a tiny workforce.<p>You can fault Acorn for many things - lack of development, lack of advertising - but youcan <i>never</i> fault them for having the sheer balls to pull it off in the first place.<p>At the time the "Archimedes" was released, it was widely touted as the world'sfastest desktop machine. It also boasted a display system that could spit out loads ofdifferent resolutions. My A5000 (same video hardware) can output 640x480 at 256 colours, or800x600 at 16 colours. It doesn't sound impressive, but this was using hardware developed in themid '80s. The rest of the world (save Apple Macs) was using CGA and like; or Hercules for thetruly deranged!<p>Not a lot was made of the fact that the machines were RISC. Maybe Acorn figured the name ofthe operating system (RISC OS) was a big hint. Maybe they figured they had enough going forthe machine without getting all geeky.<p>So when, in the early '90s, Apple announced the world's first RISC desktop machine, welaughed. And Acorn ran a good-humoured advert in the Times welcoming Apple to RISC.<p>The chipset was:<ul> <li> ARM2<br> This is the central processor, and originally stood for "Acorn RISC microprocessor" (rather than "ARM RISC machine", or whatever they've called it today (may I suggest "Advanced RISC Microprocessor"?)). <br> <br> <li> MEMC1 (Anna)<br> This was the MEMory Controller. It was very soon replaced by the MEMC1a, which I do not think had a name.<br> The RiscPC generation of machines use a MMU (Memory Management Unit). <br> <br> <li> VIDC1 (Arabella)<br> This was the VIDeo Controller, though due to all it was capable of doing to pixels and sound, many knew it as the Very Ingenious Display Contraption. Certainly, the monitors that <i>cannot</i> be supported under RISC OS are few and far between. It is a trivial matter to switch from a modern 21" SVGA monitor to a television monitor.<br> The RiscPC generation of machines use the VIDC20, which takes it the logical step further. Unfortunately, the VIDC is no longer able to keep up with the latest advances in display driver technology. Enter J. Kortink with his ViewFinder. <br> <br> <li> IOC (Albion)<br> This was the Input/Output Controller, and it looked after podules and keyboards and basically anything that did I/O. In a flash of inspiration, it offered an IIC interface which is available on the expansion bus. My teletext receiver is hooked into this.<br> RiscPC generation machines use the IOMD which is like a souped up IOC.</ul>The ARM250 (mezzanine / macrocell) offered the ARM chipset on one piece of silicon. It wasused in the A3010 and A3020 machines. It may have also been used in the A4000, but I'venot seen inside such a machine.<p> <p>The original operating system of the ARM-based machine was to be ARX, but it was taking too longand was running overbudget. So Arthur was designed. It has been said that Arthur's name derivesfrom the porting of the BBC MOS "<em>A R</em>ISC operating system by<em>Thur</em>sday". Sadly, it has a lot of the hang ups of the BBC micro, such as a lack ofmemory protection (like 'modules' running in SVC mode (really only the kernel should run inSVC mode)), there's a plethora of unrelated things done with the OS_Byte SWI, the service callmechanism...<br>From Arthur came RISC OS, which improved certain aspects of the system, but perhaps the mostsignificant improvement was the Desktop. Instead of a bizarre looking (and horribly coloured)thing that could only run a task at a time, it introduced proper co-operative multitasking.<br>The debate between pre-emtive and co-operative multitasking is legion, but I feel that Acornwanted co-operative. That it was a design decision instead of a cop-out. Because, while it makesit slightly harder to program and more liable to problems with errant tasks, it fits sobeautifully into Acorn's ethos. There's no process 'protection' like on Unix. You can drop toa privileged processor mode with little more than a SWI call, and a lot of stuff (that probablyshouldn't) runs in SVC mode. Because, at it's heart, RISC OS is a hacker's operating system. Notthe same type of 'hacking' that Linux and netbsd comes from - such things were not known in thehome/office computer sector in those days, but in it's way, RISC OS is practically begging foryou to whip out the disassembler and start poking around it's internals. The original Arthur PRMssaid that any serious application would be written in assembler (a view they later changed, tosuggesting serious applications would be written in C).<p> <p>When the ARM processor team split off into ARM Ltd, they adopted a new numbering system for theprocessors. Originally, the numerical suffix reflected the revision of the device, the ARM 1, theARM 2, the ARM 3 ... followed by the ARM two-and-a-half, which is 250 in the tradition ofmultiplying version numbers by a hundred.<p>Now, the single number reflects the macrocell as is always - ARM6, ARM7...<p>A processor with a twin number denotes a self-contained processor and basic interface circuitry,like the ARM60 and the VIDC20 (VIDC not strictly a processor, but part of the ARM chipset).<p>A processor with a triple number denotes the processor macrocell combined with other macrocells,or custom logic, like the ARM610 and the ARM710. Because of the simplicity of the designs, andthe predefined parts, the ARM610 went from specification to silicon in under four months. Shortdevelopment times are invaluable for custom devices where every development day matters... Italso matters that ARM's designs will arrive on time, so you don't end up with your computer orPDA (or whatever) sitting there awaiting the processor. Within ARM's converted barn, a line ofopened champagne bottles line the staircase - a testament to how many of their designs workedfrom the very first silicon implementation - which is virtually every single one of them.<p> <p> <p>So there you have it.<p>From an idea to a global leader in microprocessors (Intel has said recently it is making moreARM silicon than x86 silicon), the ARM processor's birth is wrapped in spectacular innovation.<p>While it is not entirely certain where RISC OS is heading, one thing is for sure. The beautifulprocessor in our RISC OS machines is going from strength to strength.<p>We at Heyrick wish ARM Ltd all the best...<p><hr size = 3><a href="index.html#03">Return to assembler index</a><hr size = 3><address>Copyright © 2002 Richard Murray</address></body></html>⌨️ 快捷键说明
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