The Acorn Archimedes is, or rather was, a range of computers produced
from the late 1980s to mid 1990s by Acorn Computers ltd. of
Cambridge, England. The successor to the original 6502-based BBC
Micro, the Archimedes machines were designed around the
original production 32-bit ARM2 processor and its chipset.
Initially released in August of 1987, it was the world's first microcomputer
with a RISC processor; and this was at a time when RISC was still
considered a valid design philosophy rather than a worn-out
buzzword.
Operating systems running on Archimedes hardware range from the
original stopgap Arthur operating system, through RISC OS (the
definitive Archimedes OS) to ARM Linux, NetBSD, via such
interesting waypoints as RISCiX and Nemesis.
Evolution of the Archimedes
The Archimedes specification and design evolved from various
commercial failures, the Cambridge
Workstation version of the unsuccessful Acorn
Business Computer in particular. Staff at a short-lived Acorn research facility
in Palo Alto were, inspired by work at Xerox, working on an
operating system known as ARX, which was to be the operating
system for the successor to the Acorn Business Computer. As can
probably be guessed from the 'research' label, no working operating
system ever emerged. Looking to cut their potential losses, Acorn
approached the BBC with their designs, and a plan for a stopgap
operating system (Arthur), offering the system as the
true replacement for the BBC Micro.
"The BBC said it was interested, so we put some red function keys on
it and changed the case colour. Our original research for the office
automation machine had said it had to be grey."
-- Roger Wilson
Production Systems
The BBC liked the system, and so with a market assured, Acorn put the
new machines into production. The initial product line consisted of the
A305 and A310, essentially the same systems but with 512k and 1Mb of
RAM respectively. The A410 and A440 were similar in design but
included an on-board ST506 hard disc controller. The A410 had 1Mb of
memory on board, the A440 had 4Mb, and a 20Mb hard disc drive.
These were later revised, by the addition of larger hard discs and a
faster version of the memory controller chip MEMC1a, to become
A410/1 and A440/1. At the same time, the A305 and A310 were dropped,
and the low-cost A3000 introduced (which, like the BBC Micro, and
its contemporaries the Commodore Amiga and Atari ST, had the
mainboard, disk drive and keyboard integrated into a single unit]).
The Archimedes Chipset
The Archimedes chipset used a unified 8MHz bus architecture, with a single
main bank of RAM containing all program and data memory, as well as
video and sound buffers. DMA to video and sound was handled by the
VIDC (VIDeo Controller) chip, in conjuction with the MEMC
(MEMory Controller, also known as 'Anna') chip. The MEMC provided basic DMA facilities,
memory timing and refresh, controls, as well as an inverse page
table memory mapping system, allowing sophisticated memory
management of up to 4Mb of fast page mode DRAMs. This 4Mb limit
quickly became painful to Archimedes owners. Many machines could handle more than 4Mb, but only with the addition of a separate MEMC chip for each additional 4Mb of RAM. The VIDC (also known,
for reasons probably lost in the annals of time, as 'Arabella')
could produce 12-bit colour outputs, and had a set of palette registers
which provided 16 palette entries to 4-bit (16-colour) video modes, or
provided part of the palette data for 8-bit/256-colour video
modes. The fixed part of the palette data for 256-colour modes
gave a very well-rounded display, and with a little colour dithering
(built into RISC OS) was almost indistinguishable from most 16-bit
displays.
The chipset was rounded out by the IOC ('Albion'), or Input/Output Controller,
which provided miscellaneous input/output functions, timers and timer
comparison interrupts.
The Last Archimedes
In 1990, a new flagship machine took over from the A440/1, namely the
A540. The main new feature of this machine was the use of the new
ARM3 processor instead of the ARM2 used in previous machines: this
ran at 25 or 33MHz, and featured a unified 4Kb level 2
instruction/data cache; theoretically, it made the machine up to six
times faster than the 8MHz ARM2s. This magnitude of speedup was
frequently realised in practice, helped by the density of ARM
instructions, and by reducing the CPU's dependency on main memory bandwidth also required for tasks such
as video DMA. The A540 also featured (naturally) a larger hard disc,
and an option to install an additional MEMC memory controller chip, giving
access to up to 8Mb of RAM.
1991 saw the release of the A5000, which was the first machine to
make use of a revised chipset, which replaced the MEMC and IOC chips
with an IOMD (Input/Output and Memory Device) chip, and also came
with a much revised version of RISC OS.
With the A5000, the 'Archimedes' moniker was officially dropped, making the A540 the last true 'Archimedes' machine; but
since the product line continues, so shall I...
Hot on the heels of this machine was the A4, Acorn's only RISC OS
laptop to see the light of day. Its design was based on an Olivetti
laptop chassis with a 640x480 grayscale LCD monitor, and the same
chipset as the A5000. A commercial failure due to its high price and
lack of flexibility; although this could be said of almost any Acorn
machine since the BBC Micro, it was particularly true of the
conspicuously overpriced A4.
Mass Market Failures
The next machines, announced in late summer 1992, were aimed at
reducing costs, both retail and manufacture. The basic Archimedes
chipset had fallen behind the times. Silicon processes were shrinking
while Acorn used essentially the same chips, without dramatically
improving performance. The shrinking process sizes, however, gave an
opportunity for a significant cost saving without extra work to
further improve performance. The entire chipset, ARM2, VIDC, MEMC1a and
IOC were integrated onto a single chip, vastly reducing the
complexity of system boards and the chip count. Also, since memory
technology had improved in the interim, the system bus and system
chip run at 12MHz instead of the previous 8MHz). This all-in-one system
chip, the ARM250, was used in the A3010, A3020 (both models
billed as replacements for the A3000, differentiated by the
expandability of RAM, inclusion of an on-board IDE controller only
on the A3020, and the fact that the A3010 had green function keys
rather than the traditional red.) and the A4000 which was in
appearance much like a slim-line A5000.
Then things got quiet for a good long time, until the RiscPC was
unveiled in 1994. The RiscPC was a breakthrough in its day, and
brought workstation class features to the home computer market:
massively expandable memory capabilities, a multi-processor bus
capable of accepting both multiple ARM processors or an Intel 486
as a second processor to run Windows software. Processor cards
included the new ARM6 and ARM7, and plans were in place for
ARM8, although these were largely scuppered by the introduction of
cards for Digital Semiconductors' (now Intel) StrongARM
processor. The RiscPC featured a much improved video controller chip, VIDC20
which could support far higher resolution and colour depths than the
VIDC10's 256 colour limitation. For the first time on an Acorn
RISC OS machine, the video subsystem had its own private memory and
data bus, which allowed higher bandwidth displays to be used without
impacting the processor's performance unduly.
The weak point of the RiscPC's design, however, was its cripplingly
slow CPU and system bus. The bus was still 32 bits wide, and ran at
16MHz. In comparison with the baseline Pentium system's bus of 64 bits
and 33MHz, this just wasn't sufficient for a scalable system, and in
particular was not sufficient to adequately support the 2-way
multiprocessing which the bus architecture was capable of physically
supporting.
The last machine to make it out of Acorn's workstation division
before they closed completely was the A7000. This was similar in many
ways to the A4000, being a small-footprint desktop case with an
integrated system chip, the ARM7500 (or ARM7500FE with floating
point accelerator). Several models were released, some intended as
diskless network nodes, all models having ethernet interfaces built in.
Shortly before the closure of the workstation division, the
development of the RiscPC's successor, named 'Phoebe' (Yes, after the
character from Friends...) was almost completed. This machine
featured standard PC components where custom ones had previously been
required, and included such joys as PCI slots, proper 16-bit sound,
and a yellow system case. None were ever sold.
And there ends the story of one of the greatest, most innovative lines
of home computers ever built, and never sold.