Acorn were a small British Company based Cambridge, England.
Acorn Computers Ltd. was founded in 1978 by Herman Hauser and Chris Currie.
In 1979, Acorn developed the Acorn Atom, the first commonplace Acorn.

The Atom had a mere 2KB of RAM and a 6502 Microprocessor.
it was available as a kit or as a ready-made machine.
In the early 1980's, the BBC started to think realistically about running a series of shows about computers.
So, in 1980 (IIRC), the BBC contacted Acorn and Sinclear to name a few, asking all involved to create a simple computer and present it to the BBC in due course.
The decision was finally made, Acorn, with their 'Proton', won. This was brilliant news for the little company, now Acorn's machine would be marketed (Acorn weren't brilliant at marketing themselves) by the largest broadcasting corporation in the British Isles.
It is romoured (I'd like to know for definite) that the Proton was built in a mere week, if it is true, it is unchallenged in computer industry.
So, in November 1981, the British Broadcasting Corporation Microcomputer Model A (BBC A) was released. This machine was a BBC B with half the memory.
The real making of Acorn, though, was to come in early 1982, the BBC B.

Shows such as 'BBC Live' and 'The Computer Show' appeared instantly.
The UK Government approved the fitting of most British schools with BBC Micro's.
Students used them, and so, parents bought BBC Micro's.
And all this with the BBC fuelling marketing for Acorn.
These were the rosy days, in 1982-1983, Acorn was quoted on the FTSE, shared were climbing and Acorn were making huge profits.
I remember watching 'BBC Live' once, and it showed a 75% usage of BBC B's in £500 computer market and WHSmith reporting strong sales in the home brand.
Herman Hauser was interviewed in The Independent lately, saying that if Acorn had licensed its Operation System (MOS/BASIC) to other companies in the early 80's, Acorn would have a complete monopoly in the microcomputer market today. But that wouldn't be Acorn, oh no, they kept their OS safely under a hood for their eyes only.
He also said that Bill Gates came, trying to sell him (Acorn) DOS, but he told him that our OS was much better than his.

Bubbles have to burst though, and they did, in 1983, the first signs of Acorn's fall became apparent.
The Electron.
The Acorn Electron was targeted at the budget computer market, but the quality of the BBC B was gone. Electrons were released in August 1983, but bad supply interrupted and it missed its main opportunity, Christmas.
It was/is normal for families to buy computer systems for Christmas, just think though, in 1983, a Electron cost £299 (IIRC) and last Christmas, a PlayStation 2 cost £400. A full computer for £299 or a gaming console for £400 ?
In 1985, Acorn started to look elsewhere for a processor, knowing that the 8-Bit 6502 at 2Mhz was outdated and needed something much more powerful.
Acorn contacted Intel, asking for the core to the 286 processor (16-Bit) but Intel would only sell Microprocessors's and not cores.
Acorn jumped the gun though, and moved straight to 32-Bit to power their future WIMP (Desktop - Windows, Icons, Mouse and Pointer,now called the GUI).

The year was 1986, the BBC B was now four years old and it was time for a new machine.
The BBC Master 128 was a BBC with 128KB RAM, a 2Mhz 65C02 Processor and a greatly improved disc-filling system, ADFS.
Even at the launch of the Master though, it was clear that Acorn wanted more than the 8-Bit range they had.
In the industry, MicroSoft's Windows system and Apples MacOS were available and the 286, a 16-bit processor, was commonplace.
A engineer at Acorn in 1985, proposed that Acorn create their own processor. Herman liked the idea but gave something that Intel had not, no money and no people.

The Acorn RISC Machine (ARM) was one of the most simple processor ever made. Most importantly though was that it was 32-Bit RISC.
In 1987, the ARM 1 was released for testing to Acorn Employees and (I think) a few customers.
There are 15 registers and a Program Counter (PC(aka R15)) on the ARM 1. It is believed that only a few hundred of these chips were ever made, I saw one, from Dave Walker, at Wakefield 2001. It is a fine chip indeed, a marvel of modern technology.

The Acorn A305/A310.
In 1987, Acorn showed the world their new WIMP-based 32-bit Machines.
This machine had a ARM2 fitted running at 8Mhz and the A305 had only 512KB of RAM which made it virtually unusable, the A310 had 1MB.

Built into the 512KB of ROM in the machine was, though, Arthur (A Operating system by Thursday IIRC), a Windowing operating system. It was more advanced than the 8-Bit's OS, it was a shame to have such a basic (oh yes, it is known that Arthurs desktop was wrote in BASIC !) operating system on such great hardware.
Acorn released many Archimedes machines, some with up to a 12Mhz ARM3 Procesor, 16MB RAM and RISC OS 3.11

Acorn's software engineers were working on a new, great OS codenamed ARX but Acorn fell short of time/money though and in a mere two weeks, RISC OS was born. It was a great improvement over Arthur and it could really multitask.
The A5000 machines were fitted with RISC OS 3.xx which was a vast improvement over RISC OS 2 but was not such a big jump, as was Arthur to RISC OS 2.
RISC OS 3 is still used by many even today, because in the Acorn industry, items have a much longer life-span than on Wintel machines.
In 1990, Acorn thought that, if, ARM was spun out to a separate company, other companies could use their chip and, so, could turn ARM into a huge company, in fact, much larger than Acorn.
The Acorn RiscPC600 was launched in April 1994.
The Acorn Machine list, best describes the RiscPC:

"This is the next generation and architecture of machines - superceeding, but compatible with, the Archimedes range of machines. All of them feature a highly configurable and modular system that makes a bewildering variety of options available. All are founded on the 'second generation' chipset featuring VIDC20, IOMD and the newer ARM6, and better, cell processors. This new range was launched on the 15th of April 1994 with the RiscPC 600 series of machines.

The new machines feature the processor card option, the concept of which was first shown in the A540 as well as a unique second processor slot allowing the machines to have two processors in the system, at once, of radically different types. Simply by slipping a 486 chip in, on an appropriate board of course, Intel based software can be run on the machine adjacent to native ARM programs. Both processors share the system resources and can be allocated memory and the like to use. Memory managment has been improved with memory paging always being done in 4K pages.

Further more the podule interface has been extended with DMA to and from podules, extended addressing, 32bit data pathways from the IO system as well as a vastly expanded memory map for each podule. Realtime video from the IO system becomes a reality with high speed and data tranfer applications being boosted considerably."

In 1996, Acorn released the StrongARM RiscPC, the StrongARM processor was the result of ARM and DEC, it featured a 202Mhz clock and 16KB Cache. I once heard somebody say that on a RiscPC, the result is 'stonking' ! it surely is, dragging windows is smooth and any heavy-metal applications fly along.

Phoebe was the 'to-be' machine of the future, but on 'Black Thursday' in September 1998, Acorn decided to bag it's new Phoebe machine due to be released in November, sacked 90 staff from the workstation division, cancelled the Acorn World Show 98 and left the market in disarray.
It was only in early 1999 that things became clear, Acorn had left the desktop market and decided to focus on Digital Set-Top-Boxes and was renamed 'Element 14' (Element 14 is silicon) but Pace Micros bought Element 14 (IIRC) for £200,000 and they were also given RISC OS.

It looked like the end of era for the only British Computer company, but no, Phoebe was well and truly dead but RISC OS was not, in January 1999, RISCOS Ltd. was founded with the intention of bringing the half finished RISC OS 3.8 (which was to ship with Pheobe) up to scratch and releasing it as RISC OS 4.

In June-July 1999, RISC OS Ltd. released RISC OS 4.00, with support for long filenames/big drives/updated pinboard etc.
To prevent reinventing the wheel, please click here to get info about RISC OS.

So, after 20 years of Acorn, they are no more, but ARM is now the worlds largest microprocessor company, and Britain's most valuable company.
RISC OS is at the hands of Pace, and it seems that if/when the Network Computer revolution happens, RISC OS will be happily running on it.
Herman Hauser is now a multi-millionaire due to his VC company, at least somebody except Bill Gates has made a fourtune.