Section 1: Acorn Hardware and Operating Systems


1.1) What is an Acorn machine?

Acorn computers are a British built line of computers that started with 6502 based machines and now are based on ARM processors. Oddly enough Acorn computers are made by the Acorn Computer Group ( see http://www.acorn.com/ for more details.) Section 1.2 details those machines currently in manufacture and how to fetch a full list of all machines known to have been made by Acorn. Acorn machines are known for their innovation, flexibility and reliability of hardware/software.

From January 1999 Acorn Computers Ltd., has been renamed Element-14, focussing more on their Digital TV technology. More information is available from http://www.element-14.com

Reading through section 1 should give newcomers to the Acorn scene a reasonable appreciation of what the machines can do by default. They were fairly strong in education and it was Acorn's primary market, although the Acorn group of companies are making moves into other markets and have consolidated primarily on the Digital TV market for the future. For those people who wish to see RISC OS in typical use then visiting http://www.geocities.com/~banksp/Power/Acorn/RISCOS.html will provide you with some images and text about using RISC OS.

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1.2) What kind of Acorn machines are there?

The full list of Acorn made machines is somewhat lengthy now and are mostly no longer in manufacture. Accordingly this list distributed in the FAQ includes only Archimedes/ARM based machines, which are those currently manufactured by Acorn. A full list is maintained on Philip Banks email server under the filename 'MachineLst'. (see question 7.4 for how to use his email server.) This is, as far as I am aware, a complete list of all Acorn machines made with the possible exception of the latest machines for which I have yet to get the detailed specifications.

Acorn RiscPC series

This is the next generation and architecture of machines - superseding, 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 management 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 transfer applications being boosted considerably. These machines are to be the ones to carry Acorn for the next three to six years and look set to do so nicely.

Acorn Diversification

Acorn, around October 1995, restructured itself into separate companies. Consequently the originating company will be noted by the machine model name, in brackets, from here on down.

Acorn Reunification

As part of an image strengthening procedure Acorn have reconsolidated to present one company image to the world. Consequently I won't be marking which subdivision 'makes' the machine anymore - all machines are 'Acorn' machines now.

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1.3) What versions of the ARM processor are there?

The details of all current ARM chips and their capabilities are contained within a file stored on Philip Banks email server, the filename is 'ARMChips'. See question 7.4 for how to request this file.

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1.4) What are the main features of RISC OS 3 over RISC OS 2?

This information is now contained on Philip Banks email server. If you wish to know the 'new' features of RO3 over RO2 then send a request to his email server, as detailed at the start of the FAQ, for the filename 'RO3diffs'.

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1.5) What are the differences between RISC OS 3.5 and RISC OS 3.6?

Quite a few, although nothing particularly drastic OS wise, mostly improvements although the lifting of the FileCore partition limit and the incorporation of JPEG handling into the OS are quite substantial improvements. Here is the list :-

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1.6) What are the differences between RISC OS 3.6 and RISC OS 3.7?

Like RISC OS 3.6, RISC OS 3.7 is primarily changed behind the scenes and is an incremental improvement of the OS. The changes mostly have been geared towards rendering the OS StrongARM compatible. Here is the list :-

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1.7) What are the differences between RISC OS 3.7 and RISC OS 3.8/4?

RISC OS 3.8 was a developmental version of RISC OS 4 released to developers just before the closure of Acorn's Workstation division and cancellation of the Phoebe project. It has now been completed by the newly formed RISC OS Ltd and RISC OS 4 is the first new version of RISC OS to be released by a company other than Acorn, A comprehensive list of features was released onto their own web site during April 1999. This can be obtained from http://www.riscos.com/Features.html
In a nutshell some of the major new features are:-

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1.8) What are the graphics capabilities of the Acorn machines?

All the Acorn machines are highly upgradeable so that they can achieve colour depths and resolutions beyond what is listed here. What is listed here is the default capabilities of the machines as supplied by Acorn. As such it should be taken as a guide to the minimum abilities of the hardware.

It must be stressed again that these are merely the resolutions and colour depths defined by the machines operating systems. In all of the machines cases there are both hardware and software extensions that alter and increase the list.

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1.9) Is Virtual Memory possible under RISC OS?

Short answer, full VM is not possible under the current versions of RISC OS. The problem is that most of RISC OS works in SVC mode, if a DataFetch abort occurs then R14_svc is corrupted. This makes returning from the SWI somewhat problematic. This is a hardware limitation with the ARM2 and ARM3 cell chips. Hardware using ARM6 or better processor cells have special memory abort modes that alleviate this problem so future machines and incarnations of RISC OS may well have VM available. Indeed the new RiscPC machines make prime candidates for having virtual memory, however there are still re-entrancy issues that make this problematic for RISC OS. (Consider loading data from a file into paged out virtual memory...)

However limited solutions are available now. !Virtual is one such solution allowing VM for a user process using a limited subset of SWI's that are carefully 'protected' against R14_svc being corrupted. Such solutions suffer from the restricted set of SWI's they support and are mostly useful for batch style processing jobs like compilation or memory intensive processing jobs. !Virtual currently does not work with Risc PCs due to the fact that it requires a page size which is a multiple of 8kb. (This means !Virtual will not work with A305s.)

Also available commercially, from Clares Micro Supplies, is Virtualise for the RiscPC machines. This provides virtual memory on dynamic areas. As more and more software starts to use dynamic areas for data storage and manipulation this will become increasingly useful.

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1.10) What is the current status of Linux for Acorn machines?

Due to the rapid development of Linux exact details about the latest incarnation of Linux are not kept in this FAQ. However for the latest information you might like to consult these web pages, or email the people involved with the port :-

Native Linux page :-

http://www.arm.uk.linux.org/~rmk/armlinux.html (rmk@ecs.soton.ac.uk)

PC card Linux :-

http://www.ph.kcl.ac.uk/~amb/linux.html (amb@physig.ph.kcl.ac.uk)

If you have the Acorn Clan disc and would like to install ArmLinux, I have written a Dummies Guide to Installing ArmLinux which is available at http://www.vigay.com/riscos/linux.html

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1.11) What 'Easter Eggs' are present in RISC OS?

It has been a long tradition with Acorn OSes to have hidden sections that give credit to the people involved in the creation of that OS. The BBC Model B ROMs had the names of the people involved hidden in the memory space occupied by Fred, Jim and Shelia. With the release of the ARM powered machines this tradition has continued on.

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1.12) What is the current status of RiscBSD for Acorn machines?

Due to the rapid development of RiscBSD exact details about the latest incarnation of it are not kept in this FAQ. RiscBSD being a port of NetBSD, a Unix variant, to the RiscPC. However for the latest information you might like to consult these web pages, listen to the RiscBSD email list or email the people involved with development of it :-

Risc BSD page :-

http://www.ph.kcl.ac.uk/~amb/riscbsd/ (amb@physig.ph.kcl.ac.uk)

RiscBSD email list :-

listserver@ic.ac.uk place "subscribe riscbsd <RealName>" in the message body.

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1.13) What is RISC OS 3.8/Ursula?
Before Acorn decided to abandon Workstations and focus on Digital TV, work was at an advanced stage on an updated version of the operating system (codenamed Ursula) to live in their new machine (Phoebe). Developers had access the RISC OS 3.8 (as Ursula declared itself to be) for evaluation and beta testing purposes. Versions of RISC OS 3.8 were primarily available for ARM710 machines, although some StrongARM compatible versions are known to exist.
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1.14) What is Lazy Task Swapping ?
On RISC OS 3.5 and 3.6 machines there were seen to be serious delays in the desktop when running applications with large wimpslots, mostly indicated by the lack of reponsiveness. The problem was identified as being that of having to page in the entire application which, with a 4k page size was quite slow. In RISC OS 3.7 this situation was improved by changing the way in which the application is paged in.

RISC OS 4 can support a new method of paging tasks in - Lazy Task Swapping. This can also be called 'demand paging', because instead of paging the entire space in, individual pages are only given 'on demand'. This means that in a large application only sections of the application space may be physically present although the application itself will be unaware of this.

There is a problem, however, with some versions of the StrongARM processor which will (under certain circumstances) cause Lazy Task Swapping to fail. StrongARMs prior to revision T exhibit this failing and Lazy Task Swapping is consequently be disabled.
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