RISC iX


RISC iX is a discontinued Unix operating system designed to run on a series of workstations based on the Acorn Archimedes microcomputer. Heavily based on 4.3BSD, it was initially completed in 1988, a year after Arthur but before RISC OS. It was introduced in the ARM2-based R140 workstation in 1989, followed up by the ARM3-based R200-series workstations in 1990.

Features

Acorn chose BSD 4.3 as the basis for RISC iX due to its academic origins, these being considered as making the software more appropriate for Acorn's principal target market of tertiary education. SunOS and NeXTSTEP systems were given as examples of other "modern high-performance workstations that use BSD". Other reasons for choosing BSD included better integration of networking and connectivity tools in comparison to System V.
Although Acorn had licensed Sun Microsystems' NeWS in 1987, broad industry adoption of the X Window System, including Sun's belated endorsement, resulted in X11 technologies featuring in RISC iX. RISC iX 1.2 upgraded the X11 server to release 4, and was certified to conform to the X/Open Portability Guide 3 Base profile.
Peculiarly, the system console featured a two-cursor text copying mechanism inspired by Acorn's own earlier 8-bit range including the BBC Micro. One reason given for the inclusion of this feature was to be able to provide command line editing facilities for shells that did not offer it and to compensate for the exclusion of shells that did.
The system implemented transparent demand paging of compressed executable programs, allowing the constituent pages of these compressed executables to be loaded into memory by the existing demand paging mechanism and then expanded in place for execution, taking advantage of the availability of sparse files to reduce the disk space occupied by these pages. Shared library support, enabling processes to share library code, was also introduced to work around other "unpleasant" consequences of the hardware's 32 KB page size, one of these being the excess space occupied by processes residing in main memory, especially in situations where separate pages need to be allocated.
Despite these remedies, the workstations offering RISC iX were regarded as being hampered by the memory management unit using 32 KB pages. The MEMC, providing the MMU capabilities in the system architecture, was designed to be simpler to implement than contemporary MMUs, providing a 128-entry lookup table that effectively partitioned physical RAM into 128 equally sized pages, with a 4 MB address space divided into 128 pages resulting in the 32 KB page size employed in these systems. A "logical" or virtual page could only be usefully mapped to a single physical page through this mapping. This approach coincidentally recreated that employed by the University of Manchester Atlas virtual memory architecture.
The hardware supporting RISC iX also did not have direct memory access capabilities for disk operations, meaning that the CPU would spend time servicing interrupts related to disk transfers resulting in "a definite reduction in, but not a complete loss of, available CPU power during disk transfers". However, by reducing the amount of data being fetched, the executable decompression technique did reduce CPU involvement in performing disk transfers, albeit at the expense of incurring CPU usage in the decompression of retrieved pages. Positive outcomes of the decompression scheme also included reduced loading on storage devices, of importance for networked storage, and generally improved disk transfer performance.

Distribution

RISC iX was either supplied preinstalled on new computer hardware or was installed onsite from a portable tape drive by Granada Microcare, who would take the installation tape away with them. Upgrades to RISC iX 1.2 from earlier versions started at £349 for R140 machines, and new installations for A400-series machines started at £999. Installations required 100 MB of space on suitable hard drive or network storage, with hard drive and SCSI card bundles being offered from £1699 for R140 machines and from £2326 for A400-series machines.
Once installed a backup of the core operating system to three floppy disks was possible, allowing future reinstallation through the use of remote filesystems or backup media to transfer files to a machine.

Hardware

According to documentation concerning RISC iX 1.2 availability, the operating system could be used on the R140, R225 and R260, being pre-installed on the R260, accessible via a fileserver on the R225, and as an upgrade from RISC iX 1.15 or earlier on the R140. The A540, being practically identical to the R260, could support RISC iX as delivered, whereas A400-series machines required an Acorn SCSI card, with older A400-series machines also needing a memory controller upgrade and "all the appropriate field change orders" to have been performed. A300-series machines and the A3000 were not supported, largely due to potential compatibility issues with upgrades needed to bring these machines up to the required specification, in addition to operating temperature considerations with the A3000. Subsequent Archimedes machines, such as the A5000 and A30x0 models, were introduced without any prominent indication of RISC iX compatibility, although the A5000 expansion hardware was designed to support the same form of expansion card interrupt management as the A540, R-series and A400/1-series, specifically to be able to support.
Several machines were designed specifically to run RISC iX.

M4

An unreleased machine, built internally by Acorn for the development of RISC iX. Reputedly only two or three were built and one of them has subsequently been destroyed. All known examples are owned by The National Museum of Computing.

A680 Technical Publishing System

Prototyped for an Olivetti product but unreleased, the A680 contained an ARM2 processor, 8 MB RAM, a 70 MB hard drive running from an onboard SCSI controller, and either a 40 MB cartridge tape drive or a single 2 MB floppy drive. Up to four "podule" expansion cards could be fitted, although one slot was occupied by the laser beam printer expansion card supporting a directly driven low-cost laser printer as an alternative to a PostScript printer connected via the serial port. The system was meant to run Frame Technology's FrameMaker under the "Acorn UNIX" operating system and NeWS graphical environment. To support 8 MB of RAM, dual memory controller units were employed.
The A680 was reportedly the first target for RISC iX and differed in certain ways from the Archimedes and R-series models. For instance, no other machine from Acorn Computers featured integrated SCSI. However, it is rumoured that overheating from the SCSI controller was one reason for the machine to never be released.

R140

Based on the A440/1, the R140 uses the same 8 MHz ARM2 processor and 4 MB RAM, also providing a 60 MB ST506 hard drive, with the option of adding a second hard drive using the same internal controller. A SCSI adaptor was available for other storage peripherals. Since the hardware is based on the Archimedes series, Acorn's podule expansions could be added, although appropriate drivers would have needed to be written.
At the time of initial release in 1989, the cost of the R140 was £3,500 for a standalone workstation without Ethernet connectivity. For the additional cost of the Ethernet expansion, a network-capable workstation could be configured. A floating point expansion card based on the WE32206 could also be added. A discount introduced at the start of 1990 offered the R140 bundled with Ethernet expansion and either a 14-inch colour monitor with PC emulation software or a 19-inch monochrome monitor for £2999 plus VAT.
Supplied with RISC OS 2 in ROM, the machine would boot that OS then could either automatically boot RISC iX totally removing RISC OS from memory or continue running RISC OSoptionally being rebooted into RISC iX at any time.
An ordinary A440/1 with at least 4 MB RAM and a suitable hard drive could also run RISC iX.

R260

Based on the A540, the R260 originally contained a 30 MHz ARM3 processor, 8 MB RAM SCSI adapter and a 100 MB or 120 MB SCSI hard drive. It booted in the same style as the earlier R140, but was normally configured for customers to boot straight into RISC iX. The machine was supplied with an Ethernet adapter.
The system was released in 1990 priced at £3995 plus VAT, having been announced with a price of £5000 plus VAT. A floating point accelerator or "arithmetic co-processor", the FPA10, was made available in 1993 for the R260, as well as for the A540 and A5000 machines, priced at £99 plus VAT. These machines were designed to support the FPA device via a dedicated socket on the processor card, and offered a peak throughput of 5 MFLOPS at 26 MHz.
A similarly configured A540 could run RISC iX. Production of the A540 and R260 was discontinued in mid-1993.

R225

The R225 was a diskless version of the R260. It required a network file server or an R260 to boot. The system was released alongside the R260 priced at £1995 plus VAT, having been announced with a price of £3000 plus VAT.

Peripherals

As well as industry-standard Ethernet, Acorn's own Econet was supported, facilitating connectivity between Econet and IP-based Ethernet networks. Moreover, the Econet interface on a RISC iX workstation could be treated as a "Unix networking" interface, permitting TCP/IP requests to be sent over Econet to hosts capable of handling them. In 1991, with Ethernet becoming more widespread on campus networks, Acorn offered a Network Gateway Starter Pack featuring the R140 equipped with Econet and Ethernet adapters at a price of £2499, with a licence for the TCP/IP Protocol Suite included to allow Archimedes computers to be able to communicate with such Ethernet-based networks via the gateway.
Similar Econet gateway capabilities were eventually extended to computers running RISC OS with Acorn's TCP/IP Protocol Suite product and with the broader Acorn Universal Networking suite of technologies, and a device driver update eventually provided a similar means of routing TCP/IP communications over Econet networks for RISC OS machines.