RiscPC

The RiscPC is a RISC OS-based computer originally released by Acorn in 1994. Emphasis was placed on easy expandability, and the ability to run PC software alongside RISC OS software.

Revisions

Mk1

Mainboard code : 0197,000 ISS.

Mk3

Mainboard code : 1208,000

Processors

ARM

The ARM is the microprocessor and the heart of the RiscPC. It comes on a little CPU card which is located on the left side of the machine, directly behind the CD-ROM slot. Every RiscPC has two slots to install a CPU card in each of them; but there is no way to use two of them under RISC OS. One of these slots has to be filled with an ARM CPU-Card since this is the major card that runs the RISC OS Operating System. The second slot does not need to be filled; but its possible to install a compatibility card with an x86 processor here.

The ARM chips in the Risc PC is of at least ARMv3. Such CPUs are capable of using the complete 32 bit address range if they are switched into the so called "32 bit mode" at start. The other variant to use these microprocessors is a so called "26 bit mode". Here the address range is 26 bits wide as it has been in the original ARM implementation that has been used in the Archimedes computers. Furthermore its possible to use a mixed mode where programs (executables) are restricted to 26 bit memory addresses but data can use the complete 32 bit addressing. For compatibility and legacy reasons some versions of RISC OS in Risc PCs operate in 26 bit mode. This doesn't affect the word length of the registers - registers are 32 bit words ever since on the ARM architecture.

The inclusion of an "wholesome 32 bit" mode into the ARM chips led to a separate register to hold the processor flags - the CPSR (Current Program Status Register).

The memory management is now included directly "on-chip" as an MMU (memory managment unit). This comes together with on-chip cache as standard.

ARM chips of these generations are able to drive an coprocessor, e.g. an FPA (floating point accelerator). BUT: normally this interface has been left out from the chips that are used to build the processor cards (ARM600 with copro interface, ARM610 without). Therefore most, if not all, non-special CPU cards doesn't have the ability to calculate hardware accelerated floating point mathematics.

ARM chips of these generations, and onwards, are switchable (at start) to big or little endian mode. RISC OS uses little-endianess for its byte order.

ARM610

This is the direct successor to the formerly used ARM 2 and ARM 3 CPU which came in the Archimedes range of computers. ARM610 comes with a unified cache of 4 kByte in size. It runs at 30 MHz. There are variants produced by VLSI (VY86C610) and GEC Plessy Semiconductors (P610ARM/KG/FPNR). ARM610 card needs at least RISC OS 3.5 to run.

ARM710

This is the successor to the 600 (610) line of ARM CPUs. The cache size increased to 8 kByte. It runs on 40 MHz. The minimal RISC OS version that includes support for these CPUs is RISC OS 3.6

ARM810

This has been announced as an upgrade path for the ARM610 CPU cards of the original RiscPCs. When it became available the StrongARM already was in existence - that's why there aren't cards with ARM810 sold to an wider audience; people used the much faster StrongARM instead.

StrongARM

Main page : StrongARM

Kinetic StrongARM

Main page: Kinetic

x86

There are some variations of x86 Cards. The first card had a 486SXL Texas Instruments Chip onboard. Better "powered" Cards came with 486DX, 486DX2 or 486DX4 Chips. The most powerful had an 5x86 IBM Chip onboard [1], that is a somwhat modified Cyrix 5x86 [2], that runs faster than a "normal" 486 CPU at comparable frequency. PC Cards had been mod'ed by users and dealers to reach higher speeds. One CPU to do this had been the (Scoket 3 Pin compatible) AMD Am5x86 [3][4].

The board itself came in two variants. The later one, called "Gemini II", had been prepared to install a cache extension on their backside.

To use a CPU card some software is needed - the Aleph1 PCPro v3.06 is the most recent (official) available. It can be found at [5].

RISC OS Versions

The RiscPC is compatible with Acorn RISC OS 3.5 through 3.71, RISC OS 4.x, RISC OS 5.x, and RISC OS 6.x.

Links

• Wikipedia entry for RiscPC
• PC Cards at the Chris Whytehead Collection
• Wikipedia entry for Cyrix 5x86
• Official AMD infosite for Am5x86
• Video "RISCy Business - The Acorn RiscPC - ARM in a desktop" of and with a RiscPC
• Video "Acorn RiscPC 600" RiscPC Overview in three Parts, Part1 - Hardware
  • Part2 - Software and Games
  • Part3 - Upgrades
• Video RiscPC launch ceremony, 1994
• Video Omnibus 600, a RiscPC usecase

---

---