About the Archimedes and RiscPC machines

Archimedes page

Last page update: 2 Aug 1998

About the Archimedes and RiscPC machines

One year ago, I obtained an ARM-310 system with one 800K floppy drive. The ARM-310 is one of the first of Acorn's line of personal computers based on the ARM processor series. Some oddities about the ARM-310: It is one of the few models which uses penlites for the CMOS instead of a rechargeable battery. If your CMOS gets corrupted, try holding down DEL when you turn on the computer to restore the factory settings. It runs RISCOS2.1 in ROM, but the ROM is upgradable to RISCOS3.1. However, the upgrade requires some small hardware patches, but I'm still looking for more information about this.

The ARM-310's 8 Mhz ARM-2 processor has a RISC architecture, which, in this case, doesn't mean that one has to clobber in a dozen brain-damaging instructions to write down a single operation, but rather, that the instruction set is so universal that a small set of instructions covers all functionality. As an example, it doesn't have stack instructions, but instead, any of the 14 free registers may be used as a stack pointer using the LDM and STM instructions, and all four possible stack schemes (full/empty, and up/down) are possible. As a nice spin-off, it is also possible to read and write data blocks quickly (I managed approx. 16 megabyte/second video bus throughput using them). As another example, the program counter is one of the 16 universal registers, and hence, any processing may be done on it, automatically adding a return-from-subroutine, program-counter-relative addressing, and single-instruction jump-table access. It also means that in order to write optimal code, there are only a few rules you need to know. The cycles listed are for an ARM-310, and may be less on newer models.

There are only four types of instructions to learn: Register processing (and compares), memory transfer, multiple memory transfer, and branches/SWIs. The operand format of each is completely symmetrical, the only exception being the multiply instructions MUL and MLA. Any instruction may be executed conditionally according to the current state of the flags. Any register processing instruction may optionally change the flags.
Cram as much functionality as possible into each instruction. Every instruction is 32 bits long, and takes an average of 1.25 cycles to execute, except for memory transfers, which are 3 cycles plus 1 cycle added for each memory access. This means memory access is relatively expensive, unless one transfers multiple registers at a time: transfer one register, get the second one (almost) for free!
Don't jump! On an ARM-310, a jump will take 4 cycles, as it flushes the pipeline. Use the conditional execution instead where possible.
Finally, the thing you probably have to get used to most (if you have experience with other CPUs) is the format of the immediate operands. These are encoded into a 12-bit floating point format: 8 bits mantissa and 4 bits exponent. The immediate value is generated from these using Value = Mantissa, ROL'ed by 2*Exponent. This allows them to cover the whole 32-bit range logarithmically, while only taking up 12 bits in the opcode. However, this implies that not all numbers in the 32-bit range are immediately assignable! Some special pseudo-opcodes are available to make life easier, but often, a careful choice of constants enables you to fit most of your immediates within the proper range. Sometimes it is useful allocating an extra register to hold an often-needed constant value.

For a good and complete reference (only 16 pages!) try: Robin Watts et al's ARM docs. For more technical documentation, see also miscellaneous technical info

Software projects

Upcoming games 'n demos

I have been working on a little `hello world' game to get to grips with the machine. Actually, I started on it almost a year ago, but the project was in the fridge until recently. To show you I'm not talking shit, here are some screenshots!

Meanwhile, Reinoud Zandijk and I have been organising some `computer freaking' sessions. We've already produced some rough games and demos for the Acorn, which we are planning to make available on this homepage. Right now, Reinoud is working on a Vector game. It doesn't really seem to have a name, though it is rumoured that some call it `Whoops!'. The player, who is controlled in an `asteroids' manner, has to take care of the pods lying around the screen. Meanwhile, enemies try to make life hard by hanging around the pods and attacking the player. The pods are the player's lives; a life is lost if a pod is shot accidentally, and the player is created from a pod after getting killed.

The player is wreaking havoc on the enemy ships

Arcvic - a Vic-20 emulator

Last week I've written a Vic-20 emulator on my ARM machine. If you're interested, see the Arcvic homepage.

Archimedes page