Once upon a time when the A305 with an ARM2 was released using Arthur as an OS (mostly written in BASIC) it was orders of magnitude faster than any other machine costing less than 10k...
...how time flies.

So let's see what we can do to get a bit more grunt:

ARM2:
Best of luck with trying to overclock one of these! If your machine has an ARM2 you're better off looking to improving memory bandwidth (See 'Turbocharging the A3000') or replace it with an ARM3.

ARM250:
Acorn's original 'system on a chip' does have some options. You may have an early 'mezzanine board' equipped machine which theoretically could take an ARM3. Simtec did briefly produce a Turbo Ram Upgrade which was to the A3010 what the Turbo Board on the projects page is the A3000 - it also clocked the CPU and VIDC cores faster.
The very good news is that Tom Walker has worked out how to reproduce this hack for the A4000 and A3020. Hopefully I will have a similar page up for the A3010.

ARM3:
Now we're getting somewhere. The ARM3 was generally rated for 25mhz or occasionally 33mhz. More is possible but probably not a lot more. In general CPU's in plastic packages may get up to 33mhz if you're very lucky, those in proper ceramic packages may get to 40mhz+ If you've get an ARM3 it's likely you either have an ARM2 model with an upgrade plugged onto the CPU socket or an A5000/R260/A540...
...the former is dead-easy to overclock, the latter depends on how the system as a whole was designed.
See 'A5000 Overclocking' and 'A5000 Clock Splitting'.

Here's the fundamentals as demonstrated by my ARM3 A3000:
-You need to locate the crystal oscillator responsible for driving your CPU. For the ARM3 this will be a metal can that's rated at twice the speed of your CPU. ie: A 25mhz ARM3 will have a 50mhz crystal.
-Next you'll want to remove the crystal from the circuit board (PCB). Most likely this will require a little work with a soldering iron - this is EASY, BUT if you've never done this before do practice a bit first and I'd recommend you watch a few You-Tube videos.
-Replace the crystal with a socket. DIL-8 or DIL-14 depending on if your crystal is square or rectangular. Don't forget to be absolutely sure which way round the crystal is meant to be!
-Buy a selection of crystals that are progressively faster than the one you've got: 50% or even 25% overclocks aren't going to happen so be realistic with your purchase(s)!
-In with the old crystal and fire up your 'pute. Working? Yes? Good, you didn't break it...
...if there's no life switch off FAST and check your dodgy soldering.
-Next try progressively faster crystals (switching off between swaps, duh!) until you either run into stability problems or it won't boot properly.
-It's wise at this point to replace your maximum-working crystal for the next-slowest you've got for the sake of your computer's long-term health.
-Heat-sinks can be useful to improve stability despite these CPU's not getting hot like certain 'modern' ones.

Here's my 25mhz ARM3 doing 33mhz:

FPA10:
Yes, my A3000 has one of these. An uncommonly fitted floating point co-processor for these computers...
...about 100x faster with than software, but not used a great deal in RiscOS. These are all rated at 25mhz and DO NOT overclock at all - except mine!

Arm610:
Found on the CPU card of early RiscPC's these are usually rated at 30 or 33mhz. Like the ARM3 the crystal is always twice the speed of the CPU it's driving and can be changed. I'm not aware of an ARM610 that will go faster than about 46mhz, and then only rarely.

Arm710:
Really as the 610 but a bit faster - 40mhz. These also overclock just a little bit. The card will take either DIL-8 or DIL-14 crystals.

StrongARM SA-110:
This is the reason people rarely overclock the 610 or 710. 40mhz+ vs 200mhz+ no contest really! This CPU came in a few versions for the RiscPC: 200, 233mhz StrongARM from Acorn and the 233, 300mhz Kinetic from Castle...
...Oh, and the 306mhz version in the Omega.
Depending on luck and processor revision they will do anything up to about 400mhz.
More? Not a chance!
See 'Overclocking the StrongARM'.

ARM7500(FE):
Like the ARM250 this is an ARM core + chip-set all in one. The FE variants also have the only floating point core that works at faster speeds than 25mhz-ish. They are found in 32-56mhz versions and are reputed to be quite good over-clockers. 64mhz for the faster-rated versions is a reasonable to shoot for.
How?
A fair few clones used this chip but if we concentrate on the A7000(+) from Acorn and later Castle the procedure should go as follows: Look at the A7000(+) motherboard. You'll see two or three crystals near the ARM7500 - if only two then there will be an obvious place on the motherboard where another crystal can be fitted. Basically you'll want to fit a socket for the crystal located between the CPU, ROMS, and backplane connector. Then progressively try faster ones...
...This is in theory quite doable with the standard A7000 that is 'missing' this crystal but I can't remember what jumper needs changing to enable this - can anyone help..?


What, more...?
Well, if anyone wants to donate an Iyonix I'll work out how to overclock these too...
...in time I hope to bring some guides for hacking the Microdigital Omega but as they are rare as hen's teeth and easy to break I'll need some brave pills before tackling mine!

Charlie (2009)