Overclocking the A2386 Bridgeboard

The Commodore A2386 Bridgeboard contains an Intel 386SX CPU at either 16, 20 or 25 MHz. This article deals specifically with the 25MHz version although the other versions should also be overclockable. Given that current PCs now have vastly faster CPUs compared to the now rather elderly A2386, any speed improvement is most welcome. Fortunately there are many ways to speed up a Bridgeboard, and if they are all utilised, the A2386 can actually exceed some performance parameters of a good 486. In fact the hard drive access speed on my Bridgeboard is actually faster than that of my Pentium 225MMX! If you haven't done so already, check out Sebastien Boisvert's excellent Bridgeboard FAQ, which can be found on Aminet. It contains just about everything there is to know about Bridgeboards.

Starting out

Before you reach for the soldering iron, there are five ways to start gaining speed. Firstly, referring to the layout above, make sure the pipeline jumper, J101 is fitted. This gives about a 10% speed boost. The next thing to do is to reduce the amount of CPU time used to refresh the RAMs. Early PCs were designed at a time when dynamic RAMs had to be refreshed every 15nS. More modern memory can go for a lot longer before needing a refresh. This timing can be changed with a program called calcqf, which can easily be found on any public domain or shareware site. This MS-DOS utility will calculate the best refresh rate and generate an executable file called "qfresh.com" which can be executed in autoexec.bat. Refer to the documentation with the program. Qfresh will give around another 5-6% extra speed. The next thing to do (if you are unfortunate enough to have to run Windows) is to get some extra memory. It is possible to fit 16MB of SIMM memory to a Bridgeboard with an ingenious hack. Check it out in the Bridgeboard FAQ. Video speed can be increased by fitting a modern ISA VGA card that can clock the ISA bus itself. The best of these cards is the "Diamond SpeedStar Pro" Fitting one of these can double your video speed, although it is possible to get even more performance as you will see below. Finally, get a cheap ($20) ISA IDE controller card and use a dedicated IDE hard drive. The speed improvement compared to using a shared Amiga drive is astounding. These IDE controllers also include serial and parallel ports as an extra bonus.

Upgrading the CPU

One of the best ways to speed up a Bridgeboard is to upgrade the CPU to a 486. Again, the Bridgeboard FAQ has all the details. I have tried two different upgrade modules, one based on a 50MHz TI486SXLC2 chip called "Make-it 486" and the other on a 75MHz clock tripled IBM chip called the "Rev to 486" For use with Windows, I found that the Make It 486, although only clock doubled to 50MHz was much faster than the Rev To 486. This is because of its much faster memory performance as well as the fact that the ISA bus was also faster. One of the reasons for the difference in memory performance is that the Rev to 486 does not support pipeline mode (refer to jumper j101). On the other hand, the Rev to 486 does have much greater raw processing power, so if you have an unaccelerated VGA card, and are using DOS, this would be the chip for you. Note that both chips do not support an FPU on the Bridgeboard unless they are in single speed mode (25MHz). According to a report I recently heard, however, the Cyrix FasMath 387sx FPU chip will work with the Rev to 486 upgrade.

Overclocking the CPU

The Bridgeboard uses standard 14-pin DIL form factor oscillator modules. The nominal frequency is half that of the module - unless you are using one of the 486 upgrade chips. As usual it is best to fit a socket when changing the oscillator. The CPU oscillator is the one next to the ISA edge connector. Refer to the diagram above. The standard 386 version was overclocked to 30MHz, using a 60MHz oscillator. At this speed it is suggested that a heatsink be glued to the top of the 386 as well as the two PAL chips indicated on the diagram in blue. The Make it 486 was able to be overclocked to 63MHz, using a 63MHz oscillator. Again, heatsinks are necessary. The Rev to 486 refused to be overclocked at all, although I have heard of one user getting over 90MHz out of it! (in that case the oscillator needed would have been 60MHz). I noticed that if the oscillator was increased past 63MHz, I got an intermittent corrupted BIOS, which could be cured by removing the battery temporarily. As in all overclocking cases, you may achieve different figures to me.

Overclocking the ISA bus

As mentioned previously, it is possible to overclock the ISA bus by using a certain type of VGA card, which provides about twice the data transfer rate. It is possible to get more by changing the ISA oscillator (see diagram). This is a 16 MHz crystal which is divided by 2 to give the ISA bus frequency of 8MHz. Increasing the oscillator to a maximum of about 33MHz gives a further big increase in ISA transfer rates, and removing it entirely gives the massive rate of 10MB/sec, which is three times the standard rate! This is assuming you are not using the Rev to 486 chip, which would only give you a rate of around 6MB/sec. Remember that you will gain no improvement at all if you are using an ordinary old VGA card - you will be stuck with the standard 3MB/sec. In any case, as far as I know, all of the last generation of ISA VGA cards have this bus master feature for faster ISA. As mentioned earlier, however, the Diamond Speed Star Pro is the fastest of these cards and is the one I recommend. In addition, it is possible to fit 2MB of display memory to it, unlike most ISA VGA cards, which are limited to a maximum of 1MB.

Clock Division Ratios

386SX CLOCK=Oscillator frequency / 2

TI486SXLC2 CLOCK=Oscillator frequency / 2 or, Oscillator frequency x 1 (user selectable)

IBM486SXLC3 CLOCK=Oscillator frequency / 2, or Oscillator frequency x 1, or Oscillator frequency x 1.5 (user selectable)

ISA BUS FREQUENCY=Oscillator frequency / 2

Points to watch

* Keep the CPU and PAL chips cool - use a heatsink

* Remember, don't fit an FPU if you are using one of the upgrade CPUs, unless you are operating without clock doubling or tripling enabled.

* Don't use a shared Amiga hard drive. Its too slow. Get a cheap (less than $20) IDE controller card and a cheap IDE drive.

* Use Linux......or better still the Amiga! (Windows uses 30% of your CPU just to run itself).

* Watch the orientation of any oscillators you change. Getting it wrong will destroy the oscillator.

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Introduced August 7th 1998. Revised Feb 3rd 1999. Version 1.2.