Overclocking the A2286 Bridgeboard

The Commodore A2286 Bridgeboard contains an Intel 80286 CPU at 8 MHz. This board was designed to allow the option of running at 10 MHz, though I don't know if they were ever sold in 10 MHz form. Before you start, check out Sebastien Boisvert's excellent Bridgeboard Guide . It contains just about everything there is to know about Bridgeboards.

Starting out

Before you reach for the soldering iron, there are three ways to start gaining speed. Firstly, if you are using a PC hard disk emulated on the Amiga's hard drive, you can greatly improve performance by allocating a much larger number of buffers for the Amiga DOS partition housing it. 200 is a good amount to try, though feel free to go higher if you have plenty of Amiga RAM. You can edit this value in HDtoolbox, or by using the 'Addbuffers' command in startup-sequence. For example if your emulated PC hard drive file is located in DH1:, you add this line to startup-sequence (without quotes): "Addbuffers dh1: 200".

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 DOS 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.

Finally, as in all bridgeboards, real 16 bit ISA VGA cards and IDE controllers are always faster than emulating them with the inbuilt Amiga display and hard disk emulation.

Fixing the notorious A20 gate bug.

The A2286 Bridgeboard is notorious for being very slow when it is communicating over the shared RAM with the Amiga. This affects AREAD, AWRITE, etc, and also access to the makeAB drive (if used). Keystrokes can also be lost during access to these. This problem also causes Windows to be very slow, which the A2286 was very notorious for. This problem is caused by a hardware bug where the 8042 keyboard chip gets accessed whenever the A2286 sends an interrupt to the Amiga, causing it to waste a lot of CPU time. To correct this bug, pin 6 of the 8042 chip (U72) needs to be isolated by removing the chip from its socket, then carefully bending pin 6 out so it is not making contact when the chip is replaced. A wire lead is then carefully soldered to pin 6, with the other end being soldered to pin 30 on J17A. Refer to the picture below.

Fixing the floppy drive controller clock to 16 MHz.

In its standard 8 MHz form, the A2286 has a 16 MHz system clock. After being divided by various ratios, this clock drives the CPU, FPU, ISA bus and the floppy drive controller. The floppy drive is intolerant of a non standard clock, so any variation will cause failure. In order to allow the floppy drive to work correctly in the 10 MHz version of the A2286, Commodore provided a means of clocking the floppy controller separately. In order to enable this, a 16 MHz oscillator needs to be installed in the empty position provided next to the floppy cable header. Two jumpers next to the new oscillator also need to be altered. JS4A needs to be cut, and JS4B needs to be connected by using a blob of solder. This is the reverse of the default state. See below. Once this modification is performed, you are now free to alter the system frequency.

Overclocking the CPU & ISA bus.

The system clock is derived from a 16 MHz oscillator located at 'osc 1' on the top edge of the main board. It is partially covered by the smaller 'sandwich board'. This 16 Mhz clock is divided by two to generate the 8 MHz CPU clock. This same clock is also used for the ISA bus, so altering this clock will also alter the ISA bus clock frequency. In most cases this should not be a problem, but be aware it is possible that certain ISA cards may not like a higher speed ISA bus.

In order to overclock your A2286, you simply need to change this oscillator. As already mentioned, the A2286 is actually designed to run at 10 MHz, so no problems should be encountered by increasing to at least this, assuming your 80286 CPU is OK with this. In my case, I had replaced my 8 MHz CPU with a 12MHz one, and I had successfully used a 25.175 MHz oscillator, giving a CPU clock just a shade over 12 MHz. I was not able to go much past this speed. It should be noted that the A2286 is not really good for heat dissipation and there is no room for heatsinks due to the sandwich board covering the CPU (and FPU). I would consider 12 MHz to be the upper limit for reliable and safe operation.

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Introduced 8th December 2023. Version 1.0.