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IBM AT & XT/286 Upgrades

Great Hierophant

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Am I right about the following, or is there something else that needs be taken into consideration :

I suspect that there are far more 6MHz IBM AT machines and boards out there than 8MHz AT or 6MHz XT/286 boards. An inexpensive way to make them faster is to upgrade them to 8MHz.

The IBM AT is modular with two clocks, one for the CPU (12MHz divided by 2) and one for the rest of the I/O chips (14.318180MHz divided).

In order to obtain a higher clock speed, you must switch out the crystal for a 16MHz one of the same type. Additionally you must make sure that your various components are rated for the higher speed :

CPU must be an 80286-8 or better and be of the PGA type
RAM must be 125ns or faster (shouldn't be an issue, but you may need to piggyback two chips if you have less than 512K and have the first version of the motherboard or use a memory board to backfill the RAM).
DMA chips must be 8237A-5 (they should be but you need to check since DMA runs at 4MHz in an 8MHz machine and 3MHz in a 6MHz machine and the stuck 8237A is only rated for 3MHz).

Now that the system is comparatively fast, that BIOS needs some work so additional drive types, the Model M keyboard and 3.5" drives are properly supported. You could burn the 11/15/85 BIOS (for the 8MHz AT) and use that. In fact, if you have the 06/10/85 BIOS, you need to do this or your system will fail the speed check.

Suppose you had an XT/286 system, you could do the exact same thing, except it also has a speed check for 6MHz. I do not know whether the AT 11/15/85 BIOS will work in the system due to the lack of wait state hardware and other minor differences, so you may need to burn a generic AT compatible BIOS.
 
You may need to replace the BIOS on a 5170--long before IBM started offering 8 MHz versions of the 5170, people were swapping crystals. It got to be ridiculous after awhile--some were running the CPU as fast as 12MHz with a heatsink. If you looked in the back section of Computerworld or PCWeek, you could find lots of ads offering sets of crystals (replace yours with the fastest one that works) and there was at least one PLL clock that allowed more-or-less continuous variation of the clock speed.

IBM countered by adding a speed check in one revision of the BIOS so that if a user re-cyrstalled a 5170, it would not pass POST. Of course, a patch was quickly worked out and soon bootleg patched copies of the BIOS were making the rounds. Eventually, they dropped the charade.

I see no reason why the same trick shouldn't work on the XT/286--if anything, it should be a better candidate.
 
You definitely have to replace the bios in a Rev 3 (and I think Rev 2) 5170 motherboard in order to gain the advantages already listed, as well as to use floppy controllers outside of the IBM one, IIRC, as you'll get a 601 error if you do so (the Rev 1 board, the one w/ the piggy-backed RAM, doesn't have this limitation)

You can view the specifics in our Wiki, however be aware that this link may change in the future if/when we fix our wiki to be more serviceable.

Even with the faster clock speed, unless one went up to the 10+ mhz processors, I'd put odds on the XT/286 being faster overall because of the way that it accesses the RAM.
 
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I see no reason why the same trick shouldn't work on the XT/286--if anything, it should be a better candidate.

The BIOS just counts the number of DRAM refreshes which occured during a loop and then checks if it's within a given range. In the XT/286, this code starts at F000:05A8 and ends with F000:05CE. F000:05CF is the first instruction of the following step of the startup routine.

In other words, if you hardcode a jump to 05CF at 05A8, and ajust the checksum accordingly, then you should overcome the problem.
 
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