386 chip "hangs" when in protected mode and 387 in place SC

[Michael Z Slater]

>I am not quoting but it goes something like this.  If you are in protected mod
e
>and you are executing a 32 bit fp instr and access a DMA device the CPU will
>"freeze" or "hang up".  This is only happens with certain releases of the
>chip and is apparently not a problem with newest release.  SCO has a 
>workaround that works on some machines (of course not on ours) ... IF it 
>doesnt work on yours then you can disable the 387 until you are able to
>replace the CPU...   8-(
>
>Intel has documented this and I believe its referred to as "Erratum 21"
>
>I need pointers on identifying these "defective" 386 chips and was wondering
>if anyone has leads on diagnostic software that would verify my suspicions
>regarding this "hanging" of the system... Also .. If I make a visual 
>inspection of the chips what markings would tell me that it was the
>type that "hangs" in protected mode.

You are correct, this is the famous errata 21.

You can identify a chip that does NOT have this bug by the DX suffix
following the 80386 part number.  (Intel began using this suffix at the
same time that they began producing the corrected version.)

There is a hardware fix that, I believe, will work on any machine.  It
requires adding one PAL chip on one of the 386 inputs.  Several companies
have sold adapters that plug into the 386 socket and insert the required
logic.  They sold for about $100, so they are cheaper than replacing the
processor.  Bell Technologies (since purchased by Intel) used to sell them;
I don't know if they still do.

Michael Slater, Microprocessor Report   mslater at cup.portal.com
707/823-4004   fax: 707/823-0504