IBM 5100

From: Jim Battle <>
Date: Tue Aug 17 00:35:54 2004

Nico de Jong wrote:

> From: "Tony Duell" <>
>>It is, of course, possible to make a processor where the microcode can be
>>changed by the user. Many early graphics workstations (the PERQ, many of
>>the Xerox machines, etc) had microcode that was loaded from disk when the
>>machine booted. WCS (Writeable Control Store) options were available for
>>a few models of PDP11 to allow the user to write microcode.

Actually, intel and AMD x86 chips all have a small WCS for patching bugs
that are discovered after the chips are out in the field. The BIOS
loads these patch words on boot up.

To make this more vintage, I will relate it to something I know and
qualifies as vintage.

The Wang 2200 had boards full of low-density ROMs (at least originally
-- over time they replaced the ROM boards with a single board as the
density of available ROMs went up). Because doing a field upgrade of
70+ mask ROM chips would have been really expensive (no usable EPROM
then), Wang had a very interesting ucode patch mechanism somewhat like
that employed by AMD and Intel.

After a system was out in the field, if a ucode patch had to be
employed, a "patch" board would go out to update the system. The patch
board was a small PLA made from diodes and resistors, the output of
which would (1) assert a signal to disable the ROM boards and (2)
generate an address to a patch ROM that supplied the replacement ucode.
Apparently that was cheaper than retrofitting all the ROMs.

When the 2200 VP model came around (1975 or so) they changed to a WCS
Received on Tue Aug 17 2004 - 00:35:54 BST

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