Modifying microcode

Jon Elson elson at
Wed May 30 21:26:09 CDT 2018

On 05/30/2018 11:48 AM, Paul Koning via cctalk wrote:
>> On May 30, 2018, at 11:11 AM, Camiel Vanderhoeven via cctalk <cctalk at> wrote:
>> Depending on your definition of small, the MicroVAX 1, and the VAX 8000 series (not that small). In both cases though, the ROM chips are a custom DEC design.
> Didn't the 780 get its microcode loaded by the console LSI-11?  And the 730 used bit slice processors (AMD 2901) as I recall, so that had to have its microcode external.
The early 780 had most microcode in ROM, and had a small 
writable control store for special OS-required options and 
patches.  Later machines had more WCS, but I think they 
still had some non-writable control store.  I remember our 
early 780 had the WCS board replaced during a field upgrade, 
and maybe the fixed control store was replaced, too.


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