Modifying microcode

Kyle Owen kylevowen at
Wed May 30 12:22:16 CDT 2018

On Wed, May 30, 2018 at 12:05 PM, Jon Elson <elson at> wrote:

> On 05/30/2018 08:19 AM, Kyle Owen via cctalk wrote:
>> I'm thinking about trying to find a microcoded architecture to play with
>> before I design something around the Intel 3000 series.
> Intel 3000?  WHY!

Well, the Tesla clones were cheap and readily available on eBay! :)

> I'd get an FPGA development board and download Xilinx's webpack software.
> It would not take real long to design the basic microcode engine, and then
> you could develop some application microcode in parallel with the hardware,
> adding whatever feature to the hardware you needed when the need came up in
> the microcode.

Yup, I've played around a lot with my Basys 3 board. But I like the idea of
writing microcode for an existing design (one that has a software
simulation as well as real hardware would be preferable), even though it
could likely run much faster on an FPGA.

> I've got a MicroVAX 3800, so I suppose I could run MICRO2 to assemble the
>> aforementioned microcode. But then what? I assume PALs would have to be
>> burned to implement the new microcode. Or is it more complicated than
>> that?
>> PALs?  I don't think the 3800 microcode was in PALs.  I think it was in
> the CPU chip.  There may have been a patch array that allowed a very small
> number of microcode words to be overridden.

Yes, you appear to be correct; it's all internal.


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