Modifying microcode

Camiel Vanderhoeven camiel.vanderhoeven at
Wed May 30 10:11:48 CDT 2018

On 5/30/18, 5:03 PM, "Kyle Owen" <kylevowen at> wrote:


On Wed, May 30, 2018 at 9:34 AM, Camiel Vanderhoeven <camiel.vanderhoeven at> wrote:

The microcode for the MicroVAX 2 (for which the MICRO2 assembler was used) and the CVAX (which is the CPU in your 3800) is implemented as a mask ROM on the CPU chip itself. No way to change it, and no way you can use MICRO2 to assemble the microcode for the CVAX.


Gotcha. Which small VAXes had external ROM/PALs for microcode store?


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.

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