On Tue, Jan 12, 2016 at 03:06:27PM -0500, James Knott wrote:
Many years ago, I worked as a computer tech, maintaining various models of minicomputer. One thing some computers had then was accessable microcode. With the Data General Eclipse line, the microcode was stored in ROM, but if the Writable Control Store (WCS) option was installed, you could add new or even modify existing instructions. The DEC VAX 11/780 loaded it's microcode from a floppy at every boot, so it was a simple matter to provide an updated version.
It was "fun" trouble shooting a problem by stepping through the microcode. ;-)
The alpha couldn't do memory management without extra instructions loaded at boot (usually by the firmware, although milo could fake it). Those instructions were written in a special low level instruction language for the alpha which had access to internal registers for the MMU and TLB that normal instructions couldn't touch, and created instructions software could execute. So Digital kept that idea going. -- Len Sorensen