On Thu, Jul 24, 2014 at 10:22 PM, Eric Smith <spacewar at gmail.com> wrote:
...
The VAX-11/780 introduced the concept of patchable control store,
where most of the control store was PROM but there was a small amount
of patch RAM. Costs of high speed RAM declined quickly enough that
most later non-microprocessor VAXen used entirely RAM control store.
With the advent of VAX microprocessors, the larger die area required
for RAM vs ROM shifted the economics shifted back to favor ROM with a
small RAM patch area.
Intel started supporting a RAM patch area, I believe, after the Pentium bug
so that they could field-update during the low-level boot process to
(quietly!) overcome any similar problem in the future. Does anyone have
more specifics on this feature, and does this capability currently exist in
any of their more modern CPU architectures?
-----
paul