18 Sep
2015
18 Sep
'15
10:10 p.m.
On 9/18/2015 9:51 PM, tony duell wrote:
Simple answer. The control store functionality had to fit in the available space
on one hex
sized card, and 16K density DRAM was the only option. Allocating additional
space to use
4K or 1K SRAM would have pushed the CPU design to an additional board, which was not
a design option at the time.
Don
The VAX-11/730
would have to have the microcode on TU58, as that is the
console media. Quite possible that it actually do load the microcode
from there at power up. I've heard that those machine were slow in so
many ways... :-)
The 11/730 has no microcode ROM as far as I can see. However part
of the
instruction decoding is done in hardware, so implementing a totally different
instruction set would be inefficient
The craziest thing I have found in the 11/730 is that the control store is
dynamic RAM (lots of 16K*1 chips). Every few milliseconds the CPU is
halted while the control store is refreshed. There is space for 4K more
of control store (1K of that is fitted if you have the integrated disk
controller or floating point board) which is SRAM. Quite why the whole
lot isn't SRAM is beyond me.
-tony