Dave McGuire wrote:
They've also been used for much more ambitious
things: The VAX-11/730
and DECsystem-2020, for example, are both built around Am2901 bit-slice
chips, running microcode from fast RAM which implements the VAX and
PDP-10 instruction sets, respectively. That microcode is loaded at boot
time, from TU58 in the case of the VAX-11/730, or from a TU45 9-track
magtape on the DECsystem-2020. Before that microcode is loaded, the
machines aren't really VAXen or PDP-10s...as they have no idea how to
implement the architecture or execute the instructions, until the
microcode is loaded. With a good bit-slice chip like an Am2901, one can
build a processor to implement any architecture one wants.
Providing the word length is a multiable of 4 bits that is.
Did going bit slice make the machines more profitable for DEC
rather than lower the price for the end user?
Many, many machines were built in this way...I only
use the VAX-11/730
and DECsystem-2020 as examples because I have them sitting here.
Further, the Am2901 isn't the only bit-slice CPU around...there were
many others. Intel made at least one, I think it was the 3002.
That was a 2 bit bit slice. TI had a weird 4 bit slice. That is all
the ones I can think of.
-Dave
--Dave McGuire
Port Charlotte, FL