On 4/22/24 19:14, Bill Gunshannon via cctalk wrote:
On 4/22/2024 2:30 PM, Paul Koning wrote:
On Apr 22, 2024, at 2:09 PM, Bill Gunshannon via cctalk
<cctalk@classiccmp.org> wrote:
Following along this line of thought but also in regards
all our
other small CPUs....
Would it not be possible to use something like a Blue
Pill to make
a small board (small enough to actually fit in the CPU
socket) that
emulated these old CPUs? Definitely enough horse power
just wondered
if there was enough room for the microcode.
Microcode?
Well, that's what I would have called it. :-)
It would bring an even more interesting concept to the
table. The
ability to add modifications to some of these chips to
see just where
they might have gone. While I don't mind the VAX, I
always wondered
what the PDP-11 could have been if it had been developed
instead. :-)
bill
Of course the VAX started out as a modified PDP-11; the
name makes that clear. And I saw an early document of
what became the VAX 11/780, labeled PDP-11/85. Perhaps
that was obfuscation.
I have never seen anything but the vaguest similarity to
the PDP-11 in
the VAX. I know it was called a VAX-11 early on but I
never understood
why.
Umm, the VAX was a very obvious extension of the PDP-11
instruction layout to 32 bits. The PDP-11 had a 3 bit
register address and 3 bit addressing mode. On the VAX
these were each extended to 4 bits. On the 11, the opcode
field was 4 bits, although more bits were available on unary
instructions. On the VAX, the opcode could be either 8 or
16 bits.
Quoting from the VAX11/780 Hardware Handbook Preface
"VAX-11/780 is DIGITAL's 32 bit extension to its 11 family
of minicomputers." This is the first sentence in the book.
As somebody who programmed PDP-11s and VAXes in assembly
language (Macro 11 and VAX Macro) I found the similarities
VERY strong. Just that the 32-bit architecture took the
constraints of the 16-bit PDP-11 away.
Jon
