The PLA scheme (another invention from the fertile mind of Bill Roberts,
architect of the LSI11 and UDA50 proto and founder of Emulex) was
basically a microcode and gate conservation scheme. It provided enormous
compression for the decode phase of the PDP11 and then got exploited to
a fare-thee-well by clever microcoders like Burt Hashizume (F11) and
Keith Henry (J11).
Sometime during the 80s, when it became clear that the process problems
with the J11 were intractable, and it could not be shrunk for higher
performance, I outlined a design for a single chip PDP11. Because ROM
densities and transistor counts were so much higher by then, I felt it
could all be done in ROM, using the usual "indirection pointers" like Rs
and Rd for source and destination registers and a fairly simple initial
decoding scheme to break out the interesting cases. I didn't save
anything about it in my online archive, so I don't know what happened to
the design notes. The idea didn't go anywhere, of course; the PDP11 was
clearly on its last legs by then.
/Bob
On 7/1/2019 2:36 PM, simh-requ...@trailing-edge.com wrote:
Message: 2 Date: Mon, 1 Jul 2019 10:50:51 -0600 From: Eric Smith
<space...@gmail.com> To: simh@trailing-edge.com Subject: Re: [Simh]
Which PDP-11 to choose Message-ID:
<CAFrGgTRM4n22yugG60SLh_xT=ttk4ifamon9rfq5fgmyz-l...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8" <snip>
IIRC Bob has written that no one has succeeded at building an alternate J11
hardware implementation, e.g., in an FPGA., because the microcode is not
entirely ROM. There is a fairly large (for the time) PLA forming part of
the control store. The PLA could be transformed into a ROM, but IIRC it has
_many_ inputs, so the ROM would be YUGE.
Almost 20 years ago I wrote a program to translate the PLA into VHDL
directly instantiating Xilinx 4LUT primitives to see how much resources it
would consume in e.g. a Spartan 3 FPGA. I don't recall the numbers, but it
didn't seem insurmountable at the time, and with today's bigger FPGAs and
6LUTs, it's even less of a problem.
Also, I think the synthesis tools are good enough that just giving the PLA
equations to synthesis would be fine, and my scheme of programmatically
transforming the equations to LUT instantiations is totally unnecessary.
Eric
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