View Full Version : Extending the PDP-8 architure?

February 16th, 2017, 07:39 PM
This might be off-topic here, but I thought it would be the best place to ask.

I was reading about 6502's and came across some projects that had extended it from 8 bits to 16 and 32 bits. Has anything like that been done with the PDP-8? I do know about Data General and their story, but do not know how close it would be to a PDP-8 extended.

Curious about this and trying to imagine a PDP-8 in 24 or 36 bits.:)

February 16th, 2017, 07:45 PM
Not that I've ever heard of. There were extensions to the PDP-8 for a few things (multiply/divide, extended memory addressing,) but never a full architectural upgrade, because DEC had other computer lines for people who wanted something with more oomph to it, and the -8's main appeal was that it was a simple, small, low-cost minicomputer for people who didn't need something fancier.

February 16th, 2017, 08:33 PM
That is what I thought. Would be an interesting hobby project, for someone smart and stubborn enough.

February 16th, 2017, 08:54 PM
The HP 2100 / 1000 instruction set in some ways seems similar to the PDP-8 instruction set extended to 16-bits.

"The HP 2100 is one of many 8- and 16-bit machine architectures said to be inspired by the PDP-8. These can be characterized by use of RAM instead of registers, page-oriented addressing, and a small number of accumulators (such as A and B) rather than a relatively large number of regular registers (such as R0-R7 found on the PDP-11). This philosophy can save money when RAM is less expensive than registers."

February 17th, 2017, 06:04 AM
The HP 2100 / 1000 instruction set in some ways seems similar to the PDP-8 instruction set extended to 16-bits.

This line evolved into the BPC+IOC+EMC triad that was the CPU for the 9825, 9835 and 9845 lines of computers from HP, as described by the 9825A US patent 4,075,679. The BPC chip itself was used as an embedded controller for various peripherals, such as the 9875 tape drive and 9872 plotters.

If you are curious about the details and want a smaller download than that patent, search out the 9835 assembler ROM manual.

February 17th, 2017, 07:57 AM
Hi All;

A couple of things, One if it was built with TTL, like my PDP8i Project, then it could be relatively easy to expand, to a longer bit word..

But, I would think that doing it in a FPGA type of board would be much easier to attempt and to build..