Brad Parker just posted about his FPGA implementation of a PDP-11 archived on
2014-10-03
, which boots so far RT-11, RSTS V4, BSD 2.9 and Unix V6. There was a question how fast an FPGA solution might be compared to a PDP-11/93.

I've also implemented a PDP-11 on an FPGA. It is a full 11/70 with split I&D, MMU and cache. No FPP so far. Available peripherals are so far DL11, LP11, KW11L, PC11, and RK11. All I/O is channeled over via 'remote-register-interface' onto a single bi-directional byte stream interface, so the FPGA board needs a backend PC with a server program to handle the I/O requests.

The design is FPGA proven, runs on Digilent S3 and NEXYS2 boards, the former with 1 MB 10 ns SRAM, the later with 16 MB 70ns PSDRAM.

Resource consumption is

  S3 board      xc3s1000   2471 slices or 33%
  NEXYS2 board  xc3s1200e  2624 slices or 30%

The implementation was verified against many XXDP maindec's. There are some open issues, especially some details of trap and double error handling aren't correct yet. In practice this is of little importance, the FPGA system happily boots and runs BSD 2.11, a system using 22bit addressing and split I&D space. {Note: you need patch 447 for 2.11BSD to get FPP emulation and RK support working}

On Performance: The design runs at 50 MHz. I've run parts of the Byte Unix benchmark on the FPGA systems. Given that the FPP is only emulated by the 2.11BSD kernel it makes only sense to look at the integer benchmarks. The Dhrystone benchmark 'dhry2reg' gives about '11500 lps' on both boards. For comparison see Michael Schneiders benchmark collection which gives about '830 lps' for a 11/53. There is little Dhrystone difference between the two boards despite the very different memory access times. The 8 kB cache with 32 bit cache lines really helps on the NEXYS2 board.

I'm in the middle of homogenizing some internal interfaces and of some code cleanup, also the backend handler needs a re-write in C++ (currently perl). When that's done I'll make the whole package (VHDL sources, test benches, backend) available on OpenCores.

Finally a comment to Dave Mitton's remark

"Now what would be really cool would be to make 4 CPUs and re-create an 11/74 quad."
See http://www.miim.com/faq/hardware/multipro.html#castor
The reason why I picked a 11/70 and not a J11 as target is because my goal is a 11/74. I've implemented the IIST already and tested against the IIST Diagnostic I could find in XXDP (riiab0). A dual core will fit into a single xc3s1200e of the NEXYS2 board. The work needed is quite clear and doable (changes on cache, mmu, and cpu core for asrb). However, I've no plans to implement the CIS, so it will always be a subset of a 11/74. But for sure fun to do and run.