Adding support for PDP- Major States within the CPU.

[poetnerd]

• Context

• Oscar Vermeulen has produced kits that offer a 3/4 scale model of traditional DEC hardware with software emulated on a Raspberry Pi: PiDP-10, PiDP-11, and the first, and still very popular PiDP-8/i. The joy of these kits is two-fold: experiencing old computer hardware and software architectures, and playing with the lights and switches of very-well emulated front panels. See: PiDP-8/I: RECREATING THE PDP-8/I
  

• The PiDP-8/i looks like the old PDP-8/i with the major registers and major states on display.

• SIMH doesn't emulate down to the Major States. It's an instruction-level emulation. Indeed, the first foundational principle reported at [About the Open SIMH project(https://opensimh.org/about/) says, but not cycle-accurate, simulation.

• The PiDP-8/i software project has developed, tested and benchmarked a fork of the OPEN SIMH PDP-8 emulation that supports the major states. This is of value to the PiDP-8/i project because it means that the "SING STEP" button on the emulated PDP-8/i front panel actually works and steps through the Fetch/Defer/Execute Major States.

• Benchmarking says that performance with emulation of Major States is half the performance of the instruction-level emulation.

• We would really like to get out of the business of maintaining a fork of SIMH. But we recognize that there's a performance penalty, and the proposed change is a step down the road of "cycle-accurate simulation."

• QUESTION: Might the PDP8 SIMH maintainers be willing to accept a pull request to add Major State support?

• the output of "sim> SHOW VERSION" while running the simulator which is having the issue

PDP-8 simulator Open SIMH V4.1-0 Current
Simulator Framework Capabilities:
32b data
32b addresses
no Ethernet
Idle/Throttling support is available
Virtual Hard Disk (VHD) support
RAW disk and CD/DVD ROM support
Asynchronous I/O support (Lock free asynchronous event queue)
Asynchronous Clock support
FrontPanel API Version 12
Host Platform:
Compiler: GCC Apple LLVM 15.0.0 (clang-1500.1.0.2.5)
Simulator Compiled as C (Release Build) on May 19 2024 at 13:01:35
Build Tool: autosetup+gmake
Memory Access: Little Endian
Memory Pointer Size: 64 bits
Large File (>2GB) support
SDL Video support: No Video Support
No RegEx support for EXPECT commands
OS clock resolution: 1ms
Time taken by msleep(1): 1ms
OS: Darwin Mac-mini.lan 22.6.0 Darwin Kernel Version 22.6.0: Mon Feb 19 19:43:13 PST 2024; root:xnu-8796.141.3.704.6~1/RELEASE_ARM64_T6020 arm64
Processor Name: Apple M2 Pro
tar tool: bsdtar 3.5.3 - libarchive 3.5.3 zlib/1.2.11 liblzma/5.0.5 bz2lib/1.0.8
curl tool: curl 8.5.0 (aarch64-apple-darwin22.6.0) libcurl/8.5.0 OpenSSL/3.2.1 zlib/1.3.1 brotli/1.1.0 zstd/1.5.5 libidn2/2.3.7 libpsl/0.21.2 (+libidn2/2.3.4) nghttp2/1.58.0
git commit id: ffe537a
git commit time: 2024-02-20T18:20:13-08:00

• how you built the simulator or that you're using prebuilt binaries

We successfully build from source cloned from https://github.com/open-simh/simh.git

• the simulator configuration file (or commands) which were used when the problem occurred.

No problems encountered.

• the expected behavior and the actual behavior

We propose internal changes to PDP8/pdp_8cpu.c to allow emulation of the Fetch/Defer/Execute major states which the PiDP-8/i hardware would be able to display,
and control through the SING STEP front panel switch.

• you may also need to provide specific pointers to data files that may be necessary to demonstrate the problem

[markpizz]

I will be glad to take your changes and include it in the https://github.com/simh/simh repo. In all likelyhood, the results could then also be migrated to the https://github.com/open-simh/simh repo.