Plexus P/20 Analysis

AI-assisted reverse engineering analysis of the Plexus P/20 minicomputer's boot ROMs, UNIX operating system, and disk contents.

The original hardware, ROM dumps, disk images, and documentation come from Adrian's Digital Basement — see Adrian's Plexus P/20 repository for the source material.

Watch the Plexus P/20 video series:

• Part 1: What is this rare multi-user UNIX workstation? (Plexus P/20)
• Part 2: First power up of the Plexus P/20 dual processor UNIX system
• Part 3: Booting UNIX on the Plexus P/20 didn't go to plan
• Part 4: We made some unbelievable discoveries about this Plexus P/20!
• Part 5: The Plexus P/20 is now fully operational!

System Overview

• CPU: Motorola MC68000/68010, 10 MHz (dual-processor: DMA + Job)
• RAM: 2 MB
• Bus: Intel Multibus I
• OS: AT&T UNIX System V Release 2 (UNIX/1.2: Sys5.2r8)
• Boot ROM: 64KB across U15/U17, dated June 4, 1985
• Disk: OMTI 5200 SCSI-to-MFM, 10 heads x 753 cylinders x 17 spt
• Kernel: /unix, 180,208 bytes, dated Nov 27 1985

Repository Contents

Documentation (docs/)

File	Description
docs/boot_loader.md	Complete U15 boot loader analysis — command parser, filesystem, a.out loader, SCSI driver
docs/filesystem.md	Detailed s5fs filesystem implementation from ROM disassembly
docs/debug_monitor.md	U17 debug monitor — all commands, memory layout, SCSI operations
docs/disk_contents.md	Complete catalog of all 4,584 files across rootfs and stocku partitions
docs/registers.md	Hardware register map — memory-mapped I/O addresses
docs/ROM_boot_menu.md	Boot ROM menu interface and commands
docs/ROM_debug.md	ROM debug monitor details
docs/notable_files.md	Curated guide to the most interesting files on the disk
docs/Jumper List Mainboard.md	Mainboard jumper configuration reference

ROM Analysis

File	Description
ROM_analysis.md	Function-by-function ROM analysis with full hardware context and cross-references
U15_disasm.txt	Raw MC68000 disassembly of U15 boot loader ROM
U17_disasm.txt	Raw MC68000 disassembly of U17 self-test/debug ROM

Tools

File	Description
s5fs_tool.py	Read/write tool for AT&T System V s5fs filesystem disk images — list, extract, cat, create, mkdir, rm

Extracted Source Code (extracted-source/)

All source code, shell scripts, headers, and configuration files extracted from the disk image, preserving original directory structure. 820 files total.

extracted-source/
  rootfs/              # Main filesystem (521 files)
    .profile, .cshrc, .login
    etc/               # Shell scripts & configs (passwd, rc, inittab, etc.)
    root/              # root home directory profiles
    usr/src/           # Kernel source, LP source, VPM source, build scripts
    usr/include/       # 293 C headers (full sys/ tree)
    usr/lib/           # acct/, cron/, sa/, spell/, uucp/ scripts
    usr/spool/         # lp/model/ printer scripts, cron/crontabs/
    usr/local/         # SCCS scripts, 20-20 CAD configs
  stocku/              # Recovery partition (299 files)
    etc/               # Shell scripts & configs
    usr/src/           # Kernel config source (conf.c, low.s, rlow.s)
    usr/include/       # 218 C headers
    usr/lib/           # acct/, ctrace/, printer models

Key files of interest:

• extracted-source/rootfs/usr/src/:mkuts68 — Kernel build script (supports Lundell/P20, Robin/P35, Pirate, Schroeder variants)
• extracted-source/stocku/usr/src/uts/m68/cf/conf.c — Kernel device configuration
• extracted-source/stocku/usr/src/uts/m68/cf/low.s — MC68000 interrupt vectors (Lundell/P20)
• extracted-source/stocku/usr/src/uts/m68/cf/rlow.s — MC68010 interrupt vectors (Robin/P35)
• extracted-source/rootfs/etc/passwd — User accounts
• extracted-source/rootfs/etc/rc — System boot script

Machine Variants

The kernel build system supports four Plexus models:

• Lundell (P/20) — MC68000, USART console, Multibus I/O
• Robin (P/35) — MC68010, SCSI, onboard serial ports
• Pirate — variant
• Schroeder — variant

How the Analysis Was Done

1. ROM binaries were disassembled with m68k-linux-gnu-objdump
2. Function boundaries were identified from the disassembly
3. Functions were sent to LLMs with full hardware context for analysis
4. Results were compiled into the ROM analysis documents
5. Disk image contents were extracted and cataloged from the original tar archives