PiDP-8/I Software: os-image

This directory contains the PiDP-8/I specific configuration for rpi-image-gen, the Raspberry Pi Foundation's modern image generation tool, plus a few layered-on files for same to ensure that the OS boots with a suitable starting configuration. This lets us build the PiDP-8/I OS images on a suitably powerful Raspberry Pi without the need for manual SD card manipulation, as was required by the legacy bosi tool, which was last used to build images based on Raspberry Pi OS Buster Lite, long since obsolete.

Concept Map

The bosi stages map to rpi-image-gen concepts as follows:

bosi stage	rpi-image-gen equivalent
`do_init` (package install)	`mmdebstrap: packages:` in layer YAML
`do_build` (fossil clone + build)	`customize-hooks:` in layer YAML
`do_prepare` (sanitize)	handled automatically by rpi-image-gen
`do_image` (capture SD card)	`rpi-image-gen build` output

Hardware Support

The original bosi tool worked on any Raspberry Pi that could run the PiDP-8/I I/O board itself. Because rpi-image-gen does not support ARMv6, the minimum supported target is now the Pi Zero 2 W (BCM2710, ARMv8 quad-core Cortex-A53). This also retires the NLS (No Lamp Simulator) image variant, since the Zero 2 W is fast enough to run the ILS. This aligns with developer priorities, who want a sufficiently fast build host that the OS/8 RK05 disk images are not painful to rebuild.

Supported targets:

Target	Device layer override
Pi Zero 2 W	`IGconf_device_layer=rpizero2w`
Pi 3	`IGconf_device_layer=rpi3`
Pi 4	`IGconf_device_layer=rpi4`
Pi 5	`IGconf_device_layer=rpi5`

The default device layer is rpi-generic64-device, a thin wrapper around rpi-generic64 that declares the device provider capability per the recommended rpi-image-gen pattern. See layer/rpi-generic64-device.yaml and upstream issue #202.

Project Structure

os-image/
├── README.md                            this file
├── config/
│   └── pidp8i-trixie.yaml               main build config (Debian Trixie, arm64)
├── files/
│   ├── bashrc-password-check.sh         Force password change on first interactive login.
│   ├── pidp8i-firstboot-system.service  Configures wifi if wpa_supplicant-wlan0.conf present.
│   ├── pidp8i-firstboot-user .service   runs 'pidp8i install' on first boot, then reboots
│   └── wpa_supplicant-wlan0.conf        WiFi credentials template — copy to the
│                                        boot partition and fill in before first boot
└── layer/
    ├── pidp8i-common.yaml.in           main layer: packages, build, install, config
    |                                   processed by auto.def to pick current branch.
    └── rpi-generic64-device.yaml    device provider wrapper for rpi-generic64

Prerequisites

A qualifying Raspberry Pi to use as a build host. The recommended path is a Pi 4 or 5 running a 64-bit version of Raspberry Pi OS 12 or higher. The hardware can be as old as a Pi 2 or a Pi Zero 2 W, though this will make the build time painfully long. This will not work on a 32-bit OS.

Alternatively, rpi-image-gen supports QEMU and containerized builds, though these are not formally tested for PiDP-8/I images.

A clone of rpi-image-gen:

$ cd ~
$ git clone https://github.com/raspberrypi/rpi-image-gen

A clone of this project:

$ cd ~
$ sudo apt install fossil
$ fossil clone https://tangentsoft.com/pidp8i

Refinements on this are in our contribution doc.

Required packages installed on the build host:

$ cd ~/rpi-image-gen
$ sudo ./install_deps.sh ~/pidp8i/os-image/depends

is restricted to 64-bit ARM targets. It is therefore known to work with the latest Pi 5 down to the Pi Zero 2 W.

Building

On the Pi 4+ build host:

cd ~/rpi-image-gen
sudo ./rpi-image-gen build -c pidp8i-trixie.yaml \
    -S ~/pidp8i/os-image

To target a specific device:

sudo ./rpi-image-gen build \
  -S ~/pidp8i/os-image \
  -c pidp8i-trixie.yaml \
  -- IGconf_device_layer=rpizero2w

The output image is written to ~/rpi-image-gen/work/deploy-<date>/.

If /tmp on your build box is a tmpfs, you may run out of scratch space. You can fix it like so:

sudo mkdir -m 1777 /media/usb/tmp
sudo TMPDIR=/media/usb/tmp ./rpi-image-gen build …

When iterating on that build process, you may prefer to maintain a parallel clone of the above on a more powerful development system, then sync it up to the Pi to test before committing a change:

fossil patch push pi5:pidp8i

Deploying

On the software deployment host:

r=$(date +%F)
s=pidp8i-trixie.img.zst
d=$(echo $s | sed -e s/-/-${r}-/ -e s/zst/xz/)
cd ~/src/tangentsoft.com/trunk
ssh pi5 "cat rpi-image-gen/work/deploy-${r}/${s}" | zstd -d | xz -9 > dl/${d}
make synch

Notable Implementation Details

User service installation: pidp8i install requires a live systemd user session bus and cannot run in the chroot. It is deferred to a one-shot systemd user service (pidp8i-firstboot.service) that runs on first boot, installs the simulator service, and reboots. From the second boot onwards the simulator starts automatically.

GPIO permissions: The pdp8 binary accesses /dev/gpiomem directly with no library dependencies. A udev rule sets GROUP=gpio MODE=0660 on the device node. The pidp8i user is added to the gpio group by the rpi-user-credentials layer.

linger: loginctl enable-linger cannot run in the chroot. The linger file is created directly at /var/lib/systemd/linger/pidp8i.

Password change on first login: We use a sentinel file (~/.password-not-changed) checked by .bashrc rather than passwd -e to force a password change on first interactive login. The reason is that passwd -e causes PAM to block the linger-triggered systemd user session at boot, preventing the simulator from autostarting without any login. Since .bashrc only runs for interactive shells, the sentinel file approach is invisible to systemd and leaves linger unaffected.

PATH: The Makefile's PATH update uses $SUDO_USER which is unset in the chroot. The layer adds /opt/pidp8i/bin and /opt/pidp8i/share/man to the pidp8i user's .profile directly.

WiFi: The image ships with a wpa_supplicant-wlan0.conf template on the boot partition (FAT32, visible on any OS). The user fills in their SSID and password before first boot. The firstboot service moves it to /etc/wpa_supplicant/ on first boot. Alternatively, sudo raspi-config → System Options → Wireless LAN can be used after connecting via ethernet.