Gloworm OS

Started as part of Computie May 23, 2020

Split into its own repository August 07, 2023

Gloworm is a simple Unix-like OS written in C for retro hardware, specifically the Computie68k series of computers. It's largely based on reading Operating Systems Design and Implementation 2ed by Andrew S. Tanenbaum, although I've gone with a monolithic design, at least for the time being, for the simplicity of it. It can do preemptive multitasking using the 68681 timer. It also has an implementation of the minix version 1 filesystem, which uses RAM (through a device driver) to store the data, or a Compact Flash card connected as an IDE device. The second serial port can be configured as a SLIP device, with a basic implementation of UDP and TCP through a BSD sockets-style API, and an NTP command is provided for updating the system time on boot (when booting from disk).

The above video shows connecting over serial from a modern PC and resetting the hardware to run the monitor in ROM which displays the welcome message. The kernel is already loaded in RAM, so running the boot command will boot the kernel which then starts the shell. From the shell, some basic file operations and the ps command are shown.

Operating System Booting From Monitor/Compact Flash on 68k-SMT

The above video shows connecting over serial from a modern computer after first powering the board on. The monitor runs first, giving the ">" prompt. The bootloader has been burnt into flash at address 0x20000. From the monitor, the boot loader is run, which then loads the kernel from the attached compact flash card. Each period (.) character printed represents 1 kilobyte of data loaded from disk). The boot loader then jumps to the loaded kernel, which displays boot messages before running the init process from disk. The init process first runs sh /etc/rc", which runs the ntpdate command to update the system time. It then runs an interactive shell.

Some commands are shown after boot, and then the httpd program is run (which has forking disabled for the time being). From another computer, the curl command is run to issue a request to the board. The httpd program responds with the data "This is a secret message"

Running the Kernel

To compile the kernel for transfer over serial, run:

make kernel.load

It can be sent over serial like the monitor. You might want to modify the kernel configuration in src/kernel/main.c if you're using a board without the CompactFlash card, or you don't want to use networking.

Building A FlashCard Root Image

The image can be built using a loopback device and following make commands:

make create-image
make mount-image
make build-system
make umount-image

Which will produce a 20MB disk image in minix-build.img using the build/ directory as the mountpoint, and the minix1 filesystem. It does not include a partition table, so that needs to be created on the flash drive by other means. It can be written to the partition-specific block device file using dd on a unix-like desktop.

In order to boot off the CompactFlash directly, the boot.load script can be loaded over serial and written to an alternate location such as 0x020000. It must be a location that is outside of the Flash chip's sector in which monitor is written, or else the system will be unbootable. The boot.bin image doesn't contain a vector table like the monitor.bin image does.

After sending the boot.load file over serial, without running it, in the monitor, run the following:

eraserom 20000
writerom 20000
verifyrom 20000

You can then boot from the monitor at any time by running:

boot 20000

The bootloader will print a period character for every 1K of the kernel image loaded from the disk into RAM, and then it will run the kernel after. If it works, you should see the heartbeat LED flashing to indicate the kernel is running and multiprocessing is enabled and switching.

Configuring Networking

If the second serial port is connected to a desktop, it will act like a SLIP connection, which can be bridged to the desktop's network, where /dev/ttyUSB1 is the serial device connected to the second serial port of the board, and 192.168.1.x is the local network, and enp3s0 is the internet-connected network interface on the linux desktop:

sudo slattach -s 38400 -p slip /dev/ttyUSB1
sudo ifconfig sl0 192.168.1.2 pointopoint 192.168.1.200 up
# (this is automatically added on my machine, but might be required) sudo route add -host 192.168.1.200 sl0
sudo arp -Ds 192.168.1.200 enp3s0 pub
sudo iptables -A FORWARD -i sl0 -j ACCEPT
sudo iptables -A FORWARD -o sl0 -j ACCEPT
sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"

The device will have IP 192.168.1.200 which also needs to be configured in the kernel's src/kernel/main.c function at the bottom.

Get the Source

git clone git@github.com:transistorfet/gloworm