DiscoBSD

2.11BSD-based UNIX-like Operating System for STM32 and PIC32 Microcontrollers

DiscoBSD is a 2.11BSD-based UNIX-like operating system for microcontrollers, with a focus on high portability to memory constrained devices without a memory management unit.

The current and sixth official release of DiscoBSD is DiscoBSD 2.5, released on August 11, 2025.

This microcontroller-focused operating system is an independent continuation of RetroBSD, a 2.11BSD-based OS targeting the MIPS-based PIC32MX7. DiscoBSD is multi-platform, as it also supports Arm Cortex-M4 STM32F4 devices.

Source code to the system is freely available under a BSD-like license.

History

DiscoBSD began as an undergraduate Directed Study in the winter of 2020 at the University of Victoria, Canada, as a case study of RetroBSD to port the operating system to the Arm Cortex-M4 architecture, and to enable portability in the hosting environment and target architectures and platforms. The paper Porting the Unix Kernel details this initial porting effort.

Work on DiscoBSD has progressed in earnest since the completion of the Directed Study, with the DiscoBSD/stm32 port booting multi-user in August 2022. The system is quite usable on supported development boards.

And work continues...

DiscoBSD Resource Requirements

A basic, minimal system uses 128 Kbytes of flash and 128 Kbytes of RAM.

The kernel is loaded into the flash and only uses 32 Kbytes of RAM. User programs each use the remaining 96 Kbytes of RAM, via swapping. Devices with more RAM can be used to run larger user programs.

An SD card, at least 512 Mbytes in size, is required for the root file system.

Installing and Running

Installation consists of loading the kernel into the microcontroller’s flash memory, and imaging the SD card with the file sdcard.img.

On Windows host systems use a disk imaging utility such as Rufus.

On Unix-like host systems with dd run:

$ dd bs=1M if=sdcard.img of=/path/to/SD/card

The board-specific kernel unix must be loaded into the MCU’s flash memory. Formats are ELF unix.elf, binary unix.bin, and Intel HEX unix.hex.

Flashing a DiscoBSD/stm32 kernel firmware into target board’s flash memory:

On Windows host systems use STM32CubeProgrammer for flash programming.

On Unix-like host systems use st-flash from the stlink-org project to load the binary-formatted kernel unix.bin into flash memory at 0x08000000.

$ st-flash --reset write unix.bin 0x08000000

Flashing a DiscoBSD/pic32 kernel firmware into target board’s flash memory:

On all supported host systems use pic32prog for flash programming.

$ pic32prog unix.hex

Communication with the DiscoBSD console requires a serial port. A USB to TTL device or the built-in VCP USB serial port on development boards can be used.

$ cu -l /dev/cuaU0 -s 115200

Depending on the host system, other serial port utilities such as screen, minicom, putty, or teraterm may be used.

Log in to DiscoBSD with user root and a blank password. Shutdown DiscoBSD with the halt, shutdown, or reboot commands.

Manual pages on DiscoBSD are available through the man command.

Building

DiscoBSD is cross-built on UNIX-like host operating systems.

Currently supported host operating systems: OpenBSD, Linux, FreeBSD.

Instructions to configure an OpenBSD host development environment for Arm and MIPS targets is available here.

The build system fully supports both BSD make and GNU make.

From the source tree root, run:

$ make build

or just:

$ make

which builds sys/arch/stm32/compile/${BOARD}/unix ELF-formatted kernels and builds and populates a whole file system userland in DESTDIR.

Running:

$ make distribution

will build everything from a make build, plus generate a file system image in the file distrib/stm32/sdcard.img for imaging to an SD card.

DiscoBSD/stm32 is the default port, but DiscoBSD/pic32 may be built via:

$ make clean
$ make MACHINE=pic32 MACHINE_ARCH=mips distribution

to generate a file system image in the file distrib/pic32/sdcard.img for imaging to an SD card, sys/arch/pic32/compile/${BOARD}/unix ELF-formatted kernels, and sys/arch/pic32/compile/${BOARD}/unix.hex Intel HEX-formatted kernels.

Put the generated file system image sdcard.img onto an SD card.

The make target installfs uses the dd utility to image the SD card attached to the host operating system at SDCARD, such as /dev/rsdXc or /dev/sdX or /dev/rdiskX, replacing X with the actual drive number or letter, as the case may be.

For example, imaging an SD card attached at sd2 on an OpenBSD host operating system through the raw i/o device:

$ make SDCARD=/dev/rsd2c installfs

Note that using BSD make on a FreeBSD host requires the system makefile include directory to be specified on the command line or via the MAKESYSPATH environment variable. For example:

$ make -m /usr/share/mk

or

$ MAKESYSPATH=/usr/share/mk
$ export MAKESYSPATH

$ make

Building a DiscoBSD Release

A DiscoBSD release consists of, for each supported architecture:

• a file system image that contains the full base system
• kernels, in various file formats, for each supported development board
• the README.md file for the architecture
• this README.md file

Before making a release, first create a distribution in DESTDIR with kernels in sys/arch/${MACHINE}/compile/${BOARD} by following the steps in Building:

$ make distribution

Once all the distribution source objects exist then a release can be created; one release for each architecture.

A DiscoBSD/stm32 release, as the default architecture, is created by:

$ make release

A DiscoBSD/pic32 release is created by:

$ make MACHINE=pic32 MACHINE_ARCH=mips release

A DiscoBSD release is created from an already-populated DESTDIR, and it is placed in RELEASEDIR, distrib/obj/releasedir by default.

Releases are available, for each architecture, as a .tar.gz gzip-compressed tar archive and as a .zip zip-compressed archive.

Debugging

DiscoBSD/stm32 is debugged through OpenOCD and GDB. The make targets for debugging are ocd and gdb-ocd.

Debug a particular development board via:

$ make BOARD=F412GDISCO ocd

in one terminal, and:

$ make BOARD=F412GDISCO gdb-ocd

in another terminal.

Additional Information

Port-specific information can be found in distrib/${MACHINE}/README.md for DiscoBSD/stm32 and DiscoBSD/pic32.

References and Resources

• The RetroBSD wiki has a wealth of PIC32 architecture information.
• The Design and Implementation of the 4.3BSD UNIX Operating System.
• Lions’ Commentary on UNIX 6th Edition and recreations of it.
• Advanced Programming in the UNIX Environment 1st Edition.
• The Design of the UNIX Operating System.
• OpenBSD’s curated list of books relevant to BSD Operating Systems.
• The paper Porting the Unix Kernel details DiscoBSD’s inception.
• The Unix Heritage Society’s Unix Archive and Source Tree.
• STMicroelectronics Reference Manual and Programming Manual.
• The Definitive Guide to ARM Cortex-M3 and Cortex-M4 Processors.
• ARMv7-M Architecture Reference Manual.

Source Tree Roadmap

bin         User utilities in both single and multi-user environments.
distrib     System distributions and releases.
etc         Templates for system configuration files and scripts in /etc.
games       Useful and semi-frivolous programs. The important stuff.
include     Standard C include files.
lib         System libraries.
libexec     System daemons and utilities (executed by other programs).
sbin        System administration utilities in both single and
multi-user environments.
share       Architecture-independent shared resource data files.
sys         Kernel sources.
tools       Build tools and simulators.
usr.bin     User utilities in multi-user environments.
usr.sbin    System administration utilities in multi-user environments.

DiscoBSD/stm32 dmesg

DiscoBSD 2.5 (F412GDISCO) #1 651: Mon Aug 11 11:11:11 MDT 2025
chris@stm32.discobsd.org:/sys/arch/stm32/compile/F412GDISCO
cpu: STM32F412xE/G rev 1/C, 100 MHz, bus 50 MHz
oscillator: phase-locked loop, clock source: high speed external
uart2: pins tx=PA2/rx=PA3, af=7, console
sd0: port sdio0
sd0: type SDHC, size 31178752 kbytes
sd0a: partition type b7, sector 2, size 204800 kbytes
sd0b: partition type b8, sector 409602, size 2048 kbytes
sd0c: partition type b7, sector 413698, size 204800 kbytes
phys mem  = 256 kbytes
user mem  = 96 kbytes
root dev  = (0,1)
swap dev  = (0,2)
root size = 204800 kbytes
swap size = 2048 kbytes
Automatic boot in progress: starting file system checks.
/dev/sd0a: 1474 files, 12214 used, 191785 free
/dev/sd0c: 3 files, 3 used, 203996 free
Updating motd... done
Starting daemons: update cron
Mon Aug 11 11:11:11 MDT 2025


2.11 BSD UNIX (name.my.domain) (console)

login: root
Password:
DiscoBSD 2.5 (F412GDISCO) #1 651: Mon Aug 11 11:11:11 MDT 2025

Welcome to DiscoBSD.

erase ^?, kill ^U, intr ^C
#

DiscoBSD/pic32 dmesg

DiscoBSD 2.5 (MAX32) #1 651: Mon Aug 11 11:11:11 MDT 2025
chris@pic32.discobsd.org:/sys/arch/pic32/compile/MAX32
cpu: 795F512L 80 MHz, bus 80 MHz
oscillator: HS crystal, PLL div 1:2 mult x20
spi2: pins sdi=RG7/sdo=RG8/sck=RG6
uart1: pins rx=RF2/tx=RF8, interrupts 26/27/28, console
uart2: pins rx=RF4/tx=RF5, interrupts 40/41/42
uart4: pins rx=RD14/tx=RD15, interrupts 67/68/69
sd0: port spi2, pin cs=RC14
gpio0: portA, pins ii---ii-iiiioiii
gpio1: portB, pins iiiiiiiiiiiiiiii
gpio2: portC, pins i-ii-------iiii-
gpio3: portD, pins --iiiiiiiiiiiiii
gpio4: portE, pins ------iiiiiiiiii
gpio5: portF, pins --ii--------i-ii
gpio6: portG, pins iiii--i-----iiii
adc: 15 channels
pwm: 5 channels
sd0: type I, size 524288 kbytes, speed 10 Mbit/sec
sd0a: partition type b7, sector 2, size 204800 kbytes
sd0b: partition type b8, sector 409602, size 2048 kbytes
sd0c: partition type b7, sector 413698, size 204800 kbytes
phys mem  = 128 kbytes
user mem  = 96 kbytes
root dev  = (0,1)
swap dev  = (0,2)
root size = 204800 kbytes
swap size = 2048 kbytes
Automatic boot in progress: starting file system checks.
/dev/sd0a: 1468 files, 13586 used, 190413 free
/dev/sd0c: 3 files, 3 used, 203996 free
Updating motd... done
Starting daemons: update cron
Mon Aug 11 11:11:11 MDT 2025


2.11 BSD UNIX (name.my.domain) (console)

login: root
Password:
DiscoBSD 2.5 (MAX32) #1 651: Mon Aug 11 11:11:11 MDT 2025

Welcome to DiscoBSD.

erase ^?, kill ^U, intr ^C
#