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https://github.com/chettrick/discobsd

2.11BSD-based Unix-like OS for STM32 and PIC32 Microcontrollers
https://github.com/chettrick/discobsd

211bsd arm bsd c cortex-m discobsd embedded microcontroller operating-system pic32 stm32 unix

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2.11BSD-based Unix-like OS for STM32 and PIC32 Microcontrollers

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README 

          
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][1],

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][2] began as an undergraduate [Directed Study][3] in the winter of

2020 at the University of Victoria, Canada, as a case study of [RetroBSD][4]

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*][5] 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...



[1]: https://github.com/chettrick/discobsd/releases/tag/DISCOBSD_2_5

[2]: http://DiscoBSD.org

[3]: https://github.com/chettrick/CSC490

[4]: https://RetroBSD.org

[5]: https://github.com/chettrick/CSC490/raw/master/project_outputs/Porting_the_Unix_Kernel-CSC490-Christopher_Hettrick.pdf



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][6].



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][7] for flash programming.



On Unix-like host systems use `st-flash` from the [stlink-org project][8] 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][9] 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.



[6]: https://github.com/pbatard/rufus

[7]: https://www.st.com/en/development-tools/stm32cubeprog.html

[8]: https://github.com/stlink-org/stlink

[9]: https://github.com/majenkotech/pic32prog-autotools



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][10].



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



[10]: tools/openbsd/README.md



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](#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][11], for each architecture, as a `.tar.gz`

gzip-compressed tar archive and as a `.zip` zip-compressed archive.



[11]: https://github.com/chettrick/discobsd/releases



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][12] and [DiscoBSD/pic32][13].



[12]: distrib/stm32/README.md

[13]: distrib/pic32/README.md



References and Resources

------------------------



* The [RetroBSD wiki][14] has a wealth of PIC32 architecture information.

* [The Design and Implementation of the 4.3BSD UNIX Operating System][15].

* [Lions' Commentary on UNIX 6th Edition][16] and [recreations of it][17].

* [Advanced Programming in the UNIX Environment][18] 1st Edition.

* [The Design of the UNIX Operating System][19].

* [OpenBSD's curated list of books][20] relevant to BSD Operating Systems.

* The paper [*Porting the Unix Kernel*][5] details DiscoBSD's inception.

* The Unix Heritage Society's [Unix Archive][21] and [Source Tree][22].

* STMicroelectronics [Reference Manual][23] and [Programming Manual][24].

* [The Definitive Guide to ARM Cortex-M3 and Cortex-M4 Processors][25].

* [ARMv7-M Architecture Reference Manual][26].



[14]: https://github.com/RetroBSD/retrobsd/wiki

[15]: https://archive.org/details/designimplementa0000unse

[16]: https://www.peerllc.com/peer-to-peer-books/lions-commentary-on-unix/

[17]: http://www.lemis.com/grog/Documentation/Lions/

[18]: http://www.kohala.com/start/apue.html

[19]: https://archive.org/details/DesignUNIXOperatingSystem

[20]: https://www.openbsd.org/books.html

[21]: https://www.tuhs.org/Archive/Distributions/UCB/

[22]: https://www.tuhs.org/cgi-bin/utree.pl

[23]: https://www.st.com/resource/en/reference_manual/dm00031020-stm32f405-415-stm32f407-417-stm32f427-437-and-stm32f429-439-advanced-arm-based-32-bit-mcus-stmicroelectronics.pdf

[24]: https://www.st.com/resource/en/programming_manual/pm0214-stm32-cortexm4-mcus-and-mpus-programming-manual-stmicroelectronics.pdf

[25]: https://booksite.elsevier.com/9780124080829/

[26]: https://developer.arm.com/documentation/ddi0403



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

# 

```