MuntsOS Embedded Linux

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CHANGELOG.txt  652555Aug 23 14:10:30 2025
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README.md  34533Aug 18 09:13:11 2025
README.txt  28151Aug 18 09:13:11 2025
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MuntsOS is a ferociously reduced Linux distribution for embedded systems. It runs on several microcomputer boards, including all 64-bit Raspberry Pi models, providing a turnkey RAM resident Linux operating system. With MuntsOS installed, a small and low cost Linux microcomputer becomes a Linux microcontroller, and can be integrated into an embedded system just like a single chip microcontroller but coming with a much, much richer development ecosystem.

Other embedded system Linux distributions such as Buildroot or Yocto Linux are very cumbersome and have very steep learning curves. If you are building a test fixture or process controller or almost any other embedded system that contains a Raspberry Pi board, MuntsOS offers a very high productivity development environment and a very easy to deploy target operating system.

News

Quick Setup Instructions for the Impatient

Instructions for installing the MuntsOS cross-toolchain development environment onto a development host computer are found in Application Note #1 and Application Note #2. Or just download and run one of the following quick setup scripts:

setup-debian
setup-fedora
setup-rhel

Instructions for installing MuntsOS to a target computer are found in Application Note #3 and Application Note #15.

Documentation

The documentation for MuntsOS (mostly application notes) is available online at:

https://repo.munts.com/muntsos/doc

Embedded Linux Distribution in a Kernel

MuntsOS is a stripped down Linux distribution that includes a small compressed root file system within the kernel image binary itself. At boot time the root file system is unpacked into RAM and thereafter the system runs entirely in RAM. After MuntsOS has finished booting, it unmounts the boot media, so you don't have to worry about an orderly shutdown. Just power off the microcomputer board whenever you want to.

Each kernel release tarball contains a kernel image file (.img), which may be common to several different microcomputer boards, and one or more device tree files (.dtb) that are specific to particular microcomputer boards. Some kernel release tarballs also contain one or more device tree overlay files (.dtbo) that can make small changes to the device tree at boot time.

Prebuilt MuntsOS kernel release tarballs are available at:

https://repo.munts.com/muntsos/kernels

Extensions

The MuntsOS root file system can be extended at boot time using any of three mechanisms:

First, if /boot/tarballs exists, any gzip'ed tarball files (.tgz) in it will be extracted on top of the root file system. Typically you would use this mechanism for customized /etc/passwd, .ssh/authorized_keys, and similiar system configuration files.

Secondly, if /boot/extensions exists, any Debian package files (.deb) in it will be installed into the root file system. Note that packages from the Debian project will probably not work; they must be built specifically for MuntsOS. The startup script that installs .deb packages from /boot/extensions also installs .nupkg and .rpm packages as well as self-contained executable extension programs (e.g. remoteio_server-aarch64).

Thirdly, the system startup script /etc/rc can be configured via a kernel command line option to search for a subdirectory called autoexec.d in various places, such as SD card, USB flash drive, USB CD-ROM or NFS mount. If an autoexec.d subdirectory is found, each executable program or script in it will be executed when the system boots.

The idea is to build a MuntsOS kernel (which takes a long time) once and install it to the target platform. Then application specific software can be built after the fact and installed as tarball files in /boot/tarballs; Debian, RPM, and NuGet package files or executable extension programs in /boot/extensions; or executable programs and scripts in /boot/autoexec.d.

Prebuilt MuntsOS ext6.6.58-ti-arm64-r29ension packages are available at:

https://repo.munts.com/muntsos/extensions

Thin Servers

Boot Files + Kernel Files + Extensions = Thin Server

The Thin Server is a system design pattern that is little more than a network interface for a single I/O device. Ideally, a Thin Server will be built from a cheap and ubiquitous network microcomputer like the Raspberry Pi. The software must be easy to install from a user's PC or Mac without requiring any special programming tools. It must be able to run headless, administered via the network. It must be able to survive without orderly shutdowns, and must not write much to flash media. It must provide a network based API (Application Programming Interface) using HTTP as a lowest common denominator.

MuntsOS, with its operating system running entirely from RAM, serves well for the Thin Server, and the two concepts have evolved together over the past few years. The simplest way to use MuntsOS is to download one of the prebuilt Thin Server .zip files and extract it to a freshly formatted FAT32 SD card. You can then modify autoexec.d/00-wlan-init on the SD card to pre-configure it for your wireless network environment, if desired, before inserting it in the target board. After booting MuntsOS, log in from the console or via SSH (user "root", password "default") and run sysconfig to perform more system configuration.

Note: Some platforms require the boot flag to be set on the FAT32 boot partition on the SD card or on-board eMMC. The ROM boot loader in the CPU will ignore any partitions that are not marked as bootable.

MuntsOS Application Notes 3 and 15 contain more detailed instructions about how to install a MuntsOS Thin Server.

Prebuilt MuntsOS Thin Servers are at available at:

https://repo.munts.com/muntsos/thinservers

Boards

BeaglePlay

The BeaglePlay is a small Linux microcomputer board with industry standard interfaces for add-on I/O modules (a mikroBUS socket, a QWIIC socket, and a Grove socket) instead of a general purpose expansion header. It has a Texas Instruments AM6254 ARMv8 Cortex-A53 quad core CPU and comes with 2 GB of RAM. The BeaglePlay has one USB-A receptacle for peripheral devices and one USB-C receptacle for power and tethering. It has a 10/100/1000BASE-T wired Ethernet interface, a 10BASE-T1L single pair Ethernet interface (intended for a factory automation network and worthless for any other purpose), and a CC1352P7 wireless microcontroller capable of supporting a wide variety of radio networks. For more information read the target platform notes in Application Note #19.

The BeaglePlay has a couple of serious design defects: The AM6254 CPU hardware watchdog timers are unusable and the ADC102S051 A/D converter has only 10 bit resolution and lacks a Linux kernel driver. Furthermore, the manufacturer kernel source repository does not often pull changes from the corresponding stable or longterm kernel trees and therefore lacks many upstream changes.

USB Gadgets

You will need to edit /boot/config.txt to enable USB Gadget mode. Change the OPTIONS word to 0x172C for a USB HID gadget, 0x072E for a USB Ethernet gadget, or 0x03AC for a USB serial port gadget. See Application Note #10 for more information about the OPTIONS word.

Orange Pi Zero 2W

The Orange Pi Zero 2W is a small Linux microcomputer with a form factor very similiar to the Raspberry Pi Zero 2 W, making it ideal for embedded system projects. It has a 1500 MHz Allwinner H618 ARMv8 Cortex-A53 quad-core CPU and comes with 1 to 4 GB of RAM and on-board Bluetooth and WiFi radios. It is available for sale on Amazon for $21.99 (1 GB RAM) to $33.99 (4 GB RAM). See Application Note #20 for more information.

The much larger RAM is a big advantage and I have been able to purchase as many as I want without limits when the Raspberry Pi Zero 2 W has been unavailable. Unfortunately, the manufacturer kernel source repository has not been maintained and is currently frozen at 6.1.31.

USB Gadgets

You will need to edit /boot/config.txt to enable USB Gadget mode. Change the OPTIONS word to 0x172C for a USB HID gadget, 0x072E for a USB Ethernet gadget, or 0x03AC for a USB serial port gadget. See Application Note #10 for more information about the OPTIONS word.

Raspberry Pi

The Raspberry Pi is a family of low cost Linux microcomputers selling for USD $15 to $80, depending on model. There have been five generations of Raspberry Pi microcomputers, each using a successively more sophisticated Broadcom ARM core CPU. The first two generations (32-bit ARMv6 Raspberry Pi 1 and 32-bit ARMv7 Raspberry Pi 2) are now obsolete.

Some Raspberry Pi models have an on-board Bluetooth radio that uses the serial port signals that are also brought out to the expansion header. By default, MuntsOS port disables the on-board Bluetooth radio, in favor of the serial port on the expansion header.

All of the following 64-bit Raspberry Pi models use the same AArch64 cross-toolchain.

Raspberry Pi 3

The Raspberry Pi 2 Model B Revision 1.2 with the 900 MHz BCM2710 ARMv8 Cortex-A53 quad-core CPU can be treated as a power conserving Raspberry Pi 3 Model B− and is useful for industrial applications where wired Ethernet is preferred.

The Rasbperry Pi 3 Model B has a 1200 MHz BCM2710 ARMv8 Cortex-A53 quad-core CPU and has 1 GB of RAM along with on-board Bluetooth and WiFi radios.

The Raspberry Pi 3 Model A+ has the same form factor as the Raspberry Pi 1 Model A+, with only one USB host receptacle and no wired Ethernet. It has a 1400 MHz BCM2710 ARMv8 Cortex-A53 quad-core CPU and has 512 MB of RAM along with on-board Bluetooth and WiFi radios.

The Raspberry Pi 3 Model B+ has a 1400 MHz BCM2710 ARMv8 Cortex-A53 quad-core CPU and has improved power management and networking components.

The Raspberry Pi Zero 2 W has the same form factor as the Raspberry Pi Zero W, with a 1000 MHz BCM2710 ARMv8 Cortex-A53 quad core CPU and 512 MB of RAM along with on-board Bluetooth and WiFi radios. This small, light, and inexpensive board is probably one of the best Linux microcomputers available for implementing embedded systems.

All Raspberry Pi 3 models use the same ARMv8 kernel, with different device trees.

USB Gadgets

MuntsOS also offers a second, different Raspberry Pi 3 kernel with USB host support disabled and USB Gadget peripheral support enabled. This kernel only runs on 3 A+, Zero 2 W, and certain CM3 carrier boards which lack the USB hub present on Raspberry Pi 3 Model B and B+ boards. The single USB controller that is part of the BCM2710 CPU is wired directly to the USB-A receptacle on the 3 A+ or the USB Micro-A receptacle on the CM3 I/O board or the Raspberry Pi Zero 2 W.

The Raspberry Pi 3 USB Gadget kernel supports USB Ethernet, Raw HID, and Serial Port gadgets, selected by bits in the OPTIONS word passed on the kernel command line (as configured in /boot/cmdline.txt). See Application Note #10 for more information about the OPTIONS word. Raspberry Pi 3 USB Gadget Thin Servers have USB Network Gadget selected by default.

You can supply power to and communicate with a compatible Raspberry Pi 3 (A+, CM3, or Zero 2W) running the USB Gadget kernel through the USB receptacle. The absolute minimum possible usable Raspberry Pi kit consists of a Raspberry Pi Zero 2 W, a micro-USB cable, and a micro-SD card with one of the MuntsOS Raspberry Pi 3 USB Gadget Thin Servers installed.

Raspberry Pi 4

The Raspberry Pi 4 Model B has a 1500 MHz BCM2711 ARMv8 Cortex-A72 quad-core CPU and is available with 1 to 8 GB of RAM. It diverged significantly from the Raspberry Pi 1 B+ form factor, with the USB and Ethernet receptacles reversed, two micro-HDMI receptacles instead of a single full size HDMI receptacle, and a USB-C power receptacle instead of micro-USB. Two of the USB receptacles are 3.0 and two are 2.0. A major improvement is a Gigabit Ethernet controller connected via PCI Express instead of the USB connected Ethernet used for all earlier models. The Raspberry Pi 4 Model B uses the same wireless chip set as the 3+.

There are also a myriad of Raspberry Pi 4 Compute Modules, with varying combinations of wireless Ethernet, RAM and eMMC.

All Raspberry Pi 4 models use the same ARMv8 kernel, with different device trees.

USB Gadgets

You will need to edit some boot configuration files to enable USB Gadget mode. First, change dtoverlay=dwc2,dr_mode=host to dtoverlay=dwc2,dr_mode=peripheral in /boot/config.txt to change the USB-C receptacle from USB host to USB peripheral. Then change the OPTIONS word in /boot/cmdline.txt to 0x172C for a USB HID gadget, 0x072E for a USB Ethernet gadget, or 0x03AC for a USB serial port gadget. See Application Note #10 for more information about the OPTIONS word.

The Raspberry Pi 4 family consumes significantly more power than the Raspberry Pi 3 and not all host computers will be able to supply enough current to a single USB receptacle to support a Raspberry Pi 4 in USB Gadget mode.

Raspberry Pi 5

The Raspberry Pi 5 Model B yields another 2-3x increase in performance over the Raspberry Pi 4, at the expense of greater power consumption. It has a 2400 MHz BCM2712 ARMv8 Cortex-A76 quad-core CPU and is available with 4 or 8 GB of RAM. The Ethernet receptacle and USB receptacles have swapped sides, so it has a form factor that is sort of a cross between the Raspberry Pi 1 B+ (same grouping of Ethernet and USB receptacles) and the Raspberry Pi 4 (same dual micro-HDMI receptacles and USB-C power receptacle).

There are also a myriad of Raspberry Pi 5 Compute Modules, with varying combinations of wireless Ethernet, RAM and eMMC.

All Raspberry Pi 5 models use the same ARMv8 kernel, with different device trees.

The Raspberry Pi 5 introduced a breaking PWM (Pulse Width Modulated) output API change: It has four hardware PWM outputs on pwmchip2 (all previous Raspberry Pi models had two PWM outputs on pwmchip0) with different pin mapping. Notably, PWM chip 2 channel 2 is mapped to GPIO18 instead of PWM chip 0 channel 0 on previous Raspberry Pi boards. See RP1 Peripherals page 15 for more information.

USB Gadgets

You will need to edit some boot configuration files to enable USB Gadget mode. First, change dtoverlay=dwc2,dr_mode=host to dtoverlay=dwc2,dr_mode=peripheral in /boot/config.txt to change the USB-C receptacle from USB host to USB peripheral. Then change the OPTIONS word in /boot/cmdline.txt to 0x172C for a USB HID gadget, 0x072E for a USB Ethernet gadget, or 0x03AC for a USB serial port gadget. See Application Note #10 for more information about the OPTIONS word.

The Raspberry Pi 5 family consumes even more power than the Raspberry Pi 4 and not all host computers will be able to supply enough current to a single USB receptacle to support a Raspberry Pi 5 in USB Gadget mode.

Cross-Toolchains

I build a custom Ada/C/C++/Fortran/Go/Modula-2 GCC cross-toolchain for each MuntsOS platform family. Each GCC cross-toolchain requires a number of additional software component libraries, which are packaged and distributed separately but installed into the same directory tree as the parent cross-toolchain. I also build Free Pascal cross-compilers. Each of these rely on the libraries contained in the corresponding GCC cross-toolchain package.

Cross-toolchain packages containing GCC 14.2.0, including support for Ada 2022, and built for Debian Linux (x86-64 and ARM64) development host computers are available at either:

https://repo.munts.com/debian12 (Debian package repository)
https://repo.munts.com/muntsos/toolchain-debs (just the package files).

x86-64 RPM packages containing the exact same binaries, and known to work on Fedora 40 and RHEL 9.1 and its derivatives, are available at:

https://repo.munts.com/muntsos/toolchain-rpms

Alire Crates

muntsos_aarch64

Adding the muntsos_aarch64 crate to an Alire Ada program project transforms said project into one that produces a cross-compiled AArch64 program for MuntsOS. The muntsos_aarch64 crate depends upon the Linux distribution meta-package muntsos-dev-aarch64 that pulls in the rest of the MuntsOS AArch64 cross-toolchain packages. See Application Note #7 for a complete example using the alr command line tool.

Please note that the other MuntsOS library crates in Alire (e.g. muntsos_beaglebone) are unusable due to breaking changes in alr 2.0. Unfortunately, Alire project policies prohibit removing obsolete crates, so muntsos_beaglebone et al remain in the repository as broken and abandoned orphans.

Microsoft .Net

libsimpleio libsimpleio-templates

With the dotnet runtime extension installed, MuntsOS can run architecture independent .Net programs produced by dotnet build, dotnet publish, dotnet pack or the equivalent actions in Microsoft Visual Studio. Many if not most of the library packages published on NuGet can be used in such programs.

The NuGet library package libsimpleio provides libsimpleio.dll, a .Net Standard 2.0 library assembly that binds to the Linux shared library libsimpleio.so that is an integral part of MuntsOS. The NuGet library package libsimpleio-templates provides a .Net Core console application project template csharp_console_libsimpleio that, while not strictly necessary, greatly simplifies creating an .Net Core console embedded system application project for MuntsOS.


dotnet new install libsimpleio-templates
mkdir myprogram
cd myprogram
dotnet new csharp_console_libsimpleio
dotnet new sln
dotnet sln add myprogram.csproj

See Application Note #8 for a complete example using C# to flash an LED. See also the API specification for libsimpleio.dll.

The combination of Visual Studio + NuGet + libsimpleio.dll delivers a very high productivity development environment for creating embedded systems software to run on MuntsOS. With RemObjects Elements, a commercial Visual Studio addon product, you can even compile Object Pascal, Java, Go, and Swift programs, all using libsimpleio.dll, to .Net program assemblies that run on MuntsOS.

Git Repository

The source code for MuntsOS is available at:

https://github.com/pmunts/muntsos

Use the following command to clone it:

git clone https://github.com/pmunts/muntsos.git

File Repository

Prebuilt binaries for MuntsOS (extensions, kernels, thin servers, and cross-toolchain packages) are available at:

https://repo.munts.com/muntsos

Questions or comments to Philip Munts phil@munts.net