A RISC-V operating system devoted to running fizz/buzz processes.

FizzBuzz OS (or just fbos) is an excuse to better grasp the fundamentals of low level RISC-V machines. In practice, this is a small Operating System kernel that is able to launch three processes: one that simply prints "Fizz", another that prints "Buzz", and a final third which prints "FizzBuzz". These processes are unaware of each other and it's up to the kernel to schedule them by using the timer interrupts as given by openSBI (fizz on % 3 seconds, buzz on % 5 seconds, and fizzbuzz on % 15 seconds).

This kernel provides just one system call, write_and_block, which allows any program to pass the string to be written into the serial port and wait for the kernel to re-schedule it.

Build

In order to build fbos you only need GCC. That being said, if you are not on a RISC-V system you will also need cross compilation tools for it (check your distribution for this, on openSUSE I simply installed cross-riscv64-gcc14). With that installed, simply set the CROSS_COMPILE environment variable as you'd do for building the Linux kernel (e.g. in my case, using openSUSE, I set it to riscv64-suse-linux-). After all of that, just:

$ make

And that's basically it! The build process should've produced a fbos binary sitting at the root of the project.

Verbose output

You will notice that the output is suspiciously quiet. You can go back into verbose output by passing V=1 to any make target. Hence, for the build step, you can do something like:

$ make V=1

Debug mode

By default the kernel will be built with optimizations on. You can disable this by passing the DEBUG=1 option to any make target. This will also come in handy whenever you'd want to debug the kernel via QEMU+GDB. Read more on this below.

Test

QEMU

The easiest way to test the fbos binary from the build step is with QEMU. That being said, this kernel only supports the bootflow of openSBI fw_dynamic. If you have a recent enough QEMU, then that's not a problem and you can skip the next step. Otherwise you might need to specify the BIOS firmware that you'll have to compile from openSBI. If you are on that side, don't worry, it's not that hard:

$ git clone https://github.com/riscv-software-src/opensbi.git
$ cd opensbi
$ make ARCH=riscv CROSS_COMPILE=<your-cross-compile> PLATFORM=generic
# => build/platform/generic/firmware/fw_dynamic.bin

With that simply set the QEMU_BIOS environment variable with the full path of the resulting fw_dynamic.bin file.

Now make sure that you have a QEMU version that is able to emulate a RISC-V system. After that, simply run:

$ make qemu

This will open up QEMU in -nographic mode (hence the serial output will be simply redirected to stdout), and you will be able to see the whole thing working. Moreover, the qemu target can be paired with the DEBUG parameter that you can pass to make. Hence, you can also call it like so:

$ make qemu DEBUG=1

This will make QEMU wait for a GDB connection. On another terminal then type:

$ make gdb

Now you have a debugging session for this kernel with debug symbols loaded. Hence, upon starting the GDB session you can simply type:

(gdb) break _start
(gdb) continue

From there you are already out of firmware code and right into the kernel. Moreover, you can also pass the GDB_EXTRA_FLAGS variable to the make gdb target. This way you can pass extra parameters to gdb, such as:

$ make gdb GDB_EXTRA_FLAGS="-tui"

And now you have started a GDB session with a nice TUI interface.

Requirements

We do not want to support a myriad of different scenarios, but we want to keep things simple. Hence, here there are some limitations/requirements:

• You need a recent enough OpenSBI running on your firmware. I have tested this on a QEMU which has OpenSBI v1.5, but any firmware that implements a Runtime SBI version of 2.0 should be fine.
• You are supposed to pass an initrd always. This kernel will not try to magically come up with a made up file system or try to fetch something from an existing one. An initrd is already provided for you on the default make target, and that's what you are supposed to be passing to the kernel.
• RISC-V is ambivalent on the endianness. Not this kernel. Out of simplicity we require a little-endian RISC-V system.

Special thanks to

SUSE for organizing Hack Week 24. This project was mainly developed during this time.

I have also taken lots of valuable input by reading Popovic's blog, so thanks a lot for writing such clear articles on a rather obscure topic. In a similar way, I have also taken the time to read a lot of code from the Linux Kernel. My understanding of both RISC-V and the Linux Kernel itself has vastly improved with this exercise, so I'd also like to take the chance to be grateful to the many people who have contributed to this vast undertaking that is the Linux Kernel.