RaspiOS

This is a little playground for AArch64 assembly on Raspberry Pi 3B+ (BCM2837B0 ARM Cortex A-53). It can be used and tweaked to bootstrap development on the chip. Current features are:

• run from U-boot
• run from EL3 or EL2
• UART functionality
• print string and hex with customizable callbacks (here as UART)
• exception / interrupt vector table
• timer functionality
• memory heap allocation
• basic MMU mapping
• basic GPU interface

Cross Compiler

RaspiOS uses the aarch64-none-elf cross compiler available on ARM toolchain download site. This should be installed automatically by the build system. However, you can always try and avoid installation anxieties by running ./install-cc.sh before starting the build.

Configuration

You can use various options to build the OS, and the default values are already defined in config.txt file. Options for early start:

CONFIG_STACK_SIZE=0x1000
CONFIG_VA_BITS=39
CONFIG_SECURE=0
CONFIG_GRANULE_SIZE=4KB

The CONFIG_SECURE is about to be obsolete, and CONFIG_VA_BITS and CONFIG_GRANULE_SIZE aren't yet properly adapted to other values. There are also two more options for memory allocator debug:

• CONFIG_MEM_TESTS - run some basic unit tests on memory initialization
• CONFIG_MEM_TRACE - print detailed debug information on memory operations
• CONFIG_MBOX_TRACE - trace mailbox functionality

Note: You can disable these last options simply by commenting them out with # sign.

Build

To build RaspiOS, simply run:

mkdir build
cd build
ARCH=aarch64 cmake ..
make

Known issues

• GPU drivers are not working in QEMU (could be just temperature sensor, though)
• timer triggers IRQ handler twice