bootloader

An experimental x86 bootloader written in Rust and inline assembly.

Written for the second edition of the Writing an OS in Rust series.

Design

TODO

Configuration

The bootloader exposes a few variables which can be configured through the Cargo.toml of your kernel:

[package.metadata.bootloader]
# The address at which the kernel stack is placed. If not provided, the bootloader
# dynamically searches for a location.
kernel-stack-address = "0xFFFFFF8000000000"

# The size of the kernel stack, given in number of 4KiB pages. Defaults to 512.
kernel-stack-size = 128

# The virtual address offset from which physical memory is mapped, as described in
# https://os.phil-opp.com/paging-implementation/#map-the-complete-physical-memory
# Only applies if the `map_physical_memory` feature of the crate is enabled.
# If not provided, the bootloader dynamically searches for a location.
physical-memory-offset = "0xFFFF800000000000"

# The address at which the bootinfo struct will be placed. if not provided,
# the bootloader will dynamically search for a location.
boot-info-address = "0xFFFFFFFF80000000"

Note that the addresses must be given as strings (in either hex or decimal format), as TOML does not support unsigned 64-bit integers.

Requirements

You need a nightly Rust compiler and cargo xbuild. You also need the llvm-tools-preview component, which can be installed through rustup component add llvm-tools-preview.

Build

The simplest way to use the bootloader is in combination with the bootimage tool. This crate requires at least bootimage 0.7.7. With the tool installed, you can add a normal cargo dependency on the bootloader crate to your kernel and then run bootimage build to create a bootable disk image. You can also execute bootimage run to run your kernel in QEMU (needs to be installed).

To compile the bootloader manually, you need to invoke cargo xbuild with two environment variables:

• KERNEL: points to your kernel executable (in the ELF format)
• KERNEL_MANIFEST: points to the Cargo.toml describing your kernel

For example:

KERNEL=/path/to/your/kernel/target/debug/your_kernel KERNEL_MANIFEST=/path/to/your/kernel/Cargo.toml cargo xbuild

As an example, you can build the bootloader with example kernel from the example-kernel directory with the following commands:

cd example-kernel
cargo xbuild
cd ..
KERNEL=example-kernel/target/x86_64-example-kernel/debug/example-kernel KERNEL_MANIFEST=example-kernel/Cargo.toml cargo xbuild --release --features binary

The binary feature is required to enable the dependencies required for compiling the bootloader executable. The command results in a bootloader executable at target/x86_64-bootloader.json/release/bootloader. This executable is still an ELF file, which can't be run directly.

Run

To run the compiled bootloader executable, you need to convert it to a binary file. You can use the llvm-objcopy tools that ships with the llvm-tools-preview rustup component. The easiest way to use this tool is using cargo-binutils, which can be installed through cargo install cargo-binutils. Then you can perform the conversion with the following command:

cargo objcopy -- -I elf64-x86-64 -O binary --binary-architecture=i386:x86-64 \
  target/x86_64-bootloader/release/bootloader target/x86_64-bootloader/release/bootloader.bin

You can run the bootloader.bin file using QEMU:

qemu-system-x86_64 -drive format=raw,file=target/x86_64-bootloader/release/bootloader.bin

Or burn it to an USB drive to boot it on real hardware:

dd if=target/x86_64-bootloader/release/bootloader.bin of=/dev/sdX && sync

Where sdX is the device name of your USB stick. Be careful to choose the correct device name, because everything on that device is overwritten.

Features

The bootloader crate can be configured through some cargo features:

• vga_320x200: This feature switches the VGA hardware to mode 0x13, a graphics mode with resolution 320x200 and 256 colors per pixel. The framebuffer is linear and lives at address 0xa0000.
• recursive_page_table: Maps the level 4 page table recursively and adds the recursive_page_table_address field to the passed BootInfo.
• map_physical_memory: Maps the complete physical memory in the virtual address space and passes a physical_memory_offset field in the BootInfo.
  • The virtual address where the physical memory should be mapped is configurable by setting the physical-memory-offset field in the kernel's Cargo.toml, as explained in Configuration.

Advanced Documentation

See these guides for advanced usage of this crate:

• Chainloading
• Higher Half Kernel - TODO

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.