serial_debug will output all the kprintf and dbgprintf data to COM1 at
8-N-1 57600 baud. this is particularly useful for debugging the boot
process on live hardware.
Note: it must be the first parameter in the boot cmdline.
This introduces very basic handling of the kernel command line to choose
the root filesystem at startup. Given that we currently only support a
single IDE hard drive, it's hard-coded to look for `/dev/hda` at the start
of the argument.
If there is nothing following this, or if the parameter is empty,
init_stage2 will try to load the ext2 filesystem from the start of the
device. This is intended to be the default behaviour when running
development builds, as it is faster to set up and doesn't require a
working grub installation.
If `/dev/hda` is followed by a number, init_stage2 will try to read an MBR
partition header from the drive, then load the requested partition. It
will reject non-numeric trailing data, and anything outside of partitions
one through four.
This implements a passthrough disk driver that translates the read/write
block addresses by a fixed offset. This could form the basis of MBR
partition support if we were to parse the MBR table at boot and create that
OffsetDiskDevice dynamically, rather than seeking to a fixed offset.
This also introduces a dependency in the form of grub. You'll need to have
32-bit grub binaries installed to build the project now.
As a bonus, divorcing Serenity from qemu's kernel loading means we can now
*technically* boot on real hardware. It just... doesn't get very far yet.
If you write the `_disk_image` file to an IDE hard drive and boot it in a
machine that supports all the basic PC hardware, it *will* start loading
the kernel.
