Looks like cprintf modifies ptr3, so make sure to save it if you are
calling that.
There also may have been an issue with how the previous sector address
was calculated, the one which does not account for carries seems to
work.
I think there may still be some issues with a drive that is formatted
wrong, but it works consistently right now.
When modifying the boot sector, it now leaves alone the partition tables
and the bios parameter block. This does reduce the amount of code from
510 bytes to 380, but this is find as it only takes about 29 bytes to
load a sector into memory.
By doing this we don't need to call bios functions, we can just jsr
directly to the addresses. This does mean that everytime the bios
changes the bootloader has to change, but ideally all the bootloader
does is load the bios and then get remapped out of memory. Any important
drivers like file system can be loaded from the bootloader.
This also means that the runtime functions are located in the bios for
the bootloader, so the rom will have to stay mapped in until the kernel
is started, at which point it will have its own runtime and the rom and
bootloader are no longer needed.
