Sources of information:
  x86 architecture: Appendix A of https://pdos.csail.mit.edu/6.828/2016/xv6/book-rev9.pdf
  boot process xv6: Appendix B of that
  files in xv6    : bootasm.S, bootmain.c, Makefile, .h files
  boot process in general: https://en.wikibooks.org/wiki/X86_Assembly/Bootloaders
  OS development  : http://wiki.osdev.org/Main_Page
  BIOS interrupts : https://en.wikipedia.org/wiki/BIOS_interrupt_call
  int 10h video   : https://courses.engr.illinois.edu/ece390/books/labmanual/graphics-int10h.html
  x86 assembly (GAS) : https://en.wikibooks.org/wiki/X86_Assembly
  BIOS keyboard   : https://en.wikipedia.org/wiki/INT_16H

Things of note in the Makefile:
  dd for file copying - putting bootblock, then kernel, then up to 10k zeroes

Commands for the demo:
  make qemu-nox-gdb
  gdb -x .gdbinit kernel
  debugger commands:
    b *0x7c00
    display/i $pc
    stepi
    x/i 0x7c00
  dd if=bootblock of=/dev/sd_SOMETHING

x86 registers of note: (e is for extended, x is also for extended)
  pc  - program counter (memory address of next instruction to run)
  eax - accumulator (used for temporary things).  Note ax is 16 bits,
        al and ah are 8 bits each from ax.
  ds, ss, cs - data segment, stack segment, code segment registers (offsets)

x86 instructions of note:
  cli clear interrupts
  xor 
  jnz jumps if not zero (checks z flag in some register)
  mov transfers data between registers and memory
  out
  in

Boot sequence rules from the BIOS standard stuff:
  * Loads first 512 sector of disk, checks that last 2 bytes are 0x55aa.
    If so, assumes that 512 bytes is a boot loader.  Reads that 512
    bytes to memory address 0x7c00, and starts running there.
  * Note - since we only have 512 bytes to work with, written in assembly to
    be efficient.  Also, bootasm.S is assembly because doing CPU-ish stuff.
  * When we start running, are in "real 8086 mode".  That uses 20 bit
    memory addresses formed from an offset and segment start.  Set segment
    starts to 0.  Also, need to enable the A20 bit by messing with the
    keyboard controller ports.  And put CPU into "protected" mode, which
    allows us to access more than 1M of memory.
  * The normal boot process would have the boot loader read from the
    disk for the rest of the stuff to load the OS, etc.  This is in
    bootmain.c