# Stage 1 for optical media -- the El Torito "no emulation" boot sector.
#
# This exists because bootasm.S cannot boot a CD. It loads stage 2 by talking
# straight to the primary IDE controller's ATA registers, and a CD is not an
# ATA disk: it answers to the ATAPI packet interface instead, and under
# El Torito it may not even be the drive at 0x1F0. Rather than teach a
# 512-byte sector two protocols, optical media get their own first stage.
#
# It has almost nothing to do, because El Torito's "load segment / sector
# count" fields do the reading for us: tools/mkiso.sh asks for
# 1 + STAGE2_SECTORS virtual sectors at 0x7C00, so by the time this runs the
# BIOS has already placed stage 2 at 0x7E00 -- exactly where bootasm.S would
# have put it. The only real work is recording *which* medium we came from,
# since stage2main.c has to read the kernel back with the matching protocol.
#
# The flag goes next to the verbose-boot flag, one byte past it, for the same
# reason that one sits where it does: it is clear of the 256-byte VBE mode
# info block at 0x30000 that stage2.S fills in a moment from now.

#define STAGE2_ADDR            0x7E00
#include "../../include/schboot.h"

.code16
.global start
start:
  cli
  xorw    %ax, %ax
  movw    %ax, %ds
  movw    %ax, %ss
  movw    $0x7c00, %sp

  # "You were booted from a CD." stage2main.c reads this to decide between
  # ATA and ATAPI; without it the loader would read the kernel off whatever
  # happens to be sitting at 0x1F0, which on a CD boot is usually nothing.
  movw    $SCHBOOT_MEDIA_SEG, %ax
  movw    %ax, %es
  movw    $SCHBOOT_MEDIA_OFF, %di
  movb    $SCHBOOT_MEDIA_CD, %al
  movb    %al, %es:(%di)

  # stage2.S expects zeroed segments and an inherited stack, same as it gets
  # from bootasm.S.
  xorw    %ax, %ax
  movw    %ax, %es

  ljmp    $0, $STAGE2_ADDR