# Stage 1 — the actual MBR boot sector. Hard-limited to 510 bytes (+2-byte
# 0x55AA signature) by the BIOS, so there's no room here for anything but
# "load stage 2 off disk and jump to it". All the VBE/A20/protected-mode/
# splash/kernel-loading work now lives in stage2.S + stage2main.c, which
# aren't sector-size-constrained.
#
# How many sectors of stage2 to pull in. This, the padding the Makefile gives
# stage2's binary, and the "+(STAGE2_SECTORS+1)" sector math in stage2main.c's
# readseg() all have to agree on where the kernel starts on disk -- so the
# Makefile passes -DSTAGE2_SECTORS and is the single authority. The value below
# is only a fallback for reading this file standalone.
#
# It is 48, not the old 24, because stage2 now carries the boot logo
# (include/logo.h, 8 KiB). The real ceiling is memory rather than this number:
# stage2 loads at 0x7E00 and the multiboot kernel-search buffer starts at
# 0x10000, so stage2 cannot exceed 0x8200 bytes = 65 sectors.
#ifndef STAGE2_SECTORS
#define STAGE2_SECTORS 48
#endif
#define STAGE2_ADDR    0x7E00

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

  # Read STAGE2_SECTORS sectors, starting at disk sector 1 (LBA), into
  # memory right after this boot sector (0x7E00). Raw IDE PIO (LBA28) —
  # same register convention as stage2main.c's own readsect(), just
  # inlined here since we can't call C code yet. Port numbers 0x1F2-0x1F7
  # are all > 0xFF, so they must go through %dx — `out $imm8,%al` only
  # encodes an 8-bit port and silently truncates anything larger.
  # "You were not booted from a CD." stage2main.c reads this byte to choose
  # between ATA and ATAPI when it loads the kernel, and nothing else clears
  # it -- so a warm reboot from a disc could otherwise leave the flag set and
  # send a perfectly ordinary hard-disk boot looking for an optical drive.
  # Placed one byte past the verbose flag; both sit clear of the 256-byte VBE
  # mode info block that stage2.S is about to write at 0x30000.
  movw    $0x3000, %ax
  movw    %ax, %es
  movw    $0x101, %di
  movb    $0, %al
  movb    %al, %es:(%di)
  xorw    %ax, %ax
  movw    %ax, %es

  movw    $STAGE2_ADDR, %di   # ES:DI destination, ES=0
  movw    $1, %si             # first LBA sector to read
  movb    $STAGE2_SECTORS, %bl

.Lsector:
.Lwait1:
  movw    $0x1F7, %dx
  inb     %dx, %al
  andb    $0xC0, %al
  cmpb    $0x40, %al
  jne     .Lwait1

  movw    $0x1F2, %dx
  movb    $1, %al
  outb    %al, %dx            # sector count = 1

  incw    %dx                 # 0x1F3
  movw    %si, %ax
  outb    %al, %dx            # LBA [7:0]

  incw    %dx                 # 0x1F4
  xorb    %al, %al
  outb    %al, %dx            # LBA [15:8]

  incw    %dx                 # 0x1F5
  outb    %al, %dx            # LBA [23:16]

  incw    %dx                 # 0x1F6
  movb    $0xE0, %al
  outb    %al, %dx            # drive 0, LBA mode

  incw    %dx                 # 0x1F7
  movb    $0x20, %al
  outb    %al, %dx            # READ SECTORS command

.Lwait2:
  inb     %dx, %al            # %dx still 0x1F7
  andb    $0xC0, %al
  cmpb    $0x40, %al
  jne     .Lwait2

  movw    $256, %cx           # 256 words = 512 bytes
  movw    $0x1F0, %dx
  rep insw                    # reads into ES:DI, DI += 512 automatically

  incw    %si
  decb    %bl
  jnz     .Lsector

  ljmp    $0, $STAGE2_ADDR