path: root/apps/boot.hex

# Bootable image that:
#   - loads more sectors past the first boot sector (using BIOS primitives)
#   - switches to 32-bit mode (giving up access to BIOS primitives)
#   - sets up a handler for keyboard events
#   - as an example program, prints alphabets to the top-left position on screen (by writing to memory-mapped VGA memory) as they're typed
#
# If the initial load fails, it prints 'D' to the top-left of the screen and
# halts.
#
# To convert to a disk image, first prepare a realistically sized disk image:
#   dd if=/dev/zero of=disk.img count=20160  # 512-byte sectors, so 10MB
# Now fill in sectors:
#   ./bootstrap run apps/hex < apps/boot.hex > boot.bin
#   dd if=boot.bin of=disk.img conv=notrunc
# To run:
#   qemu-system-i386 disk.img
# Or:
#   bochs -f apps/boot.bochsrc  # boot.bochsrc loads disk.img
#
# Since we start out in 16-bit mode, we need instructions SubX doesn't
# support.
# This file contains just hex bytes and comments. Zero error-checking. Make
# liberal use of:
#   - comments documenting expected offsets
#   - size checks on the emitted file (currently: 512 bytes)
#   - xxd to eyeball that offsets contain expected bytes

## 16-bit entry point

# Upon reset, the IBM PC
#   loads the first sector (512 bytes)
#   from some bootable image (see the boot sector marker at the end of this file)
#   to the address range [0x7c00, 0x7e00)

# offset 00 (address 0x7c00):
  # disable interrupts for this initialization
  fa  # cli

  # initialize segment registers
  # this isn't always needed, but is considered safe not to assume
  b8 00 00  # ax <- 0
  8e d8  # ds <- ax
  8e d0  # ss <- ax
  8e c0  # es <- ax
  8e e0  # fs <- ax
  8e e8  # gs <- ax

  # We don't read or write the stack before we get to 32-bit mode. No function
  # calls, so we don't need to initialize the stack.

# 0e:
  # load more sectors from disk
  b4 02  # ah <- 2  # read sectors from disk
  # dl comes conveniently initialized at boot time with the index of the device being booted
  b5 00  # ch <- 0  # cylinder 0
  b6 00  # dh <- 0  # track 0
  b1 02  # cl <- 2  # second sector, 1-based
  b0 01  # al <- 1  # number of sectors to read
  # address to write sectors to = es:bx = 0x7e00, contiguous with boot segment
  bb 00 00  # bx <- 0
  8e c3  # es <- bx
  bb 00 7e  # bx <- 0x7e00
  cd 13  # int 13h, BIOS disk service
  0f 82 76 00  # jump-if-carry disk-error

# 26:
  # undo the A20 hack: https://en.wikipedia.org/wiki/A20_line
  # this is from https://github.com/mit-pdos/xv6-public/blob/master/bootasm.S
  # seta20.1:
  e4 64  # al <- port 0x64
  a8 02  # set zf if bit 1 (second-least) is not set
  75 fa  # if zf not set, goto seta20.1 (-6)

  b0 d1  # al <- 0xd1
  e6 64  # port 0x64 <- al

# 30:
  # seta20.2:
  e4 64  # al <- port 0x64
  a8 02  # set zf if bit 1 (second-least) is not set
  75 fa  # if zf not set, goto seta20.2 (-6)

  b0 df  # al <- 0xdf
  e6 64  # port 0x64 <- al

# 3a:
  # switch to 32-bit mode
  0f 01 16  # lgdt 00/mod/indirect 010/subop 110/rm32/TODO
    80 7c  # *gdt_descriptor
# 3f:
  0f 20 c0  # eax <- cr0
  66 83 c8 01  # eax <- or 0x1
  0f 22 c0  # cr0 <- eax
  ea c0 7c 08 00  # far jump to initialize_32bit_mode after setting cs to the record at offset 8 in the gdt (gdt_code)

# padding
# 4e:
                                          00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

## GDT: 3 records of 8 bytes each

# 60:
# gdt_start:
# gdt_null:  mandatory null descriptor
  00 00 00 00 00 00 00 00
# gdt_code:  (offset 8 from gdt_start)
  ff ff  # limit[0:16]
  00 00 00  # base[0:24]
  9a  # 1/present 00/privilege 1/descriptor type = 1001b
      # 1/code 0/conforming 1/readable 0/accessed = 1010b
  cf  # 1/granularity 1/32-bit 0/64-bit-segment 0/AVL = 1100b
      # limit[16:20] = 1111b
  00  # base[24:32]
# gdt_data:  (offset 16 from gdt_start)
  ff ff  # limit[0:16]
  00 00 00  # base[0:24]
  92  # 1/present 00/privilege 1/descriptor type = 1001b
      # 0/data 0/conforming 1/readable 0/accessed = 0010b
  cf  # same as gdt_code
  00  # base[24:32]
# gdt_end:

# padding
# 78:
                        00 00 00 00 00 00 00 00

# 80:
# gdt_descriptor:
  17 00  # final index of gdt = gdt_end - gdt_start - 1
  60 7c 00 00  # start = gdt_start

# padding
# 85:
                  00 00 00 00 00 00 00 00 00 00

# 90:
# disk_error:
  # print 'D' to top-left of screen to indicate disk error
  # *0xb8000 <- 0x0f44
  # bx <- 0xb800
  bb 00 b8
  # ds <- bx
  8e db  # 11b/mod 011b/reg/ds 011b/rm/bx
  # al <- 'D'
  b0 44
  # ah <- 0x0f  # white on black
  b4 0f
  # bx <- 0
  bb 00 00
  # *ds:bx <- ax
  89 07  # 00b/mod/indirect 000b/reg/ax 111b/rm/bx

e9 fb ff  # loop forever

# padding
# a1:
   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

## 32-bit code from this point (still some instructions not in SubX)

# c0:
# initialize_32bit_mode:
  66 b8 10 00  # ax <- offset 16 from gdt_start
  8e d8  # ds <- ax
  8e d0  # ss <- ax
  8e c0  # es <- ax
  8e e0  # fs <- ax
  8e e8  # gs <- ax

  # load interrupt handlers
  0f 01 1e  # lidt 00/mod/indirect 011/subop 110/rm32/TODO
    00 7f  # *idt_descriptor

  # initialization is done; enable interrupts
  fb
  e9 27 00 00 00  # jump to 0x7d00

# padding
# d9:
                           00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

## 'application' SubX code: print one character to top-left of screen

# offset 100 (address 0x7d00):
# Entry:
  # eax <- *0x7ff4  # random address in second segment containing 'H'
  8b  # copy rm32 to r32
    05  # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32
    # disp32
    f4 7f 00 00
  # *0xb8000 <- eax
  89  # copy r32 to rm32
    05  # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32
    # disp32
    00 80 0b 00

e9 fb ff ff ff  # loop forever

# padding
# 111:
   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

# 120:

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00

# final 2 bytes of boot sector
55 aa

## sector 2
# not loaded on boot; loaded by load_disk

# offset 200 (address 0x7e00): interrupt descriptor table
# 32 entries * 8 bytes each = 256 bytes (0x100)
# idt_start:
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# offset 9: keyboard, following https://alex.dzyoba.com/blog/os-interrupts
  20 7d  # offset[0:16]
  08 00  # segment selector (gdt_code)
  00  # unused
  8e  # 1/p 00/dpl 0 1110/type/32-bit-interrupt-gate
  00 00  # offset[16:32]

00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
# idt_end:

# offset 300 (address 0x7f00):
# idt_descriptor:
  ff 00  # idt_end - idt_start - 1
  00 7e 00 00  # start = idt_start

# padding
                  00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 48 0f 00 00 00 00 00 00 00 00 00 00  # spot the 'H' with attributes
# offset 400 (address 0x8000)

# vim:ft=conf