gistfile1.asm

# Tell the assembler to output 16-bit code; x86-compatible CPUs start in the 16-bit Real Mode.
.code16gcc

# Export the _start symbol, which is by convention the entry point for the .text
# section. Our makefile places the beginning of the text section at the start of
# the MBR. At runtime, the BIOS loads the MBR into address 0x7c00.
.globl  _start
_start:
  mov $(startup_message - _start + 0x7c00), %bx
  call print_str_BX

  # Let's enter protected mode.
  # http://wiki.osdev.org/Protected_Mode
  cli
  lgdt (gdt_descriptor - _start + 0x7c00)

  mov %cr0, %eax
  or $1, %eax # set Protected bit #0
  mov %eax, %cr0

  # now we are in 16-bit protected mode. flush the instruction queue
  mov $0x60, %ax
  jmp in_prot16

in_prot16:

  # make a long jump to:
  # load CS with a selector to a 32-bit executable code descriptor.
  #mov $1, %cs
  #mov $0, %ip
  ljmp $1, $0
  ljmp *(in_prot32 - _start + 0x7c00)

in_prot32:
.code32
  # done. ?

  movb $'X', 0xB8000
  movb $7, 0xB8001

  # It's unclear what the processor will do when we just stop doing anything here.
  # It'll probably start executing null byte instructions or something else silly.
  # So, just busy loop here.
hang:
  jmp hang


.code16gcc
startup_message:
.string "Hello from the grid.\r\n\0"

gdt_loaded_msg:
.string "GDT loaded.\r\n\0"

gdt_descriptor:
  .word 3 * 4 # gdt size
  .word gdt - _start + 0x7c00

gdt:
  # null entry
  .word 0,0
  .word 0,0

  # OS code segment
  .word 0xffff # limit[15..0]
  .word 0x0000 # base[15..00]
  .byte 0x00   # base[23..16]
  .byte 0b10011010 # P(1) DPL(00) S(1) 1 C(0) R(1) A(0)
  .byte 0b11001111 # G(1) D(1) 0 0 Limit[19..16]
  .byte 0x00   # base[31..24]

  # overlapping OS data segment
  .word 0xffff # limit[15..0]
  .word 0x0000 # base[15..00]
  .byte 0x00   # base[23..16]
  .byte 0b10010010 # P(1) DPL(00) S(1) 0 E(0) W(1) A(0)
  .byte 0b11001111 # G(1) B(1) 0 0 Limit[19..16]
  .byte 0x00   # base[31..24]


print_str_BX:
  # BX (Base Index) is one of the few 16-bit registers that can be
  # used for address arithmetic. I'm not yet sure why, but it apparently has something to
  # do with segments (x86 in Real Mode has a 20-bit address space, but only 16-bit registers).
  # Cf. http://f.osdev.org/viewtopic.php?f=13&t=18374

  # Load the next byte to print into AL, exit if it's \0.
  mov (%bx), %al
  test %al, %al
  jz print_done

  # Invoke INT10/AH=0E, which is by convention a BIOS call to print the character in AL.
  mov $0x0e, %ah
  int $0x10

  # Advance the counter, and rinse
  inc %bx
  jmp print_str_BX

print_done:
  ret