## Bare bone boot.s from wiki.osdev.org

# multiboot header
.section .rodata.multiboot
.align 4

# magic number
.long 0x1BADB002

# flags: align, meminfo
.long 0x3

# checksum: -(magic+flags)
.long -(0x1BADB002 + 0x3)

# the initial kernel stack
.section .kernel_stack
.global os_stack
.size os_stack, 4096

# NOTE: New Start

# .section .gdt
# gdt_start:
#     .quad 0x0000000000000000 # null descriptor
#     .quad 0x00cf9a000000ffff # code segment descriptor: base=0, limit=4GB, 32-bit code
#     .quad 0x00cf92000000ffff # data segment descriptor: base=0, limit=4GB, 32-bit data
# gdt_end:
#
# gdt_descriptor:
#     .word gdt_end - gdt_start - 1 # limit
#     .long gdt_start # base
#
# .section .idt
# idt_table:
#     .space 256*8 # 256 entries x 8 bytes each
# idt_descriptor:
#     .word 256*8-1 # limit
#     .long idt_table # base

# NOTE: New End

#.Lstack_bottom:
os_stack:
    .byte 0
    .skip 65565 # 64 KiB
    # .skip 16384 # 16 KiB
    # .skip 4094 # 4 KiB
    .byte 0
    .Lstack_top:


# The linker script specifies _start as the entry point to the kernel and the
# bootloader will jump to this position once the kernel has been loaded. It
# doesn't make sense to return from this function as the bootloader is gone.
.section .text.startup
.global _start
.type _start, @function
_start:
# NOTE: Old Start

	# Welcome to kernel mode!
	# To set up a stack, we simply set the esp register to point to the top of
	# our stack (as it grows downwards).
	movl $.Lstack_top, %esp
	# We are now ready to actually execute C code. (see ./startup.cc)
	call os_main

	# In case the function returns, we'll want to put the computer into an
	# infinite loop. To do that, we use the clear interrupt ('cli') instruction
	# to disable interrupts, the halt instruction ('hlt') to stop the CPU until
	# the next interrupt arrives, and jumping to the halt instruction if it ever
	# continues execution, just to be safe. We will create a local label rather
	# than real symbol and jump to there endlessly.
	cli
	hlt
.Lhang:
	jmp .Lhang

# NOTE: Old End

# NOTE: New Start

#     cli
#
#     lgdt gdt_descriptor # GDT
#     lidt idt_descriptor # IDT Stub
#
#     # set up segment registers (flat 32-bit)
#     movw $0x10, %ax
#     movw %ax, %ds
#     movw %ax, %es
#     movw %ax, %fs
#     movw %ax, %gs
#     movw %ax, %ss
#     movl $.Lstack_top, %esp # set stack
#
#     # call main C function
#     call os_main
#
#     cli
#     hlt
# .Lhang:
#     jmp .Lhang

# NOTE: New End

# Set the size of the _start symbol to the current location '.' minus its start.
# This is useful when debugging or when you implement call tracing.
.size _start, . - _start