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# https://www.gnu.org/software/grub/manual/multiboot/multiboot.html#Boot-information-format
# http://wiki.osdev.org/Bare_Bones
# Declare constants used for creating a multiboot header.
.set ALIGN, 1<<0 # align loaded modules on page boundaries
.set MEMINFO, 1<<1 # provide memory map
.set GFXINFO, 1<<2 # provide gfx info
.set FLAGS, ALIGN | MEMINFO | GFXINFO # this is the Multiboot 'flag' field
.set MAGIC, 0x1BADB002 # 'magic number' lets bootloader find the header
.set CHECKSUM, -(MAGIC + FLAGS) # checksum of above, to prove we are multiboot
# Declare a header as in the Multiboot Standard. We put this into a special
# section so we can force the header to be in the start of the final program.
# You don't need to understand all these details as it is just magic values that
# is documented in the multiboot standard. The bootloader will search for this
# magic sequence and recognize us as a multiboot kernel.
.section .multiboot
.align 4
.long MAGIC
.long FLAGS
.long CHECKSUM
.long 0 # we dont need this for ELF
.long 0
.long 0
.long 0
.long 0
.long 0 #gfx_stuff 0=enable!
.long 1280
.long 1024
.long 32
# Currently the stack pointer register (esp) points at anything and using it may
# cause massive harm. Instead, we'll provide our own stack. We will allocate
# room for a small temporary stack by creating a symbol at the bottom of it,
# then allocating 16384 bytes for it, and finally creating a symbol at the top.
.section .bootstrap_stack, "aw", @nobits
stack_bottom:
.skip 16384 # 16 KiB
stack_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
.global _start
.global stack_top
.global stack_bottom
.type _start, @function
_start:
# To set up a stack, we simply set the esp register to point to the top of
# our stack (as it grows downwards).
movl $stack_top, %esp
push %ebx #pass address of the multiboot information data structure
push %eax #pass eax, so kernel can check for magic number
call kernel_main
# should never be reached
cli
hlt
.Lhang:
jmp .Lhang
# 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
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