126 lines
4.3 KiB
Plaintext
126 lines
4.3 KiB
Plaintext
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/* Inspired by from https://www.gnu.org/software/grub/manual/multiboot2/multiboot.html#boot_002eS */
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/* boot.S - bootstrap the kernel */
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/* Copyright (C) 1999, 2001, 2010 Free Software Foundation, Inc.
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#define ASM_FILE 1
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#include <multiboot2.h>
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.section .multiboot
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.align 4
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/* Multiboot header. */
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multiboot_header:
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/* magic */
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.long MULTIBOOT2_HEADER_MAGIC
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/* ISA: i386 */
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.long MULTIBOOT_ARCHITECTURE_I386
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/* Header length. */
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.long multiboot_header_end - multiboot_header
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/* checksum */
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.long -(MULTIBOOT2_HEADER_MAGIC + MULTIBOOT_ARCHITECTURE_I386 + (multiboot_header_end - multiboot_header))
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multiboot_header_end:
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.section .bss
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.align 4096
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stack_bottom:
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.skip 16384 # 16 KiB
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stack_top:
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.section .text
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.global _start
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.type _start, @function
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_start:
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/*
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The bootloader has loaded us into 32-bit protected mode on a x86
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machine. Interrupts are disabled. Paging is disabled. The processor
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state is as defined in the multiboot standard. The kernel has full
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control of the CPU. The kernel can only make use of hardware features
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and any code it provides as part of itself. There's no printf
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function, unless the kernel provides its own <stdio.h> header and a
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printf implementation. There are no security restrictions, no
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safeguards, no debugging mechanisms, only what the kernel provides
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itself. It has absolute and complete power over the
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machine.
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*/
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/*
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To set up a stack, we set the esp register to point to the top of our
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stack (as it grows downwards on x86 systems). This is necessarily done
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in assembly as languages such as C cannot function without a stack.
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*/
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mov $stack_top, %esp
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mov $stack_bottom, %ebp
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/*
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This is a good place to initialize crucial processor state before the
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high-level kernel is entered. It's best to minimize the early
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environment where crucial features are offline. Note that the
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processor is not fully initialized yet: Features such as floating
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point instructions and instruction set extensions are not initialized
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yet. The GDT should be loaded here. Paging should be enabled here.
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C++ features such as global constructors and exceptions will require
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runtime support to work as well.
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*/
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/* Reset EFLAGS. */
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pushl $0
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popf
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/*
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Enter the high-level kernel. The ABI requires the stack is 16-byte
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aligned at the time of the call instruction (which afterwards pushes
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the return pointer of size 4 bytes). The stack was originally 16-byte
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aligned above and we've since pushed a multiple of 16 bytes to the
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stack since (pushed 0 bytes so far) and the alignment is thus
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preserved and the call is well defined.
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*/
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/* Push the pointer to the Multiboot information structure. */
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pushl %ebx
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/* Push the magic value. */
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pushl %eax
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call kmain
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/*
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If the system has nothing more to do, put the computer into an
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infinite loop. To do that:
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1) Disable interrupts with cli (clear interrupt enable in eflags).
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They are already disabled by the bootloader, so this is not needed.
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Mind that you might later enable interrupts and return from
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kernel_main (which is sort of nonsensical to do).
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2) Wait for the next interrupt to arrive with hlt (halt instruction).
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Since they are disabled, this will lock up the computer.
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3) Jump to the hlt instruction if it ever wakes up due to a
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non-maskable interrupt occurring or due to system management mode.
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*/
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cli
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1: hlt
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jmp 1b
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/*
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Set the size of the _start symbol to the current location '.' minus its start.
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This is useful when debugging or when you implement call tracing.
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*/
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.size _start, . - _start
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.global _stack_bottom
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_stack_bottom:
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.long stack_bottom
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.global _stack_top
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_stack_top:
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.long stack_top
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