; Declare constants for the multiboot header.
MAGIC equ 0x1BADB002
FLAG_VIDEO_MODE equ (1 << 2)
FLAGS equ FLAG_VIDEO_MODE
CHECKSUM equ -(MAGIC + FLAGS) & 0xFFFFFFFF
HEADER_ADDR equ 0 ; if flags[16] is set
LOAD_ADDR equ   0 ; if flags[16] is set
LOAD_END_ADDR equ   0 ; if flags[16] is set
BSS_END_ADDR equ 0 ; if flags[16] is set
ENTRY_ADDR equ 0 ; if flags[16] is set
MODE_TYPE equ 0 ; if flags[2] is set
WIDTH equ 0 ; if flags[2] is set
HEIGHT equ 0 ; if flags[2] is set
DEPTH equ 0x20 ; if flags[2] is set

; Declare a multiboot header that marks the program as a kernel. These are magic
; values that are documented in the multiboot standard. The bootloader will
; search for this signature in the first 8 KiB of the kernel file, aligned at a
; 32-bit boundary. The signature is in its own section so the header can be
; forced to be within the first 8 KiB of the kernel file.
section .multiboot
align 4
    dd MAGIC
    dd FLAGS
    dd CHECKSUM
    dd HEADER_ADDR
    dd LOAD_ADDR
    dd LOAD_END_ADDR
    dd BSS_END_ADDR
    dd ENTRY_ADDR
    dd MODE_TYPE
    dd WIDTH
    dd HEIGHT
    dd DEPTH


; The multiboot standard does not define the value of the stack pointer register
; (esp) and it is up to the kernel to provide a stack. This allocates room for a
; small stack by creating a symbol at the bottom of it, then allocating 16384
; bytes for it, and finally creating a symbol at the top. The stack grows
; downwards on x86. The stack is in its own section so it can be marked nobits,
; which means the kernel file is smaller because it does not contain an
; uninitialized stack. The stack on x86 must be 16-byte aligned according to the
; System V ABI standard and de-facto extensions. The compiler will assume the
; stack is properly aligned and failure to align the stack will result in
; undefined behavior.
section .bss
align 16
stack_bottom:
resb 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.
; Declare _start as a function symbol with the given symbol size.
section .text
global _start:function (_start.end - _start)
_start:
    ; The bootloader has loaded us into 32-bit protected mode on a x86
    ; machine. Interrupts are disabled. Paging is disabled. The processor
    ; state is as defined in the multiboot standard. The kernel has full
    ; control of the CPU. The kernel can only make use of hardware features
    ; and any code it provides as part of itself. There's no printf
    ; function, unless the kernel provides its own <stdio.h> header and a
    ; printf implementation. There are no security restrictions, no
    ; safeguards, no debugging mechanisms, only what the kernel provides
    ; itself. It has absolute and complete power over the
    ; machine.

    ; To set up a stack, we set the esp register to point to the top of our
    ; stack (as it grows downwards on x86 systems). This is necessarily done
    ; in assembly as languages such as C cannot function without a stack.
    mov esp, stack_top

    ; This is a good place to initialize crucial processor state before the
    ; high-level kernel is entered. It's best to minimize the early
    ; environment where crucial features are offline. Note that the
    ; processor is not fully initialized yet: Features such as floating
    ; point instructions and instruction set extensions are not initialized
    ; yet. The GDT should be loaded here. Paging should be enabled here.
    ; C++ features such as global constructors and exceptions will require
    ; runtime support to work as well.

    ; Enter the high-level kernel. The ABI requires the stack is 16-byte
    ; aligned at the time of the call instruction (which afterwards pushes
    ; the return pointer of size 4 bytes). The stack was originally 16-byte
    ; aligned above and we've since pushed a multiple of 16 bytes to the
    ; stack since (pushed 0 bytes so far) and the alignment is thus
    ; preserved and the call is well defined.
        ; note, that if you are building on Windows, C functions may have "_" prefix in assembly: _kernel_main
    extern main

    push eax
    call main

    ; If the system has nothing more to do, put the computer into an
    ; infinite loop. To do that:
    ; 1) Disable interrupts with cli (clear interrupt enable in eflags).
    ;    They are already disabled by the bootloader, so this is not needed.
    ;    Mind that you might later enable interrupts and return from
    ;    kernel_main (which is sort of nonsensical to do).
    ; 2) Wait for the next interrupt to arrive with hlt (halt instruction).
    ;    Since they are disabled, this will lock up the computer.
    ; 3) Jump to the hlt instruction if it ever wakes up due to a
    ;    non-maskable interrupt occurring or due to system management mode.
    cli
.hang:  hlt
    jmp .hang
.end:

    %include "CPU/interrupt.asm"