/*
 * Minimal Multiboot2 boot test — proves QEMU x86_64 can boot our kernel.
 *
 * SPDX-License-Identifier: MIT
 */
.section .multiboot, "ax"
.align 8

#define MULTIBOOT2_MAGIC    0x36d76289
#define MULTIBOOT2_ARCH     0
#define MULTIBOOT2_HDR_LEN  24

mb2_header:
    .long MULTIBOOT2_MAGIC
    .long MULTIBOOT2_ARCH
    .long -(MULTIBOOT2_MAGIC + MULTIBOOT2_ARCH)
    .word MULTIBOOT2_HDR_LEN
    .word 0

.section .text, "ax"
.code32

.global _start
_start:
    /* Set up stack */
    movl $(_stack_top), %esp

    /* Save Multiboot2 info */
    pushl $0
    pushl %eax           /* magic */
    pushl %ebx           /* mboot info addr */

    /* Jump to 64-bit code */
    ljmp $0x08, $start64

.section .text64, "ax"
.code64

start64:
    /* Set up 64-bit segments */
    movw $0x10, %ax
    movw %ax, %ds
    movw %ax, %es
    movw %ax, %ss
    xorw %ax, %ax
    movw %ax, %fs
    movw %ax, %gs

    /* Restore Multiboot2 info */
    popq %rdi           /* magic */
    popq %rsi           /* info addr */
    addq $8, %rsp       /* skip padding */

    /* Write 'UOS' to VGA text buffer (0xb8000) to prove we're alive */
    movq $0x204F5355204F5355, %rax   /* "UOS UOS" */
    movq %rax, (%rax)

    /* Write to serial port COM1 (0x3F8) */
    movw $0x3F8, %dx
    movb $'U', %al
    outb %al, (%dx)
    movb $'O', %al
    outb %al, (%dx)
    movb $'S', %al
    outb %al, (%dx)
    movb $' ', %al
    outb %al, (%dx)
    movb $'O', %al
    outb %al, (%dx)
    movb $'K', %al
    outb %al, (%dx)
    movb $0x0A, %al     /* newline */
    outb %al, (%dx)
    movb $0x0D, %al     /* carriage return */
    outb %al, (%dx)

    /* Halt */
    cli
.Lhalt:
    hlt
    jmp .Lhalt

.section .bss, "aw"
.align 16
_stack_bottom:
    .space 4096
_stack_top: