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.global _start
.extern long_mode_start
.section .text
.code32
/* Entry point. It puts the machine into a consistent state and starts long mode. */
_start:
mov $stack_top, %esp // Setup the stack.
call boot_check_multiboot
call boot_check_cpuid
call boot_check_long_mode
call boot_setup_page_tables
call boot_enable_paging
lgdt (boot_gdtdesc)
jmpl $0x8, $boot_long_mode_start
hlt
boot_check_multiboot:
cmp $0x36d76289, %eax
jne .boot_no_multiboot
ret
.boot_no_multiboot:
mov $'M', %al
jmp boot_error
boot_check_cpuid:
pushfd
pop %eax
mov %eax, %ecx
xor $(1 << 21), %eax
push %eax
popfd
pushfd
pop %eax
push %ecx
popfd
cmp %eax, %ecx
je .boot_no_cpuid
ret
.boot_no_cpuid:
mov $'C', %al
jmp boot_error
boot_check_long_mode:
mov $0x80000000, %eax
cpuid
cmp $0x80000001, %eax
jb .boot_no_long_mode
mov $0x80000001, %eax
cpuid
test $(1 << 29), %edx
jz .boot_no_long_mode
ret
.boot_no_long_mode:
mov $'L', %al
jmp boot_error
boot_setup_page_tables:
mov $page_table_l3, %eax
or $0b11 , %eax /* present, writable */
mov %eax, (page_table_l4)
mov $page_table_l2, %eax
or $0b11, %eax /* present, writable */
mov %eax, (page_table_l3)
mov $0, %ecx /* counter */
.boot_pages_loop:
mov $0x200000, %eax /* 2MiB */
mul %ecx
or $0b10000011, %eax /* present, writable, huge page */
mov %eax, +page_table_l2(,%ecx,8)
inc %ecx /* increment counter */
cmp $512, %ecx /* checks if the whole table is mapped */
jne .boot_pages_loop /* if not, continue */
ret
boot_enable_paging:
/* pass page table location to cpu */
mov $page_table_l4, %eax
mov %eax, %cr3
/* enable PAE */
mov %cr4, %eax
or $(1 << 5), %eax
mov %eax, %cr4
/* enable long mode */
mov $0xC0000080, %ecx
rdmsr
or $(1 << 8), %eax
wrmsr
/* enable paging */
mov %cr0, %eax
or $(1 << 31), %eax
mov %eax, %cr0
ret
boot_error:
/* print in VGA "ERR: X" where X is the error code */
movl $0x4f524f45, 0xb8000
movl $0x4f3a4f52, 0xb8004
movl $0x4f204f20, 0xb8008
movb %al, 0xb800a
hlt
.section .bss
.align 4096
page_table_l4:
.skip 4096
page_table_l3:
.skip 4096
page_table_l2:
.skip 4096
stack_bottom:
.skip 4096 * 4
stack_top:
.section .rodata
.p2align 2 /* force 4 byte alignment */
boot_gdt64:
.quad 0 /* zero entry */
.quad (1 << 43) | (1 << 44) | (1 << 47) | (1 << 53) /* code segment */
boot_gdtdesc:
.word (boot_gdtdesc - boot_gdt64 - 1) /* sizeof(gdt) - 1 */
.long boot_gdt64 /* address gdt */
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