borked.asm

[BITS 32]

global start

MB_MAGIC equ 0x1BADB002
MB_FLAGS  equ (1 << 0) | (1 << 1)
MB_CHECKSUM equ (0 - (MB_MAGIC + MB_FLAGS))

section .multiboot
	align 4
	dd MB_MAGIC
	dd MB_FLAGS
	dd MB_CHECKSUM

section .bss
	align 16
	stack_bottom:
		resb 4096*4096
	stack_top:

section .text

	GDT64:
		.Null: equ $ - GDT64
			dw 0xFFFF                    ; Limit (low).
			dw 0                         ; Base (low).
			db 0                         ; Base (middle)
			db 0                         ; Access.
			db 1                         ; Granularity.
			db 0                         ; Base (high).
		.Code: equ $ - GDT64         ; The code descriptor.
			dw 0                         ; Limit (low).
			dw 0                         ; Base (low).
			db 0                         ; Base (middle)
			db 10011010b                 ; Access (exec/read).
			db 10101111b                 ; Granularity, 64 bits flag, limit19:16.
			db 0                         ; Base (high).
		.Data: equ $ - GDT64         ; The data descriptor.
			dw 0                         ; Limit (low).
			dw 0                         ; Base (low).
			db 0                         ; Base (middle)
			db 10010010b                 ; Access (read/write).
			db 00000000b                 ; Granularity.
			db 0                         ; Base (high).
		.Pointer:                    ; The GDT-pointer.
			dw $ - GDT64 - 1             ; Limit.
			dq GDT64

	start:
		jmp cpuid_check
	kernel:
		mov esp, stack_top
		call kernel_main
		jmp hang

	cpuid_check:
		pushfd
		pop eax
		mov ecx, eax

		xor eax, 1 << 21
		push eax
		popfd

		pushfd
		pop eax
		push ecx

		popfd
		xor ecx, eax ;cpuid is supported if jne
		jne no_cpuid
		jmp long_mode_check

	no_cpuid:
		xor eax, eax
		jmp hang

	long_mode_check:
		mov eax, 0x80000000
		cpuid
		cmp eax, 0x80000001
		jb no_long_mode

		mov eax, 0x80000001
		cpuid
		test edx, 1 << 29
		jz no_long_mode

	table_clear:
		mov edi, 0x1000
		mov cr3, edi
		xor eax, eax
		mov ecx, 4096
		rep stosd
		mov edi, cr3

	set_tables:
		mov DWORD [edi], 0x2003
		add edi,  0x1000
		mov DWORD [edi], 0x3003
		add edi,  0x1000
		mov DWORD [edi], 0x4003
		add edi,  0x1000
		mov ebx,  0x00000003
		mov ecx,  512
	entry_set:
		mov [edi], ebx
		add ebx, 0x1000
		add edi, 8
		loop entry_set
	enable_paging:
		mov eax, cr4
		or eax, 1 << 5
		mov cr4, eax

	long_mode_switch:
		mov ecx, 0xC0000080
		rdmsr
		or eax, 1 << 8
		mov cr0, eax
		lgdt [GDT64.Pointer]
		jmp GDT64.Code:kernel

	no_long_mode:
		jmp hang

		hang:
			cli
			hlt

		hanging:
			jmp hanging