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For some reason, you might want to switch to 64 bit mode in your bootloader without entering [[protected mode]]. The following article practically described how to do so, mainly demonstrating it by example assembly code. Additionally, some may find it easier to switch directly to long mode on additional CPUs as they are initialized for [[Symmetric Multiprocessing]].
The following NASM code demonstrates how to boot into 64 bit mode without entering [[protected mode]]:
 
== Switching to Long Mode ==
<pre>
[ORG 0x00007C00]
[BITS 16]
 
The following can be used to switch directly to Long Mode. The code is well documented through comments, and nearly explains almost everything it does.
 
<syntaxhighlight lang="asm">
%define PAGE_PRESENT (1 << 0)
%define PAGE_WRITE (1 << 1)
 
%define CODE_SEG 0x0008
boot_loader:
%define DATA_SEG 0x0010
;Parameter from BIOS: dl = boot drive
 
ALIGN 4
IDT:
.Length dw 0
.Base dd 0
 
; Function to switch directly to long mode from real mode.
;Set default state
; Identity maps the first 2MiB.
; Uses Intel syntax.
 
; es:edi Should point to a valid page-aligned 16KiB buffer, for the PML4, PDPT, PD and a PT.
cli
; ss:esp Should point to memory that can be used as a small (1 uint32_t) stack
xor bx,bx
mov es,bx
mov fs,bx
mov gs,bx
mov ds,bx
mov ss,bx
mov sp,0x7C00
sti
 
SwitchToLongMode:
jmp 0:.clear_cs
; Zero out the 16KiB buffer.
.clear_cs:
; Since we are doing a rep stosd, count should be bytes/4.
push di ; REP STOSD alters DI.
mov ecx, 0x1000
xor eax, eax
cld
rep stosd
pop di ; Get DI back.
 
 
; Build the Page Map Level 4.
;Load kernel from floppy disk
; es:di points to the Page Map Level 4 table.
lea eax, [es:di + 0x1000] ; Put the address of the Page Directory Pointer Table in to EAX.
or eax, PAGE_PRESENT | PAGE_WRITE ; Or EAX with the flags - present flag, writable flag.
mov [es:di], eax ; Store the value of EAX as the first PML4E.
 
mov ax,0x020D ;ah = Function 0x02 ;al = Number of sectors
mov bx,startLongMode ;es:bx = Destination
mov cx,0x0002 ;cx = Cylinder and sector
xor dh,dh ;dx = Head and drive number
int 0x13 ;Int 0x13 Function 0x02 (Load sectors)
 
; Build the Page Directory Pointer Table.
lea eax, [es:di + 0x2000] ; Put the address of the Page Directory in to EAX.
or eax, PAGE_PRESENT | PAGE_WRITE ; Or EAX with the flags - present flag, writable flag.
mov [es:di + 0x1000], eax ; Store the value of EAX as the first PDPTE.
 
;Enable A20 via keyboard controller
; Build the Page Directory.
lea eax, [es:di + 0x3000] ; Put the address of the Page Table in to EAX.
or eax, PAGE_PRESENT | PAGE_WRITE ; Or EAX with the flags - present flag, writeable flag.
mov [es:di + 0x2000], eax ; Store to value of EAX as the first PDE.
push di ; Save DI for the time being.
lea di, [di + 0x3000] ; Point DI to the page table.
mov eax, PAGE_PRESENT | PAGE_WRITE ; Move the flags into EAX - and point it to 0x0000.
 
call .wait_for_keyboard_controller
 
; Build the Page Table.
mov al,0xD1 ;Out A20 control
.LoopPageTable:
out 0x64,al
mov [es:di], eax
add eax, 0x1000
add di, 8
cmp eax, 0x200000 ; If we did all 2MiB, end.
jb .LoopPageTable
 
pop di ; Restore DI.
call .wait_for_keyboard_controller
; Disable IRQs
mov al, 0xFF ; Out 0xFF to 0xA1 and 0x21 to disable all IRQs.
out 0xA1, al
out 0x21, al
nop
nop
 
lidt [IDT] ; Load a zero length IDT so that any NMI causes a triple fault.
mov al,0xDF ;Out Enable A20
out 0x60,al
 
; Enter long mode.
mov eax, 10100000b ; Set the PAE and PGE bit.
mov cr4, eax
mov edx, edi ; Point CR3 at the PML4.
mov cr3, edx
mov ecx, 0xC0000080 ; Read from the EFER MSR.
rdmsr
 
or eax, 0x00000100 ; Set the LME bit.
;Build page tables
wrmsr
mov ebx, cr0 ; Activate long mode -
or ebx,0x80000001 ; - by enabling paging and protection simultaneously.
mov cr0, ebx
 
lgdt [GDT.Pointer] ; Load GDT.Pointer defined below.
;The page tables will look like this:
jmp CODE_SEG:LongMode ; Load CS with 64 bit segment and flush the instruction cache
; Global Descriptor Table
GDT:
.Null:
dq 0x0000000000000000 ; Null Descriptor - should be present.
 
.Code:
;PML4:
dq 0x00209A0000000000 ; 64-bit code descriptor (exec/read).
;dq 0x000000000000b00f ;00000000 00000000 00000000 00000000 00000000 00000000 10010000 00001111
dq 0x0000920000000000 ; 64-bit data descriptor (read/write).
;times 511 dq 0x0000000000000000
ALIGN 4
dw 0 ; Padding to make the "address of the GDT" field aligned on a 4-byte boundary
 
.Pointer:
;PDP:
dw $ - GDT - 1 ; 16-bit Size (Limit) of GDT.
;dq 0x000000000000c00f ;00000000 00000000 00000000 00000000 00000000 00000000 10100000 00001111
dd GDT ; 32-bit Base Address of GDT. (CPU will zero extend to 64-bit)
;times 511 dq 0x0000000000000000
 
;PD:
;dq 0x000000000000018f ;00000000 00000000 00000000 00000000 00000000 00000000 00000001 10001111
;times 511 dq 0x0000000000000000
 
[BITS 64]
;This defines one 2MB page at the start of memory, so we can access the first 2MBs as if paging was disabled
LongMode:
mov ax, DATA_SEG
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ss, ax
 
; Blank out the screen to a blue color.
xor bx,bx
mov esedi,bx 0xB8000
mov rcx, 500 ; Since we are clearing uint64_t over here, we put the count as Count/4.
cld
mov rax, 0x1F201F201F201F20 ; Set the value to set the screen to: Blue background, white foreground, blank spaces.
mov di,0xa000
rep stosq ; Clear the entire screen.
; Display "Hello World!"
mov edi, 0x00b8000
mov rax, 0x1F6C1F6C1F651F48
mov [edi],rax
mov rax, 0x1F6F1F571F201F6F
mov [edi + 8], rax
 
mov rax, 0x1F211F641F6C1F72
mov ax,0xb00f
mov [edi + 16], rax
stosw
jmp Main.Long ; You should replace this jump to wherever you want to jump to.
</syntaxhighlight>
 
=== Notes about the above code ===
xor ax,ax
# The above code assumes that the caller has checked whether the CPU supports x86_64 or not.
mov cx,0x07ff
# As noted in code comments, only 2MB is mapped. The code can be easily modified to map the kernel to higher half.
rep stosw
# The code doesn't include BPB for floppies, or any such thing. This also can be easily added.
 
== Example bootsector ==
mov ax,0xc00f
stosw
 
The following example bootsector has been provided for completeness. It checks whether the CPU supports x86_64 or not. It then calls SwitchToLongMode to switch to long mode.
xor ax,ax
mov cx,0x07ff
rep stosw
 
<syntaxhighlight lang="asm">
mov ax,0x018f
%define FREE_SPACE 0x9000
stosw
 
ORG 0x7C00
xor ax,ax
BITS 16
mov cx,0x07ff
rep stosw
 
; Main entry point where BIOS leaves us.
 
Main:
;Enter long mode
jmp 0x0000:.FlushCS ; Some BIOS' may load us at 0x0000:0x7C00 while other may load us at 0x07C0:0x0000.
; Do a far jump to fix this issue, and reload CS to 0x0000.
 
.FlushCS:
mov eax,10100000b ;Set PAE and PGE
xor ax, ax
mov cr4,eax
 
; Set up segment registers.
mov edx, 0x0000a000 ;Point cr3 at PML4
mov cr3ss,edx ax
; Set up stack so that it starts below Main.
mov sp, Main
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
cld
 
call CheckCPU ; Check whether we support Long Mode or not.
mov ecx,0xC0000080 ;Specify EFER MSR
jc .NoLongMode
 
; Point edi to a free space bracket.
rdmsr ;Enable long mode
mov edi, FREE_SPACE
or eax,0x00000100
; Switch to Long Mode.
wrmsr
jmp SwitchToLongMode
 
mov ebx,cr0 ;Activate long mode
or ebx,0x80000001 ;by enabling paging and protection simultaneously
mov cr0,ebx ;skipping protected mode entirely
 
BITS 64
lgdt [gdt.pointer]
.Long:
hlt
jmp .Long
 
jmp gdt.code:startLongMode ;Load CS with 64 bit segment and flush the instruction cache
 
BITS 16
 
.NoLongMode:
mov si, NoLongMode
call Print
 
.Die:
.wait_for_keyboard_controller:
hlt
jmp .Die
 
mov cx,0xFFFF
 
%include "LongModeDirectly.asm"
.A20_loop:
BITS 16
in al,0x64
test al,2
loopnz .A20_loop
 
ret
 
NoLongMode db "ERROR: CPU does not support long mode.", 0x0A, 0x0D, 0
 
 
; Checks whether CPU supports long mode or not.
;Global Descriptor Table
 
; Returns with carry set if CPU doesn't support long mode.
gdt:
dq 0x0000000000000000
 
CheckCPU:
.code equ $ - gdt
; Check whether CPUID is supported or not.
dq 0x0020980000000000
pushfd ; Get flags in EAX register.
pop eax
mov ecx, eax
xor eax, 0x200000
push eax
popfd
 
pushfd
.data equ $ - gdt
pop eax
dq 0x0000900000000000
xor eax, ecx
shr eax, 21
and eax, 1 ; Check whether bit 21 is set or not. If EAX now contains 0, CPUID isn't supported.
push ecx
popfd
 
test eax, eax
.pointer:
jz .NoLongMode
dw $-gdt-1
dd gdt
mov eax, 0x80000000
cpuid
cmp eax, 0x80000001 ; Check whether extended function 0x80000001 is available are not.
jb .NoLongMode ; If not, long mode not supported.
 
mov eax, 0x80000001
cpuid
test edx, 1 << 29 ; Test if the LM-bit, is set or not.
jz .NoLongMode ; If not Long mode not supported.
 
ret
 
.NoLongMode:
;Fill boot sector
stc
ret
 
times 510-($-$$) db 0
dw 0xAA55 ;Boot loader signature
 
; Prints out a message using the BIOS.
 
; es:si Address of ASCIIZ string to print.
 
Print:
[BITS 64]
pushad
.PrintLoop:
lodsb ; Load the value at [@es:@si] in @al.
test al, al ; If AL is the terminator character, stop printing.
je .PrintDone
mov ah, 0x0E
int 0x10
jmp .PrintLoop ; Loop till the null character not found.
.PrintDone:
popad ; Pop all general purpose registers to save them.
ret
 
 
; Pad out file.
times 510 - ($-$$) db 0
dw 0xAA55
</syntaxhighlight>
 
The above code can be compiled (and tested using QEMU) by the following:
startLongMode:
 
<syntaxhighlight lang="bash">
cli ;Interupts are disabled because no IDT has been set up
nasm -fbin Main.asm -o LongModeDirectly # The main file's name should be Main.asm
qemu-system-x86_64 -hda LongModeDirectly # The secondary file's name should be LongModeDirectly.asm and should be in the same directory
</syntaxhighlight>
 
=== Notes about the above bootsector ===
mov edi,0x00b8000 ;Display:Put long mode kernel here.
 
mov rax,0x0720077407750750
* The above bootsector has been written just for completeness.
mov [edi],rax
* The above bootsector can't be used as an actual bootsector as it doesn't:
mov rax,0x0767076e076f076c
# Enable [[A20]]
mov [edi+8],rax
# Detect memory
mov rax,0x0764076f076d0720
# Load required files from disk using BIOS
mov [edi+16],rax
 
mov rax,0x0765076b07200765
==See Also==
mov [edi+24],rax
 
mov rax,0x076c0765076e0772
===Forum Threads===
mov [edi+32],rax
 
mov rax,0x0772076507680720
* [http://www.osdev.org/phpBB2/viewtopic.php?t=11093 Related forum thread]
mov [edi+40],rax
mov rax,0x07200720072e0765
mov [edi+48],rax
 
jmp $
</pre>
 
[[Category:X86-64]]
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