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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]].
== Demo Code ==
 
== Switching to Long Mode ==
The following NASM code demonstrates how to boot into 64 bit mode without entering [[protected mode]]:
 
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.
<pre>
[ORG 0x00007C00]
[BITS 16]
 
<syntaxhighlight lang="asm">
%define PAGE_PRESENT (1 << 0)
%define PAGE_WRITE (1 << 1)
 
%define CODE_SEG 0x0008
%define DATA_SEG 0x0010
 
ALIGN 4
boot_loader:
IDT:
;Parameter from BIOS: dl = boot drive
.Length dw 0
.Base dd 0
 
; Function to switch directly to long mode from real mode.
; 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.
;Set default state
; ss:esp Should point to memory that can be used as a small (1 uint32_t) stack
 
SwitchToLongMode:
cli
; Zero out the 16KiB buffer.
xor bx,bx
; Since we are doing a rep stosd, count should be bytes/4.
mov es,bx
push di ; REP STOSD alters DI.
mov fs,bx
mov gsecx,bx 0x1000
xor eax, eax
mov ds,bx
cld
mov ss,bx
rep stosd
mov sp,0x7C00
pop di ; Get DI back.
sti
 
jmp 0:.clear_cs
.clear_cs:
 
; Build the Page Map Level 4.
; 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.
 
;Load kernel from floppy disk
 
; Build the Page Directory Pointer Table.
mov ax,0x020D ;ah = Function 0x02 ;al = Number of sectors
lea eax, [es:di + 0x2000] ; Put the address of the Page Directory in to EAX.
mov bx,startLongMode ;es:bx = Destination
or eax, PAGE_PRESENT | PAGE_WRITE ; Or EAX with the flags - present flag, writable flag.
mov cx,0x0002 ;cx = Cylinder and sector
mov [es:di + 0x1000], eax ; Store the value of EAX as the first PDPTE.
xor dh,dh ;dx = Head and drive number
int 0x13 ;Int 0x13 Function 0x02 (Load sectors)
 
; 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.
 
;Enable A20 via port 92h
 
; Build the Page Table.
in al,92h
.LoopPageTable:
or al,02h
mov [es:di], eax
out 92h,al
add eax, 0x1000
add di, 8
cmp eax, 0x200000 ; If we did all 2MiB, end.
jb .LoopPageTable
 
pop di ; Restore DI.
; 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.
;Build page tables
;The page tables will look like this:
;PML4:
;dq 0x000000000000b00f = 00000000 00000000 00000000 00000000 00000000 00000000 10010000 00001111
;times 511 dq 0x0000000000000000
 
; Enter long mode.
;PDP:
mov eax, 10100000b ; Set the PAE and PGE bit.
;dq 0x000000000000c00f = 00000000 00000000 00000000 00000000 00000000 00000000 10100000 00001111
mov cr4, eax
;times 511 dq 0x0000000000000000
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.
;PD:
wrmsr
;dq 0x000000000000018f = 00000000 00000000 00000000 00000000 00000000 00000000 00000001 10001111
;times 511 dq 0x0000000000000000
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.
;This defines one 2MB page at the start of memory, so we can access the first 2MBs as if paging was disabled
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:
xor bx,bx
dq 0x00209A0000000000 ; 64-bit code descriptor (exec/read).
mov es,bx
dq 0x0000920000000000 ; 64-bit data descriptor (read/write).
cld
mov di,0xa000
ALIGN 4
dw 0 ; Padding to make the "address of the GDT" field aligned on a 4-byte boundary
 
.Pointer:
mov ax,0xb00f
dw $ - GDT - 1 ; 16-bit Size (Limit) of GDT.
stosw
dd GDT ; 32-bit Base Address of GDT. (CPU will zero extend to 64-bit)
 
xor ax,ax
mov cx,0x07ff
rep stosw
 
[BITS 64]
mov ax,0xc00f
LongMode:
stosw
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 ax,ax
mov cxedi,0x07ff 0xB8000
mov rcx, 500 ; Since we are clearing uint64_t over here, we put the count as Count/4.
rep stosw
mov rax, 0x1F201F201F201F20 ; Set the value to set the screen to: Blue background, white foreground, blank spaces.
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,0x018f
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 ==
 
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.
;Enter long mode
 
<syntaxhighlight lang="asm">
mov eax,10100000b ;Set PAE and PGE
%define FREE_SPACE 0x9000
mov cr4,eax
 
ORG 0x7C00
mov edx, 0x0000a000 ;Point CR3 at PML4
BITS 16
mov cr3,edx
 
; Main entry point where BIOS leaves us.
mov ecx,0xC0000080 ;Specify EFER MSR
 
Main:
rdmsr ;Enable Long Mode
jmp 0x0000:.FlushCS ; Some BIOS' may load us at 0x0000:0x7C00 while other may load us at 0x07C0:0x0000.
or eax,0x00000100
; Do a far jump to fix this issue, and reload CS to 0x0000.
wrmsr
 
.FlushCS:
mov ebx,cr0 ;Activate long mode
xor ax, ax
or ebx,0x80000001 ;by enabling paging and protection simultaneously
mov cr0,ebx ;skipping protected mode entirely
 
; Set up segment registers.
lgdt [gdt.pointer] ;load 80-bit gdt.pointer below
mov ss, 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.
jmp gdt.code:startLongMode ;Load CS with 64 bit segment and flush the instruction cache
jc .NoLongMode
 
; Point edi to a free space bracket.
mov edi, FREE_SPACE
; Switch to Long Mode.
jmp SwitchToLongMode
 
 
BITS 64
;Global Descriptor Table
.Long:
gdt:
hlt
dq 0x0000000000000000 ;Null Descriptor
jmp .Long
 
.code equ $ - gdt
dq 0x0020980000000000
 
BITS 16
.data equ $ - gdt
dq 0x0000900000000000
 
.NoLongMode:
.pointer:
mov si, NoLongMode
dw $-gdt-1 ;16-bit Size (Limit)
call Print
dq gdt ;64-bit Base Address
;Changed from "dd gdt"
;Ref: Intel System Programming Manual V1 - 2.1.1.1
 
.Die:
hlt
jmp .Die
 
times 510-($-$$) db 0 ;Fill boot sector
dw 0xAA55 ;Boot loader signature
 
%include "LongModeDirectly.asm"
BITS 16
 
[BITS 64]
 
NoLongMode db "ERROR: CPU does not support long mode.", 0x0A, 0x0D, 0
startLongMode:
 
cli ;Interupts are disabled because no IDT has been set up
 
; Checks whether CPU supports long mode or not.
mov edi,0x00b8000 ;Display:"Put long mode kernel here.__"
mov rax,0x0720077407750750 ;at the top left corner of screen
mov [edi],rax
mov rax,0x0767076e076f076c
mov [edi+8],rax
mov rax,0x0764076f076d0720
mov [edi+16],rax
mov rax,0x0765076b07200765
mov [edi+24],rax
mov rax,0x076c0765076e0772
mov [edi+32],rax
mov rax,0x0772076507680720
mov [edi+40],rax
mov rax,0x07200720072e0765
mov [edi+48],rax
 
; Returns with carry set if CPU doesn't support long mode.
jmp $ ;Hang the system
</pre>
 
CheckCPU:
== Notes about the code ==
; Check whether CPUID is supported or not.
# This code should work fine if being run as MBR (when all segment registers set to zero)
pushfd ; Get flags in EAX register.
# As noted in code comments, only 2MB is mapped
# The code doesn't check CPU type, so x86 CPU (ie .486 or before) won't fit to run
pop eax
mov ecx, eax
xor eax, 0x200000
push eax
popfd
 
pushfd
pop eax
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
jz .NoLongMode
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:
stc
ret
 
 
; Prints out a message using the BIOS.
 
; es:si Address of ASCIIZ string to print.
 
Print:
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:
 
<syntaxhighlight lang="bash">
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 ===
 
* The above bootsector has been written just for completeness.
* The above bootsector can't be used as an actual bootsector as it doesn't:
# Enable [[A20]]
# Detect memory
# Load required files from disk using BIOS
 
==See Also==
 
===Forum Threads===
 
* [http://www.osdev.org/phpBB2/viewtopic.php?t=11093 Related forum thread]
 
== See Also ==
[http://www.osdev.org/phpBB2/viewtopic.php?t=11093 Related forum thread]
 
[[Category:X86-64]]
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