Entering Long Mode Directly

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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.


Switching to Long 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.

%define PAGE_PRESENT    (1 << 0)
%define PAGE_WRITE      (1 << 1)
%define CODE_SEG     0x0008
%define DATA_SEG     0x0010
    .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.
; ss:esp    Should point to memory that can be used as a small (1 uint32_t) stack
    ; Zero out the 16KiB buffer.
    ; Since we are doing a rep stosd, count should be bytes/4.   
    push di                           ; REP STOSD alters DI.
    mov ecx, 0x1000
    xor eax, eax
    rep stosd
    pop di                            ; Get DI back.
    ; 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.
    ; 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.
    ; 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.
    ; Build the Page Table.
    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.
    ; Disable IRQs
    mov al, 0xFF                      ; Out 0xFF to 0xA1 and 0x21 to disable all IRQs.
    out 0xA1, al
    out 0x21, al
    lidt [IDT]                        ; Load a zero length IDT so that any NMI causes a triple fault.
    ; 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. 
    or eax, 0x00000100                ; Set the LME bit.
    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.
    jmp CODE_SEG:LongMode             ; Load CS with 64 bit segment and flush the instruction cache
    ; Global Descriptor Table
    dq 0x0000000000000000             ; Null Descriptor - should be present.
    dq 0x00209A0000000000             ; 64-bit code descriptor (exec/read).
    dq 0x0000920000000000             ; 64-bit data descriptor (read/write).
    dw 0                              ; Padding to make the "address of the GDT" field aligned on a 4-byte boundary
    dw $ - GDT - 1                    ; 16-bit Size (Limit) of GDT.
    dd GDT                            ; 32-bit Base Address of GDT. (CPU will zero extend to 64-bit)
[BITS 64]      
    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.
    mov edi, 0xB8000
    mov rcx, 500                      ; Since we are clearing uint64_t over here, we put the count as Count/4.
    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 [edi + 16], rax
    jmp Main.Long                     ; You should replace this jump to wherever you want to jump to.

Notes about the above code

  1. The above code assumes that the caller has checked whether the CPU supports x86_64 or not.
  2. As noted in code comments, only 2MB is mapped. The code can be easily modified to map the kernel to higher half.
  3. 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.

%define FREE_SPACE 0x9000
ORG 0x7C00
; Main entry point where BIOS leaves us.
    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.
    xor ax, ax
    ; Set up segment registers.
    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
    call CheckCPU                     ; Check whether we support Long Mode or not.
    jc .NoLongMode
    ; Point edi to a free space bracket.
    mov edi, FREE_SPACE
    ; Switch to Long Mode.
    jmp SwitchToLongMode
    jmp .Long
    mov si, NoLongMode
    call Print
    jmp .Die
%include "LongModeDirectly.asm"
NoLongMode db "ERROR: CPU does not support long mode.", 0x0A, 0x0D, 0
; Checks whether CPU supports long mode or not.
; Returns with carry set if CPU doesn't support long mode.
    ; Check whether CPUID is supported or not.
    pushfd                            ; Get flags in EAX register.
    pop eax
    mov ecx, eax  
    xor eax, 0x200000 
    push eax 
    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
    test eax, eax
    jz .NoLongMode
    mov eax, 0x80000000   
    cmp eax, 0x80000001               ; Check whether extended function 0x80000001 is available are not.
    jb .NoLongMode                    ; If not, long mode not supported.
    mov eax, 0x80000001  
    test edx, 1 << 29                 ; Test if the LM-bit, is set or not.
    jz .NoLongMode                    ; If not Long mode not supported.
; Prints out a message using the BIOS.
; es:si    Address of ASCIIZ string to print.
    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.
    popad                             ; Pop all general purpose registers to save them.
; Pad out file.
times 510 - ($-$$) db 0
dw 0xAA55

The above code can be compiled (and tested using QEMU) by the following:

    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

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:
  1. Enable A20
  2. Detect memory
  3. Load required files from disk using BIOS

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