This question is inspired by a Reddit question in r/osdev except that this question focuses on the SS register. One may say RTFM (ISA entry for MOV), but when this question comes up it can get varying answers even among OS developers.
Question: Should using the MOV instruction to set SS to 0x0000 cause a general protection fault #GP(0) in 64-bit mode? For example: If I am in 64-bit mode with a Current Privilege level (CPL) of 0, should I expect to see a #GP(0) with this code snippet:
NULL_SEL_RPL0 EQU 0
mov eax, NULL_SEL_RPL0 ; EAX = 0x0000 = NULL Selector with
; Requested Privilege Level (RPL) 0.
mov ss, eax ; Try to set SS with NULL selector 0x0000
; Should this cause a #GP(0) fault?
What the Intel x86 Instruction Set Reference says about MOV is:
Operation DEST ← SRC; Loading a segment register while in protected mode results in special checks and actions, as described in the following listing. These checks are performed on the segment selector and the segment descriptor to which it points. IF SS is loaded THEN IF segment selector is NULL THEN #GP(0); FI; IF segment selector index is outside descriptor table limits OR segment selector's RPL ≠ CPL OR segment is not a writable data segment OR DPL ≠ CPL THEN #GP(selector); FI; IF segment not marked present THEN #SS(selector); ELSE SS ← segment selector; SS ← segment descriptor; FI; FI; IF DS, ES, FS, or GS is loaded with non-NULL selector THEN IF segment selector index is outside descriptor table limits OR segment is not a data or readable code segment OR ((segment is a data or nonconforming code segment) AND ((RPL > DPL) or (CPL > DPL))) THEN #GP(selector); FI; IF segment not marked present THEN #NP(selector); ELSE SegmentRegister ← segment selector; SegmentRegister ← segment descriptor; FI; FI; IF DS, ES, FS, or GS is loaded with NULL selector THEN SegmentRegister ← segment selector; SegmentRegister ← segment descriptor; FI;
In particular this part:
IF SS is loaded THEN IF segment selector is NULL THEN #GP(0); FI;
Behaviour I expect: I am loading SS with a NULL selector (0x0000). I would presume from this documentation that I should get a #GP(0). On real hardware, VirtualBox, QEMU, and BOCHS the code snippet doesn't fault.
Observed behaviour: No fault occurs and the 64-bit code continues to execute as expected.
Why does the behaviour I expect differ from what the documentation seems to suggest?
Complete Example
To make this a Minimal Complete Verifiable Example that can test the code snippet in emulators/VMs and on real hardware I present the following code. For test purposes the test code below is being placed in stage 2 of a bootloader. Getting into long mode is non-trivial. The code creates a GDT; an IDT to cause all exceptions and IRQs to triple fault; disables interrupts on the PICs; flushed pending IRQs; enables paging; identity maps the first 2MiB of memory; enters 64-bit mode directly from real-mode. The 64-bit code snippet came from the code at the very bottom at the label longmode64:
stage2.asm:
STAGE2_OFS EQU 0x7e00 ; Origin point (VMA) of stage2
; Offset form base of memory where stage2 starts
VIDEO_TEXT_ADDR EQU 0xb8000
; Hard code beginning of text video memory
ATTR_WHITE_ON_MAGENTA EQU 0x57 ; White on magenta attribute
EFLAGS_IF_BIT EQU 9 ; Interrupt Flag (IF) bit = 9
org STAGE2_OFS ; Set origin point (VMA) of stage2
bits 16
; Stage2 Entry point
; Upon entry these have all been set:
; Direction Flag (DF) = 0
; DS=ES=GS=FS=0x0000
; SS:SP = 0x0000:0x7c00
stage2:
mov si, nolm_err ; Default error message to long mode error
call check_longmode ; Is long mode available on this CPU?
jz .error ; If not print error and stop
mov si, noa20_err ; Default error message to A20 enable error
call a20_enable ; Enable A20 line
jz .error ; If the A20 line isn't enabled then print error and stop
mov edi, PAGING_BASE_ADDR ; DS:EDI set to 4KiB aligned memory address 0x0000:0x1000
jmp switch_longmode_64 ; Switch to 64-bit mode and
; and continue at label 'longmode64_entry'
.error:
call print_string ; Print error message
.end:
cli ; Disable interrupts
.endloop:
hlt ; Halt CPU
jmp .endloop ; Loop in case we get an NMI (non-maskable interrupt)
; Function: check_longmode
; Check if long mode is available on the CPU
;
; Inputs: None
; Clobbers: EAX, ECX
; Returns: Zero Flag (ZF) set if CPU support long mode
check_longmode:
call check_386
jz .nolongmode
; Check whether CPUID is supported or not. If we can successfully
; flip bit 21 in EFLAGS then CPUID is supported.
pushfd
pop eax ; Get current EFLAGS
mov ecx, eax ; ECX = copy of original EFLAGS
xor eax, 1<<21 ; Flip bit 21
push eax
popfd ; Set new EFLAGS
pushfd
pop eax ; ECX = updated EFLAGS
push ecx
popfd ; Restore original EFLAGS
xor eax, ecx ; Are any bits different between original and new EFLAGS
jz .nolongmode ; If they are then CPUID is supported
mov eax, 0x80000000 ; Get Highest Extended Function Implemented
cpuid
cmp eax, 0x80000001 ; Check support for at least Extended Function 0x80000001
jb .nolongmode ; If not, long mode not supported
mov eax, 0x80000001 ; Get Extended Processor Info and Feature Bits
cpuid
test edx, 1 << 29 ; Test if the LM bit is set
jz .nolongmode ; If not set then long mode isn't supported
ret ; Otherwise long mode is supported return with ZF = 1
.nolongmode:
xor eax, eax ; Return with ZF = 0
ret
; Function: print_string
; Display a string to the console on display page 0
;
; Inputs: SI = Offset of address to print
; Clobbers: AX, BX, SI
print_string:
mov ah, 0x0e ; BIOS tty Print
xor bx, bx ; Set display page to 0 (BL)
jmp .getch
.repeat:
int 0x10 ; print character
.getch:
lodsb ; Get character from string
test al,al ; Have we reached end of string?
jnz .repeat ; if not process next character
.end:
ret
; Function: wait_8042_cmd
; Wait until the Input Buffer Full bit in the keyboard controller's
; status register becomes 0. After calls to this function it is
; safe to send a command on Port 0x64
;
; Inputs: None
; Clobbers: AX
; Returns: None
KBC_STATUS_IBF_BIT EQU 1
wait_8042_cmd:
in al, 0x64 ; Read keyboard controller status register
test al, 1 << KBC_STATUS_IBF_BIT
; Is bit 1 (Input Buffer Full) set?
jnz wait_8042_cmd ; If it is then controller is busy and we
; can't send command byte, try again
ret ; Otherwise buffer is clear and ready to send a command
; Function: wait_8042_data
; Wait until the Output Buffer Empty (OBE) bit in the keyboard controller's
; status register becomes 0. After a call to this function there is
; data available to be read on port 0x60.
;
; Inputs: None
; Clobbers: AX
; Returns: None
KBC_STATUS_OBE_BIT EQU 0
wait_8042_data:
in al, 0x64 ; Read keyboard controller status register
test al, 1 << KBC_STATUS_OBE_BIT
; Is bit 0 (Output Buffer Empty) set?
jz wait_8042_data ; If not then no data waiting to be read, try again
ret ; Otherwise data is ready to be read
; Function: a20_kbd_enable
; Enable the A20 line via the keyboard controller
;
; Inputs: None
; Clobbers: AX, CX
; Returns: None
a20_kbd_enable:
pushf
cli ; Disable interrupts
call wait_8042_cmd ; When controller ready for command
mov al, 0xad ; Send command 0xad (disable keyboard).
out 0x64, al
call wait_8042_cmd ; When controller ready for command
mov al, 0xd0 ; Send command 0xd0 (read output port)
out 0x64, al
call wait_8042_data ; Wait until controller has data
in al, 0x60 ; Read data from keyboard
mov cx, ax ; CX = copy of byte read
call wait_8042_cmd ; Wait until controller is ready for a command
mov al, 0xd1
out 0x64, al ; Send command 0xd1 (write output port)
call wait_8042_cmd ; Wait until controller is ready for a command
mov ax, cx
or al, 1 << 1 ; Write value back with bit 1 set
out 0x60, al
call wait_8042_cmd ; Wait until controller is ready for a command
mov al, 0xae
out 0x64, al ; Write command 0xae (enable keyboard)
call wait_8042_cmd ; Wait until controller is ready for command
popf ; Restore flags including interrupt flag
ret
; Function: a20_fast_enable
; Enable the A20 line via System Control Port A
;
; Inputs: None
; Clobbers: AX
; Returns: None
a20_fast_enable:
in al, 0x92 ; Read System Control Port A
test al, 1 << 1
jnz .finished ; If bit 1 is set then A20 already enabled
or al, 1 << 1 ; Set bit 1
and al, ~(1 << 0) ; Clear bit 0 to avoid issuing a reset
out 0x92, al ; Send Enabled A20 and disabled Reset to control port
.finished:
ret
; Function: a20_bios_enable
; Enable the A20 line via the BIOS function Int 15h/AH=2401
;
; Inputs: None
; Clobbers: AX
; Returns: None
a20_bios_enable:
mov ax, 0x2401 ; Int 15h/AH=2401 enables A20 on BIOS with this feature
int 0x15
ret
; Function: a20_check
; Determine if the A20 line is enabled or disabled
;
; Inputs: None
; Clobbers: AX, CX, ES
; Returns: ZF=1 if A20 enabled, ZF=0 if disabled
a20_check:
pushf ; Save flags so Interrupt Flag (IF) can be restored
push ds ; Save volatile registers
push si
push di
cli ; Disable interrupts
xor ax, ax
mov ds, ax
mov si, 0x600 ; 0x0000:0x0600 (0x00600) address we will test
mov ax, 0xffff
mov es, ax
mov di, 0x610 ; 0xffff:0x0610 (0x00600) address we will test
; The physical address pointed to depends on whether
; memory wraps or not. If it wraps then A20 is disabled
mov cl, [si] ; Save byte at 0x0000:0x0600
mov ch, [es:di] ; Save byte at 0xffff:0x0610
mov byte [si], 0xaa ; Write 0xaa to 0x0000:0x0600
mov byte [es:di], 0x55 ; Write 0x55 to 0xffff:0x0610
xor ax, ax ; Set return value 0
cmp byte [si], 0x55 ; If 0x0000:0x0600 is 0x55 and not 0xaa
je .disabled ; then memory wrapped because A20 is disabled
dec ax ; A20 Disable, set AX to -1
.disabled:
; Cleanup by restoring original bytes in memory. This must be in reverse
; order from the order they were originally saved
mov [es:di], ch ; Restore data saved data to 0xffff:0x0610
mov [si], cl ; Restore data saved data to 0x0000:0x0600
pop di ; Restore non-volatile registers
pop si
pop ds
popf ; Restore Flags (including IF)
test al, al ; Return ZF=1 if A20 enabled, ZF=0 if disabled
ret
; Function: a20_enable
; Enable the A20 line
;
; Inputs: None
; Clobbers: AX, BX, CX, DX
; Returns: ZF=0 if A20 not enabled, ZF=1 if A20 enabled
a20_enable:
call a20_check ; Is A20 already enabled?
jnz .a20_on ; If so then we're done ZF=1
call a20_bios_enable ; Try enabling A20 via BIOS
call a20_check ; Is A20 now enabled?
jnz .a20_on ; If so then we're done ZF=1
call a20_kbd_enable ; Try enabling A20 via keyboard controller
call a20_check ; Is A20 now enabled?
jnz .a20_on ; If so then we're done ZF=1
call a20_fast_enable ; Try enabling A20 via fast method
call a20_check ; Is A20 now enabled?
jnz .a20_on ; If so then we're done ZF=1
.a20_err:
xor ax, ax ; If A20 disabled then return with ZF=0
.a20_on:
ret
; Function: check_386
; Check if this processor is at least a 386
;
; Inputs: None
; Clobbers: AX
; Returns: ZF=0 if Processor earlier than a 386, ZF=1 if processor is 386+
check_386:
xor ax, ax ; Zero EFLAGS
push ax
popf ; Push zeroed flags
pushf
pop ax ; Get the currently set flags
and ax, 0xf000 ; if high 4 bits of FLAGS are not set then
cmp ax, 0xf000 ; CPU is an 8086/8088/80186/80188
je .error ; and exit with ZF = 0
mov ax, 0xf000 ; Set the high 4 bits of FLAGS to 1
push ax
popf ; Update the FLAGS register
pushf ; Get newly set FLAGS into AX
pop ax
and ax, 0xf000 ; if none of the high 4 bits are set then
jnz .noerror ; CPU is an 80286. Return success ZF = 1
; otherwise CPU is a 386+
.error:
xor ax, ax ; Set ZF = 0 (Earlier than a 386)
.noerror:
ret
; Function: switch_longmode_64
; Switch processor to 64-bit mode directly from real mode
; See: https://wiki.osdev.org/Entering_Long_Mode_Directly
; - Enable Interrupts (IF=1)
; - Enable paging
; - Identity Map first 2MiB of memory with a large page
; by setting up proper PML4, PDPT, and PD
; - Disable interrupts on the Master and Slave PICs
; - Flush any pending external interrupts
; - Use LIDT to load an IDT record with size of 0 to force
; all software and hardware interrupts to triple fault
; - Jump to 64-bit mode at label `longmode64_entry`
;
; Inputs: DS:EDI 4KiB aligned address where there is at least
; 12KiB of physical memory available
; Clobbers: N/A
; Returns: Jumps to label 'longmode64_entry', doesn't return
PAGE_PRESENT EQU (1<<0)
PAGE_WRITE EQU (1<<1)
PAGE_USER EQU (1<<2)
PAGEDIR_SIZE_LARGE EQU (1<<7)
PAGING_STRUCT_SIZE EQU 3*4096 ; Size of memory area to hold PML4, PDPT, and PD
PAGING_BASE_ADDR EQU 0x1000 ; Offset in first 64Kb that is the start of a 16KiB
; region that can be used for a default paging tree
PML4_OFS EQU 0x0000 ; Offset of PML4 table
PDPT_OFS EQU 0x1000 ; Offset of Page Directory Pointer Table
PD_OFS EQU 0x2000 ; Offset of Page Directory Table
switch_longmode_64:
push dword 1<<EFLAGS_IF_BIT; Reset all the EFLAG bits to 0 except IF=1
popfd
; Zero out the 12KiB buffer used for PML4, PDPT, PD.
; We are using rep stosd (DWORD) thus the count should be bytes / 4.
push di ; Temporarily store DI
mov ecx, (PAGING_STRUCT_SIZE/4)
; Number of DWORDS to set
xor eax, eax ; Value to set 0x00000000
rep stosd ; Zero the memory
pop di ; Restore DI
; DI = 4KiB aligned address to base of paging structures
; Create Page Map Level 4 Table (PML4)
lea eax, [di + PDPT_OFS] ; EAX = address of Page Directory Pointer Table (PDPT)
or eax, PAGE_PRESENT | PAGE_WRITE | PAGE_USER
; Set present flag, writable and user flags
mov [di + PML4_OFS], eax ; Store the address the PDPT to the first PML4 entry
; Create the Page Directory Pointer Table (PDPT)
lea eax, [di + PD_OFS] ; EAX = address of Page Directory (PD)
or eax, PAGE_PRESENT | PAGE_WRITE | PAGE_USER
; Set present flag, writable and user flags
mov [di + PDPT_OFS], eax ; Store page directory address as the first PDPT entry
; Create Page Directory (PD)
mov dword [di + PD_OFS], PAGE_PRESENT | PAGE_WRITE | PAGE_USER | \
PAGEDIR_SIZE_LARGE | 0 << 21
; Set first PD entry to present, writable, user, and
; large page. Identity map to the first 2MiB in
; physical memory
; Disable IRQs on the Master and Slave PICs
mov al, 0xFF ; Bits that are 1 disable interrupts, 0 = enable
out 0xA1, al ; Disable all interrupts on Slave PIC
out 0x21, al ; Disable all interrupts on Master PIC
; Flush any pending IRQs
mov ecx, 8
; Do a loop to allow pending interrupts to be processed.
; Execute enough instructions to process all 16 interrupts.
.irqflush:
dec ecx
jnz .irqflush
lidt [idtr] ; Load a zero length IDT so that any hardware
; interrupt or CPU exception causes a triple fault
; Enter long mode directly from real mode without entering compatibility mode
movzx esp, sp ; Zero extend SP to ESP
mov eax, 10100000b
mov cr4, eax ; Set CR4 PAE and PGE bits on and other features off
mov cr3, edi ; Set CR3 to address of PML4 (@ 0x00001000)
mov ecx, 0xC0000080
rdmsr ; Read EFER MST to EDX:EAX
or eax, 0x00000100 ; Set the LME bit
wrmsr ; Write back changes to EFER MSR
mov eax, cr0 ; Get current CR0
or eax, 0x80000001 ; Enable both paging and protected mode bits
mov cr0, eax ; Update CR0
jmp .flushipfq ; This JMP is to flush instruction prefetch queue
.flushipfq:
lgdt [gdtr] ; Load gdt from gdtr
jmp CODE64_PL0_SEL:longmode64_entry
; Start executing code in 64-bit mode
noa20_err db "A20 line couldn't be enabled", 10, 13, 0
nolm_err db "Processor doesn't support x86-64 mode", 10, 13, 0
; Zero length IDT record to force all interrupts to triple fault
align 4
idtr:
.len dw 0
.base dd 0
; Macro to build a GDT descriptor entry
%define MAKE_GDT_DESC(base, limit, access, flags) \
(((base & 0x00FFFFFF) << 16) | \
((base & 0xFF000000) << 32) | \
(limit & 0x0000FFFF) | \
((limit & 0x000F0000) << 32) | \
((access & 0xFF) << 40) | \
((flags & 0x0F) << 52))
; GDT structure
align 4
gdt_start: dq MAKE_GDT_DESC(0, 0, 0, 0)
; Null descriptor
gdt64_code_pl0: dq MAKE_GDT_DESC(0, 0x00000000, 10011010b, 0010b)
; 64-bit code, privilege level 0, l=1, sz=0
gdt64_data_pl0: dq MAKE_GDT_DESC(0, 0x00000000, 10010010b, 0000b)
; 64-bit data, privilege level 0, l=0, sz=0
gdt64_code_pl3: dq MAKE_GDT_DESC(0, 0x00000000, 11111010b, 0010b)
; 64-bit code, privilege level 3, l=1, sz=0
gdt64_data_pl3: dq MAKE_GDT_DESC(0, 0x00000000, 11110010b, 0000b)
; 64-bit data, privilege level 3, l=0, sz=0
end_of_gdt:
; GDT record
align 4
dw 0 ; Padding align dd GDT in gdtr on 4 byte boundary
gdtr:
dw end_of_gdt - gdt_start - 1
; limit (Size of GDT - 1)
dd gdt_start ; base of GDT
NULL_SEL_RPL0 EQU 0
NULL_SEL_RPL1 EQU 1
NULL_SEL_RPL2 EQU 2
NULL_SEL_RPL3 EQU 3
CODE64_PL0_SEL EQU gdt64_code_pl0 - gdt_start
DATA64_PL0_SEL EQU gdt64_data_pl0 - gdt_start
CODE64_PL3_SEL EQU gdt64_code_pl3 - gdt_start
DATA64_PL3_SEL EQU gdt64_data_pl3 - gdt_start
; Entry point for 64-bit mode
; Upon entry these have all been set:
; - CPU is running at Current Privilege Level (CPL) = 0 aka kernel mode
; - Interrupts are enabled (IF=1)
; - External interrupts are disabled on the Master and Slave PICs
; - Direction Flag clear (DF=0)
BITS 64
longmode64_entry:
mov eax, DATA64_PL0_SEL ; Set DS/ES/FS/GS/SS to a
; privilege level 0 data selector
mov ds, eax
mov es, eax
mov fs, eax
mov gs, eax
mov ss, eax
; Insert 64-bit code to test here
mov eax, NULL_SEL_RPL0 ; EAX = 0x0000 = NULL Selector with
; Requested Privilege Level (RPL) 0
; Should loading a NULL selector with RPL 0 (0x0000) fail
; and cause an exception / triple fault / reboot? When tested in VMs and
; real hardware it works and `LM` is printed to the display below
mov ss, eax ; Try to set SS with NULL selector
; Write the letters "LM" (long mode) to upper left hand corner of display
; starting at text video memory address 0xb8000 using white on magenta attribute
mov dword [VIDEO_TEXT_ADDR], (ATTR_WHITE_ON_MAGENTA << 8 | 'M') << 16 | \
(ATTR_WHITE_ON_MAGENTA << 8 | 'L')
hlt
stage2_end:
To assemble this file with NASM use:
nasm -f bin stage2.asm -o stage2.bin
A bootloader that this code is designed to work with can be found in this Stackoverflow answer. The code in that answer acts a test harness in the form of a bootable 1.44MiB floppy disk image. To build the disk - copy boot.asm and bpb.inc to the directory with stage2.asm (above) and use these commands:
nasm -f bin boot.asm -o disk.img
To test in QEMU you can run the command:
qemu-system-x86_64 -fda disk.img
If there is no fault the output should be similar to this:
If the code doesn't fault it should print LM in the upper left of the screen.
