通过将EFLAGS.VM设置为1,问题从32位保护模式切换到v8086模式
Mic*_*tch 8 x86 assembly x86-64 nasm osdev
我处于以当前权限级别(CPL = 0)运行的32位保护模式.我试图通过将EFLAGS.VM(位17)标志设置为1(和IOPL为0)并对我的16位实模式代码执行FAR JMP来进入v8086模式.我使用当前的标志PUSHF; 将EFLAGS.VM(第17位)设置为1; 将EFLAGS.IOPL(第22位和第23位)设置为0; 设置新的EFLAGS POPF.这个代码看起来像:
bits 32
cli
[snip]
pushf ; Get current EFLAGS
pop eax
or eax, 1<<EFLAGS_VM_BIT ; Set VM flag to enter v8086 mode
and eax, ~(3<<EFLAGS_IOPL_BITS)
; Set IOPL to 0
; IF flag already 0 because of earlier CLI
push eax
popf ; Reload new flags
jmp CODE32_SEL:v86_mode_entry
; Far JMP to v8086 entry point
; v8086 code entry point
bits 16
v86_mode_entry:
hlt ; Halt should double fault
[snip]
对于这些测试,我故意运行:
- CPL = 0时断开中断.
- 在v8086模式下运行时中断.
- 我没有IDT.
- 我没有TSS,因为我没有通过中断,门和异常在特权级别之间转换.
为了测试我是否已进入v8086模式,我做了一个HLT指令.由于我没有中断机制,我预计会发生双重故障.在hlt似乎正确执行和系统坐在那里.在BOCH中,当我到达时,hlt我注意到标志是:
Run Code Online (Sandbox Code Playgroud)eflags 0x00000046: id vip vif ac vm rf nt IOPL=0 of df if tf sf ZF af PF cf
EFLAGS.VM标志被标记为off(0),因为它被列为vm和不被列出VM.这不是我的预期.
问题:
- 我的代码有什么问题,如何更正以便输入v8086模式和
hlt双重故障？ - 在64位长模式或32位兼容性(长模式)下是否可以进入v8086模式？
此代码的最小完整可验证示例是进入受保护模式的引导加载程序,并执行上述任务:
VIDEO_TEXT_ADDR EQU 0xb8000 ; Hard code beginning of text video memory
ATTR_BWHITE_ON_GREEN EQU 0x2f ; Bright white on green attribute
ATTR_BWHITE_ON_MAGENTA EQU 0x5f ; Bright White on magenta attribute
PM_MODE_STACK EQU 0x80000 ; Protected mode stack below EBDA
EFLAGS_VM_BIT EQU 17 ; EFLAGS VM bit
EFLAGS_IOPL_BITS EQU 12 ; EFLAGS IOPL bits (bit 12 and bit 13)
; 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))
bits 16
ORG 0x7c00
; Include a BPB (1.44MB floppy with FAT12) to be more compatible with USB floppy media
; %include "bpb.inc"
boot_start:
xor ax, ax ; DS=SS=ES=0
mov ds, ax
mov ss, ax ; Stack at 0x0000:0x7c00
mov sp, 0x7c00
cld ; Set string instructions to use forward movement
; Fast method of enabling A20 may not work on all x86 BIOSes
; It is good enough for emulators and most modern BIOSes
; See: https://wiki.osdev.org/A20_Line
cli ; Disable interrupts for rest of code as we don't
; want A20 code to be interrupted. In protected mode
; we have no IDT so any interrupt that does occur will
; double fault and reboot.
in al, 0x92
or al, 2
out 0x92, al ; Enable A20 using Fast Method
lgdt [gdtr] ; Load our GDT
mov eax, cr0
or eax, 1
mov cr0, eax ; Set protected mode flag
jmp CODE32_SEL:start32 ; FAR JMP to set CS
; v8086 code entry point
v86_mode_entry:
hlt ; Halt
; 32-bit protected mode entry point
bits 32
start32:
mov ax, DATA32_SEL ; Setup the segment registers with data selector
mov ds, ax
mov es, ax
mov ss, ax
mov esp, PM_MODE_STACK ; Set protected mode stack pointer
mov fs, ax ; Not currently using FS and GS
mov gs, ax
mov ah, ATTR_BWHITE_ON_GREEN; Attribute to print with
mov al, ah ; Attribute to clear last line when scrolling
mov esi, in_pm_msg ; Print message that we are in protected mode
call print_string_pm
pushf ; Get current EFLAGS
pop eax
or eax, 1<<EFLAGS_VM_BIT ; Set VM flag to enter v8086 mode
and eax, ~(3<<EFLAGS_IOPL_BITS)
; Set IOPL to 0
; IF flag already 0 because of earlier CLI
push eax
popf ; Reload new flags
jmp CODE32_SEL:v86_mode_entry
; Far JMP to v8086 entry point
; Function: print_string_pm
; Display a string to the console on display page 0 in protected mode.
; Very basic. Doesn't update hardware cursor, doesn't handle scrolling,
; LF, CR, TAB.
;
; Inputs: ESI = Offset of address to print
; AH = Attribute of string to print
; Clobbers: None
; Returns: None
print_string_pm:
push edi
push esi
push eax
mov edi, [vidmem_ptr] ; Start from video address stored at vidmem_ptr
jmp .getchar
.outchar:
stosw ; Output character to video display
.getchar:
lodsb ; Load next character from string
test al, al ; Is character NUL?
jne .outchar ; If not, go back and output character
mov [vidmem_ptr], edi ; Update global video pointer
pop eax
pop esi
pop edi
ret
align 4
vidmem_ptr: dd VIDEO_TEXT_ADDR ; Start console output in upper left of display
in_pm_msg:
db "In 32-bit protected mode!", 0
align 4
gdt_start:
dq MAKE_GDT_DESC(0, 0, 0, 0) ; null descriptor
gdt32_code:
dq MAKE_GDT_DESC(0, 0x000fffff, 10011010b, 1100b)
; 32-bit code, 4kb gran, limit 0xffffffff bytes, base=0
gdt32_data:
dq MAKE_GDT_DESC(0, 0x000fffff, 10010010b, 1100b)
; 32-bit data, 4kb gran, limit 0xffffffff bytes, base=0
end_of_gdt:
gdtr:
dw end_of_gdt - gdt_start - 1
; limit (Size of GDT - 1)
dd gdt_start ; base of GDT
CODE32_SEL equ gdt32_code - gdt_start
DATA32_SEL equ gdt32_data - gdt_start
; Pad boot sector to 510 bytes and add 2 byte boot signature
TIMES 510-($-$$) db 0
dw 0xaa55
可以使用以下命令生成引导加载程序:
nasm -f bin v86.asm -o v86.bin
它可以在QEMU中运行:
qemu-system-i386 -fda v86.bin
TL;DR :
Question #1:
POPF doesn't actually allow you to change the VM flag per the Instruction Set Architecture reference:
When operating in protected, compatibility, or 64-bit mode at privilege level 0 (or in real-address mode, the equivalent to privilege level 0), all non-reserved flags in the EFLAGS register except RF1, VIP, VIF, and VM may be modified. VIP, VIF and VM remain unaffected.
There are two general mechanisms that can be used to set EFLAGS.VM and enter v8086 mode:
A task switch to an 80386 task loads the image of EFLAGS from the new TSS. The TSS of the new task must be an 80386 TSS, not an 80286 TSS, because the 80286 TSS does not store the high-order word of EFLAGS, which contains the VM flag. A value of one in the VM bit of the new EFLAGS indicates that the new task is executing 8086 instructions; therefore, while loading the segment registers from the TSS, - the processor forms base addresses as the 8086 would.
An IRET from a procedure of an 80386 task loads the image of EFLAGS from the stack. A value of one in VM in this case indicates that the procedure to which control is being returned is an 8086 procedure. The CPL at the time the IRET is executed must be zero, else the processor does not change VM.
Question #2:
v8086模式仅适用于32位保护模式(传统模式)的x86-64处理器.您不能在64位长模式或32位(或16位)兼容模式下使用它.您必须将处理器切换到长模式并进入以CPL = 0运行的32位保护模式(传统模式)并执行上述两种方法之一.这是一项昂贵的(表现明智的)事业,并且充满了问题.完成后,您必须切换回长模式.
如果有一些用例,并且您使用的是具有多个内核的系统 - 您可以在32位保护模式下启动其中一个内核,而Bootstrap Processor(BSP)在长模式下运行.
方法1:使用IRET进入v8086模式
This is the easiest solution. If you do an IRET from 32-bit protected mode (in CPL=0) and the EFLAGS.VM register on the stack is set, the CPU will attempt to return to v8086 mode and assumes the stack frame contains the required information to make that transition:
Run Code Online (Sandbox Code Playgroud)PROTECTED-MODE: [snip] EIP ? Pop(); CS ? Pop(); (* 32-bit pop, high-order 16 bits discarded *) tempEFLAGS ? Pop(); [snip] RETURN-TO-VIRTUAL-8086-MODE: (* Interrupted procedure was in virtual-8086 mode: PE = 1, CPL=0, VM = 1 in flag image *) IF EIP not within CS limit THEN #GP(0); FI; EFLAGS ? tempEFLAGS; ESP ? Pop(); SS ? Pop(); (* Pop 2 words; throw away high-order word *) ES ? Pop(); (* Pop 2 words; throw away high-order word *) DS ? Pop(); (* Pop 2 words; throw away high-order word *) FS ? Pop(); (* Pop 2 words; throw away high-order word *) GS ? Pop(); (* Pop 2 words; throw away high-order word *) CPL ? 3; (* Resume execution in Virtual-8086 mode *) END;
If you push these items on the stack in reverse order and do the iret you should be able to enter v8086 mode.
V86_STACK_SEG EQU 0x0000 ; v8086 stack SS
V86_STACK_OFS EQU 0x0000 ; v8086 stack SP
V86_CS_SEG EQU 0x0000 ; v8086 code segment CS
EFLAGS_VM_BIT EQU 17 ; EFLAGS VM bit
EFLAGS_BIT1 EQU 1 ; EFLAGS bit 1 (reserved , always 1)
[snip]
xor ebx, ebx ; EBX=0
push ebx ; Real mode GS=0
push ebx ; Real mode FS=0
push ebx ; Real mode DS=0
push ebx ; Real mode ES=0
push V86_STACK_SEG
push V86_STACK_OFS ; v8086 stack SS:SP (grows down from SS:SP)
push dword 1<<EFLAGS_VM_BIT | 1<<EFLAGS_BIT1
; Set VM Bit, IF bit is off, DF=0(forward direction),
; IOPL=0, Reserved bit (bit 1) always 1. Everything
; else 0. These flags will be loaded in the v8086 mode
; during the IRET. We don't want interrupts enabled
; because we have no v86 monitor via protected mode
; GPF handler
push V86_CS_SEG ; Real Mode CS (segment)
push v86_mode_entry ; Entry point (offset)
iret ; Transfer control to v8086 mode and our real mode code
I have set ES=DS=CS=FS=GS=0 and a real mode stack at V86_STACK_SEG:V86_STACK_OFS (define these as you see fit). IP is set to the offset of the v86_mode_entry label. In the code snippet above I only set 2 bits to 1 (bit 1 and VM). Bit 1 is a reserved bit in EFLAGS that is always suppose to be set to 1. All other flags in EFLAGS are 0, thus IOPL=0.
All other registers will contain the same values they had before entering v8086 mode. You may wish to zero them out to avoid leaking information into the v8086 task from 32-bit protected mode (ie: a kernel).
A minimal complete verifiable example of using this code is:
VIDEO_TEXT_ADDR EQU 0xb8000 ; Hard code beginning of text video memory
ATTR_BWHITE_ON_GREEN EQU 0x2f ; Bright white on green attribute
ATTR_BWHITE_ON_MAGENTA EQU 0x5f ; Bright White on magenta attribute
PM_MODE_STACK EQU 0x80000 ; Protected mode stack below EBDA
V86_STACK_SEG EQU 0x0000 ; v8086 stack SS
V86_STACK_OFS EQU 0x0000 ; v8086 stack SP
V86_CS_SEG EQU 0x0000 ; v8086 code segment CS
EFLAGS_VM_BIT EQU 17 ; EFLAGS VM bit
EFLAGS_BIT1 EQU 1 ; EFLAGS bit 1 (reserved, always 1)
EFLAGS_IF_BIT EQU 9 ; EFLAGS IF bit
; 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))
bits 16
ORG 0x7c00
; Include a BPB (1.44MB floppy with FAT12) to be more compatible with USB floppy media
; %include "bpb.inc"
boot_start:
xor ax, ax ; DS=SS=ES=0
mov ds, ax
mov ss, ax ; Stack at 0x0000:0x7c00
mov sp, 0x7c00
cld ; Set string instructions to use forward movement
; Fast method of enabling A20 may not work on all x86 BIOSes
; It is good enough for emulators and most modern BIOSes
; See: https://wiki.osdev.org/A20_Line
cli ; Disable interrupts for rest of code as we don't
; want A20 code to be interrupted. In protected mode
; we have no IDT so any interrupt that does occur will
; double fault and reboot.
in al, 0x92
or al, 2
out 0x92, al ; Enable A20 using Fast Method
lgdt [gdtr] ; Load our GDT
mov eax, cr0
or eax, 1
mov cr0, eax ; Set protected mode flag
jmp CODE32_SEL:start32 ; FAR JMP to set CS
; v8086 code entry point
v86_mode_entry:
sub dword [vidmem_ptr], VIDEO_TEXT_ADDR
; Adjust video pointer to be relative to beginning of
; segment 0xb800
mov si, in_v86_msg ; Print in v86 message
mov ah, ATTR_BWHITE_ON_MAGENTA
; Attribute to print with
call print_string_rm_nobios
.endloop:
jmp $ ; Infinite loop since we did code a solution to exit VM
; Function: print_string_rm_nobios
; Display a string to the console on display page 0 in real/v8086 mode
; without using the BIOS. We don't have a proper v8086 monitor so can't
; use BIOS to display.
;
; Very basic. Doesn't update hardware cursor, doesn't handle scrolling,
; LF, CR, TAB.
;
; Inputs: SI = Offset of address to print
; AH = Attribute of string to print
; Clobbers: None
; Returns: None
print_string_rm_nobios:
push di
push si
push ax
push es
mov di, VIDEO_TEXT_ADDR>>4 ; ES=0xb800 (text video mode segment)
mov es, di
mov di, [vidmem_ptr] ; Start from video address stored at vidmem_ptr
jmp .getchar
.outchar:
stosw ; Output character to display
.getchar:
lodsb ; Load next character from string
test al, al ; Is character NUL?
jne .outchar ; If not, go output character
mov [vidmem_ptr], di ; Update global video pointer
pop es
pop ax
pop si
pop di
ret
; 32-bit protected mode entry point
bits 32
start32:
mov ax, DATA32_SEL ; Setup the segment registers with data selector
mov ds, ax
mov es, ax
mov ss, ax
mov esp, PM_MODE_STACK ; Set protected mode stack pointer
mov fs, ax ; Not currently using FS and GS
mov gs, ax
mov ah, ATTR_BWHITE_ON_GREEN; Attribute to print with
mov al, ah ; Attribute to clear last line when scrolling
mov esi, in_pm_msg ; Print message that we are in protected mode
call print_string_pm
xor ebx, ebx ; EBX=0
push ebx ; Real mode GS=0
push ebx ; Real mode FS=0
push ebx ; Real mode DS=0
push ebx ; Real mode ES=0
push V86_STACK_SEG
push V86_STACK_OFS ; v8086 stack SS:SP (grows down from SS:SP)
push dword 1<<EFLAGS_VM_BIT | 1<<EFLAGS_BIT1
; Set VM Bit, IF bit is off, DF=0(forward direction),
; IOPL=0, Reserved bit (bit 1) always 1. Everything
; else 0. These flags will be loaded in the v8086 mode
; during the IRET. We don't want interrupts enabled
; because we have no v86 monitor via protected mode
; GPF handler
push V86_CS_SEG ; Real Mode CS (segment)
push v86_mode_entry ; Entry point (offset)
iret ; Transfer control to v8086 mode and our real mode code
; Function: print_string_pm
; Display a string to the console on display page 0 in protected mode.
; Very basic. Doesn't update hardware cursor, doesn't handle scrolling,
; LF, CR, TAB.
;
; Inputs: ESI = Offset of address to print
; AH = Attribute of string to print
; Clobbers: None
; Returns: None
print_string_pm:
push edi
push esi
push eax
mov edi, [vidmem_ptr] ; Start from video address stored at vidmem_ptr
jmp .getchar
.outchar:
stosw ; Output character to video display
.getchar:
lodsb ; Load next character from string
test al, al ; Is character NUL?
jne .outchar ; If not, go back and output character
mov [vidmem_ptr], edi ; Update global video pointer
pop eax
pop esi
pop edi
ret
align 4
vidmem_ptr: dd VIDEO_TEXT_ADDR ; Start console output in upper left of display
in_pm_msg:
db "In 32-bit protected mode!", 0
in_v86_msg:
db "In v8086 mode!", 0
align 4
gdt_start:
dq MAKE_GDT_DESC(0, 0, 0, 0) ; null descriptor
gdt32_code:
dq MAKE_GDT_DESC(0, 0x000fffff, 10011010b, 1100b)
; 32-bit code, 4kb gran, limit 0xffffffff bytes, base=0
gdt32_data:
dq MAKE_GDT_DESC(0, 0x000fffff, 10010010b, 1100b)
; 32-bit data, 4kb gran, limit 0xffffffff bytes, base=0
end_of_gdt:
gdtr:
dw end_of_gdt - gdt_start - 1
; limit (Size of GDT - 1)
dd gdt_start ; base of GDT
CODE32_SEL equ gdt32_code - gdt_start
DATA32_SEL equ gdt32_data - gdt_start
; Pad boot sector to 510 bytes and add 2 byte boot signature
TIMES 510-($-$$) db 0
dw 0xaa55
This example code can be modified to do the hlt and it will double fault. It does properly enter v8086 mode. I print a string while it is in 32-bit protected mode and a string after it enters v8086 mode. Since IOPL=0 the real mode code doesn't use any privileged instructions nor does it use any instructions that are Interrupt Flag (IF) sensitive, nor does it do port IO. Without a VM Monitor (GPF handler that is v8086 mode aware) you are limited to non-privileged and non interrupt flag sensitive instructions. Since the INT instruction is IF sensitive, the BIOS can not be used. I write the characters directly to the display.
Method 2: Use a hardware task switch to enter v8086 mode
如果您未在操作系统中使用硬件任务切换,我建议不要使用此机制.如果您已选择使用硬件任务切换,那么使用此方法是有意义的.1
如果使用硬件任务切换进入v8086模式,则需要在GDT中使用TSS结构和TSS条目.GDT中的TSS条目是指定包含TSS的段的基数和限制.GDT条目通常定义为:
最初标记为可用的32位TSS描述符的类型为0x09; 的S位(系统段)设置为0; 一个P位的1; 一个G位设置为0(字节粒度); 对于v8086任务,我们希望描述符特权级别(DPL)为0.这导致访问字节为0x89,标志字节为0x00.
TSS结构本身可以遵循此相关Stackoverflow答案中建议的结构类型.对于下面的示例,我们将不使用IO端口位图,因此我将其设置TSS_IO_BITMAP_SIZE为0.
一旦创建了适当的结构,就可以使用v8086任务所需的寄存器状态填充TSS.这将包括CS:IP,其中执行将在v8086任务中开始.要进入v8086任务,只需通过TSS选择器进行FAR JMP:
jmp TSS32_SEL:0 ; Transfer control to v8086 mode and our real mode code
通过TSS选择器跳转时忽略偏移量.我使用值0作为偏移量,但可以将其设置为任何值.此FAR JMP将使用TSS选择器加载任务寄存器并将任务标记为忙 ; 根据TSS结构设置CPU状态; 将控制转移到任务.一个最小的完整示例如下:
VIDEO_TEXT_ADDR EQU 0xb8000 ; Hard code beginning of text video memory
ATTR_BWHITE_ON_GREEN EQU 0x2f ; Bright white on green attribute
ATTR_BWHITE_ON_MAGENTA EQU 0x5f ; Bright White on magenta attribute
PM_MODE_STACK EQU 0x80000 ; Protected mode stack below EBDA
V86_STACK_SEG EQU 0x0000 ; v8086 stack SS
V86_STACK_OFS EQU 0x0000 ; v8086 stack SP
V86_CS_SEG EQU 0x0000 ; v8086 code segment CS
EFLAGS_VM_BIT EQU 17 ; EFLAGS VM bit
EFLAGS_BIT1 EQU 1 ; EFLAGS bit 1 (reserved, always 1)
EFLAGS_IF_BIT EQU 9 ; EFLAGS IF bit
TSS_IO_BITMAP_SIZE EQU 0 ; Size 0 disables IO port bitmap (no permission)
; 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))
bits 16
ORG 0x7c00
; Include a BPB (1.44MB floppy with FAT12) to be more compatible with USB floppy media
; %include "bpb.inc"
boot_start:
xor ax, ax ; DS=SS=ES=0
mov ds, ax
mov ss, ax ; Stack at 0x0000:0x7c00
mov sp, 0x7c00
cld ; Set string instructions to use forward movement
; Fast method of enabling A20 may not work on all x86 BIOSes
; It is good enough for emulators and most modern BIOSes
; See: https://wiki.osdev.org/A20_Line
cli ; Disable interrupts for rest of code as we don't
; want A20 code to be interrupted. In protected mode
; we have no IDT so any interrupt that does occur will
; double fault and reboot.
in al, 0x92
or al, 2
out 0x92, al ; Enable A20 using Fast Method
lgdt [gdtr] ; Load our GDT
mov eax, cr0
or eax, 1
mov cr0, eax ; Set protected mode flag
jmp CODE32_SEL:start32 ; FAR JMP to set CS
; v8086 code entry point
v86_mode_entry:
sub dword [vidmem_ptr], VIDEO_TEXT_ADDR
; Adjust video pointer to be relative to beginning of
; segment 0xb800
mov si, in_v86_msg ; Print in v86 message
mov ah, ATTR_BWHITE_ON_MAGENTA
; Attribute to print with
call print_string_rm_nobios
.endloop:
jmp $ ; Infinite loop since we did code a solution to exit VM
; Function: print_string_rm_nobios
; Display a string to the console on display page 0 in real/v8086 mode
; without using the BIOS. We don't have a proper v8086 monitor so can't
; use BIOS to display.
;
; Very basic. Doesn't update hardware cursor, doesn't handle scrolling,
; LF, CR, TAB.
;
; Inputs: SI = Offset of address to print
; AH = Attribute of string to print
; Clobbers: None
; Returns: None
print_string_rm_nobios:
push di
push si
push ax
push es
mov di, VIDEO_TEXT_ADDR>>4 ; ES=0xb800 (text video mode segment)
mov es, di
mov di, [vidmem_ptr] ; Start from video address stored at vidmem_ptr
jmp .getchar
.outchar:
stosw ; Output character to display
.getchar:
lodsb ; Load next character from string
test al, al ; Is character NUL?
jne .outchar ; If not, go output character
mov [vidmem_ptr], di ; Update global video pointer
pop es
pop ax
pop si
pop di
ret
; 32-bit protected mode entry point
bits 32
start32:
mov ax, DATA32_SEL ; Setup the segment registers with data selector
mov ds, ax
mov es, ax
mov ss, ax
mov esp, PM_MODE_STACK ; Set protected mode stack pointer
mov fs, ax ; Not currently using FS and GS
mov gs, ax
mov ah, ATTR_BWHITE_ON_GREEN; Attribute to print with
mov al, ah ; Attribute to clear last line when scrolling
mov esi, in_pm_msg ; Print message that we are in protected mode
call print_string_pm
mov ecx, TSS_SIZE ; Zero out entire TSS structure
mov edi, tss_entry
xor eax, eax
rep stosb
; v8086 stack SS:SP (grows down from SS:SP)
mov dword [tss_entry.ss], V86_STACK_SEG
mov dword [tss_entry.esp], V86_STACK_OFS
mov dword [tss_entry.eflags], 1<<EFLAGS_VM_BIT | 1<<EFLAGS_BIT1
; Set VM Bit, IF bit is off, DF=0(forward direction),
; IOPL=0, Reserved bit (bit 1) always 1. Everything
; else 0. We don't want interrupts enabled upon entry to
; v8086 because we have no v8086 monitor (a protected mode
; GPF handler)
; Set Real Mode CS:EIP to start execution at
mov dword [tss_entry.cs], V86_CS_SEG
mov dword [tss_entry.eip], v86_mode_entry
; Set iomap_base in tss with the offset of the iomap relative to beginning of the tss
mov word [tss_entry.iomap_base], tss_entry.iomap-tss_entry
%if TSS_IO_BITMAP_SIZE > 0
; If using an IO Bitmap then a padding byte has to be set to 0xff at end of bitmap
mov byte [tss_entry.iomap_pad], 0xff
%endif
jmp TSS32_SEL:0 ; Transfer control to v8086 mode and our real mode code
; Function: print_string_pm
; Display a string to the console on display page 0 in protected mode.
; Very basic. Doesn't update hardware cursor, doesn't handle scrolling,
; LF, CR, TAB.
;
; Inputs: ESI = Offset of address to print
; AH = Attribute of string to print
; Clobbers: None
; Returns: None
print_string_pm:
push edi
push esi
push eax
mov edi, [vidmem_ptr] ; Start from video address stored at vidmem_ptr
jmp .getchar
.outchar:
stosw ; Output character to video display
.getchar:
lodsb ; Load next character from string
test al, al ; Is character NUL?
jne .outchar ; If not, go back and output character
mov [vidmem_ptr], edi ; Update global video pointer
pop eax
pop esi
pop edi
ret
align 4
vidmem_ptr: dd VIDEO_TEXT_ADDR ; Start console output in upper left of display
in_pm_msg:
db "In 32-bit protected mode!", 0
in_v86_msg:
db "In v8086 mode!", 0
align 4
gdt_start:
dq MAKE_GDT_DESC(0, 0, 0, 0) ; null descriptor
gdt32_code:
dq MAKE_GDT_DESC(0, 0x000fffff, 10011010b, 1100b)
; 32-bit code, 4kb gran, limit 0xffffffff bytes, base=0
gdt32_data:
dq MAKE_GDT_DESC(0, 0x000fffff, 10010010b, 1100b)
; 32-bit data, 4kb gran, limit 0xffffffff bytes, base=0
gdt32_tss:
dq MAKE_GDT_DESC(tss_entry, TSS_SIZE-1, 10001001b, 0000b)
; 32-bit TSS, 1b gran, available, IOPL=0
end_of_gdt:
CODE32_SEL equ gdt32_code - gdt_start
DATA32_SEL equ gdt32_data - gdt_start
TSS32_SEL equ gdt32_tss - gdt_start
gdtr:
dw end_of_gdt - gdt_start - 1
; limit (Size of GDT - 1)
dd gdt_start ; base of GDT
; Pad boot sector to 510 bytes and add 2 byte boot signature
TIMES 510-($-$$) db 0
dw 0xaa55
; Data section above bootloader @ 0x7c00. Acts like a BSS section
ABSOLUTE 0x7e00
; Store the TSS just beyond the boot signature read into memory
; at 0x0000:0x7e00
tss_entry:
.back_link: resd 1
.esp0: resd 1 ; Kernel stack pointer used on ring transitions
.ss0: resd 1 ; Kernel stack segment used on ring transitions
.esp1: resd 1
.ss1: resd 1
.esp2: resd 1
.ss2: resd 1
.cr3: resd 1
.eip: resd 1
.eflags: resd 1
.eax: resd 1
.ecx: resd 1
.edx: resd 1
.ebx: resd 1
.esp: resd 1
.ebp: resd 1
.esi: resd 1
.edi: resd 1
.es: resd 1
.cs: resd 1
.ss: resd 1
.ds: resd 1
.fs: resd 1
.gs: resd 1
.ldt: resd 1
.trap: resw 1
.iomap_base:resw 1 ; IOPB offset
;.cetssp: resd 1 ; Need this if CET is enabled
; Insert any kernel defined task instance data here
; ...
; If using VME (Virtual Mode extensions) there need to bean additional 32 bytes
; available immediately preceding iomap. If using VME uncomment next 2 lines
;.vmeintmap: ; If VME enabled uncomment this line and the next
; resb 32 ; 32*8 bits = 256 bits (one bit for each interrupt)
.iomap: resb TSS_IO_BITMAP_SIZE ; IO bitmap (IOPB) size 8192 (8*8192=65536) representing
; all ports. An IO bitmap size of 0 would fault all IO
; port access if IOPL < CPL (CPL=3 with v8086)
%if TSS_IO_BITMAP_SIZE > 0
.iomap_pad: resb 1 ; Padding byte that has to be filled with 0xff
; To deal with issues on some CPUs when using an IOPB
%endif
TSS_SIZE EQU $-tss_entry
笔记
- 1依靠硬件任务切换很难移植到其他CPU; x86 CPU未针对硬件任务切换进行优化; 不保留FPU和SIMD状态; 性能可能比通过软件编写任务切换慢.x86-64处理器上的长模式甚至不支持硬件任务切换.在x86上运行的现代操作系统通常不使用CPU的硬件任务切换.