Notes on Logical Address: segment selector/descriptor/table

Segment Selector: 16-bit field  ==> segment identifier    ━┓

                                                                                            ┣━  Logical Address                  

                   offset: 32-bit field           ━┛

 

 

 

Segment Selectors    ┏━━ index: 13-bit, segment descriptor entry index in GDT/LDT

(stored in                  ┃       TI (Table Indicator) :  TI=0, Segment Descriptor is in GDT;

Segmentation     ━━┫                                          TI=1, Segment Descriptor is in LDT

registers)  ┃      RPL(Requestor Privilege Level): 2-bit, specifies the CPL when this 

                                 ┗━━                                                  segment selector is loaded into cs register

                     

 RPL field may also be used to selectively weaken the processor privilege level when accessing data segments.

 

 

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                                            ┏━━  cs: Code Segment. containing program instructions

Segmentation Registers      ┃        ss: stack segment, containing current program stack

（6）     ==>                  ━━┫       ds: data segment, containing static and external data

only to hold                         ┃       

Segment Selectors              ┗━━ es, fs, gs: general purpose, arbitrary segments

                                             

 

cs register: includes a 2-bit field that specifies CPL (current privilege level).

                                        

 

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Segment Descriptor(8-byte): describes segment characteristics, storing in LDT or GDT.

                          

                                     ┏━━  Base: 32-bit  segment linear address

Segment Descriptor   ━┫        Limit: 20-bit  segment length in byte

                                     ┗━━  Other: G, S , Type, DPL…

 

DPL(Descriptor privilege level): 2-bit field. 

minimal CPU privilege level requested for accessing the segment.

    DPL=0:  segment is accessible only when CPL=0, Kernel Mode

    DPL=3:  segment is accessible with every CPL value

 

 

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1 GDT              ==> gdtr processor register

1 LDT for each process   ==> ldtr processor register

 

 

since a Segment Descriptor is 8 bytes long, it’s address is obtained by

GDT/LDT base address + index*8

e.g.

GDT is at 0x00020000 (stored in gdtr register),

index specified by Segment Selector is 2,

Segment Descriptor is at: 0x00020000 + (2*8), or 0x00020010.

 

 

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 ; ========================================== ; pmtest2.asm ; 编译方法：nasm pmtest2.asm -o pmtest2.com ; ========================================== %include "pm.inc" ; 常量, 宏, 以及一些说明 org 0100h jmp LABEL_BEGIN [SECTION .gdt] ; GDT ; 段基址, 段界限 , 属性 LABEL_GDT: Descriptor 0, 0, 0 ; 空描述符 LABEL_DESC_NORMAL: Descriptor 0, 0ffffh, DA_DRW ; Normal 描述符 LABEL_DESC_CODE32: Descriptor 0, SegCode32Len - 1, DA_C + DA_32 ; 非一致代码段, 32 LABEL_DESC_CODE16: Descriptor 0, 0ffffh, DA_C ; 非一致代码段, 16 LABEL_DESC_DATA: Descriptor 0, DataLen - 1, DA_DRW ; Data LABEL_DESC_STACK: Descriptor 0, TopOfStack, DA_DRWA + DA_32 ; Stack, 32 位 LABEL_DESC_TEST: Descriptor 0500000h, 0ffffh, DA_DRW LABEL_DESC_VIDEO: Descriptor 0B8000h, 0ffffh, DA_DRW ; 显存首地址 ; GDT 结束 GdtLen equ $ - LABEL_GDT ; GDT长度 GdtPtr dw GdtLen - 1 ; GDT界限 dd 0 ; GDT基地址 ; GDT 选择子 SelectorNormal equ LABEL_DESC_NORMAL - LABEL_GDT SelectorCode32 equ LABEL_DESC_CODE32 - LABEL_GDT SelectorCode16 equ LABEL_DESC_CODE16 - LABEL_GDT SelectorData equ LABEL_DESC_DATA - LABEL_GDT SelectorStack equ LABEL_DESC_STACK - LABEL_GDT SelectorTest equ LABEL_DESC_TEST - LABEL_GDT SelectorVideo equ LABEL_DESC_VIDEO - LABEL_GDT ; END of [SECTION .gdt] [SECTION .data1] ; 数据段 ALIGN 32 [BITS 32] LABEL_DATA: SPValueInRealMode dw 0 ; 字符串 PMMessage: db "In Protect Mode now. ^-^", 0 ; 进入保护模式后显示此字符串 OffsetPMMessage equ PMMessage - $ StrTest: db "ABCDEFGHIJKLMNOPQRSTUVWXYZ", 0 OffsetStrTest equ StrTest - $ DataLen equ $ - LABEL_DATA ; END of [SECTION .data1] ; 全局堆栈段 [SECTION .gs] ALIGN 32 [BITS 32] LABEL_STACK: times 512 db 0 TopOfStack equ $ - LABEL_STACK - 1 ; END of [SECTION .gs] [SECTION .s16] [BITS 16] LABEL_BEGIN: mov ax, cs mov ds, ax mov es, ax mov ss, ax mov sp, 0100h mov [LABEL_GO_BACK_TO_REAL+3], ax mov [SPValueInRealMode], sp ; 初始化 16 位代码段描述符 mov ax, cs movzx eax, ax shl eax, 4 add eax, LABEL_SEG_CODE16 mov word [LABEL_DESC_CODE16 + 2], ax shr eax, 16 mov byte [LABEL_DESC_CODE16 + 4], al mov byte [LABEL_DESC_CODE16 + 7], ah ; 初始化 32 位代码段描述符 xor eax, eax mov ax, cs shl eax, 4 add eax, LABEL_SEG_CODE32 mov word [LABEL_DESC_CODE32 + 2], ax shr eax, 16 mov byte [LABEL_DESC_CODE32 + 4], al mov byte [LABEL_DESC_CODE32 + 7], ah ; 初始化数据段描述符 xor eax, eax mov ax, ds shl eax, 4 add eax, LABEL_DATA mov word [LABEL_DESC_DATA + 2], ax shr eax, 16 mov byte [LABEL_DESC_DATA + 4], al mov byte [LABEL_DESC_DATA + 7], ah ; 初始化堆栈段描述符 xor eax, eax mov ax, ds shl eax, 4 add eax, LABEL_STACK mov word [LABEL_DESC_STACK + 2], ax shr eax, 16 mov byte [LABEL_DESC_STACK + 4], al mov byte [LABEL_DESC_STACK + 7], ah ; 为加载 GDTR 作准备 xor eax, eax mov ax, ds shl eax, 4 add eax, LABEL_GDT ; eax <- gdt 基地址 mov dword [GdtPtr + 2], eax ; [GdtPtr + 2] <- gdt 基地址 ; 加载 GDTR lgdt [GdtPtr] ; 关中断 cli ; 打开地址线A20 in al, 92h or al, 00000010b out 92h, al ; 准备切换到保护模式 mov eax, cr0 or eax, 1 mov cr0, eax ; 真正进入保护模式 jmp dword SelectorCode32:0 ; 执行这一句会把 SelectorCode32 装入 cs, 并跳转到 Code32Selector:0 处 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; LABEL_REAL_ENTRY: ; 从保护模式跳回到实模式就到了这里 mov ax, cs mov ds, ax mov es, ax mov ss, ax mov sp, [SPValueInRealMode] in al, 92h ; ┓ and al, 11111101b ; ┣ 关闭 A20 地址线 out 92h, al ; ┛ sti ; 开中断 mov ax, 4c00h ; ┓ int 21h ; ┛回到 DOS ; END of [SECTION .s16] [SECTION .s32]; 32 位代码段. 由实模式跳入. [BITS 32] LABEL_SEG_CODE32: mov ax, SelectorData mov ds, ax ; 数据段选择子 mov ax, SelectorTest mov es, ax ; 测试段选择子 mov ax, SelectorVideo mov gs, ax ; 视频段选择子 mov ax, SelectorStack mov ss, ax ; 堆栈段选择子 mov esp, TopOfStack ; 下面显示一个字符串 mov ah, 0Ch ; 0000: 黑底 1100: 红字 xor esi, esi xor edi, edi mov esi, OffsetPMMessage ; 源数据偏移 mov edi, (80 * 10 + 0) * 2 ; 目的数据偏移。屏幕第 10 行, 第 0 列。 cld .1: lodsb test al, al jz .2 mov [gs:edi], ax add edi, 2 jmp .1 .2: ; 显示完毕 call DispReturn call TestRead call TestWrite call TestRead ; 到此停止 jmp SelectorCode16:0 ; ------------------------------------------------------------------------ TestRead: xor esi, esi mov ecx, 8 .loop mov al, [es:esi] call DispAL inc esi loop .loop call DispReturn ret ; TestRead 结束----------------------------------------------------------- ; ------------------------------------------------------------------------ TestWrite: push esi push edi xor esi, esi xor edi, edi mov esi, OffsetStrTest ; 源数据偏移 cld .1: lodsb test al, al jz .2 mov [es:edi], al inc edi jmp .1 .2: pop edi pop esi ret ; TestWrite 结束---------------------------------------------------------- ; ------------------------------------------------------------------------ ; 显示 AL 中的数字 ; 默认地: ; 数字已经存在 AL 中 ; edi 始终指向要显示的下一个字符的位置 ; 被改变的寄存器: ; ax, edi ; ------------------------------------------------------------------------ DispAL: push ecx push edx mov ah, 0Ch ; 0000: 黑底 1100: 红字 mov dl, al shr al, 4 mov ecx, 2 .begin: and al, 01111b cmp al, 9 ja .1 add al, '0' jmp .2 .1: sub al, 0Ah add al, 'A' .2: mov [gs:edi], ax add edi, 2 mov al, dl loop .begin add edi, 2 pop edx pop ecx ret ; DispAL 结束------------------------------------------------------------- ; ------------------------------------------------------------------------ DispReturn: push eax push ebx mov eax, edi mov bl, 160 div bl and eax, 0FFh inc eax mov bl, 160 mul bl mov edi, eax pop ebx pop eax ret ; DispReturn 结束--------------------------------------------------------- SegCode32Len equ $ - LABEL_SEG_CODE32 ; END of [SECTION .s32] ; 16 位代码段. 由 32 位代码段跳入, 跳出后到实模式 [SECTION .s16code] ALIGN 32 [BITS 16] LABEL_SEG_CODE16: ; 跳回实模式: mov ax, SelectorNormal mov ds, ax mov es, ax mov fs, ax mov gs, ax mov ss, ax mov eax, cr0 and al, 11111110b mov cr0, eax LABEL_GO_BACK_TO_REAL: jmp 0:LABEL_REAL_ENTRY ; 段地址会在程序开始处被设置成正确的值 Code16Len equ $ - LABEL_SEG_CODE16 ; END of [SECTION .s16code]