In the previous section, we told GDT usage. I would like to talk about LDT usage, huh. The following is the source code:
; ------------------------------------------------- -------
EA20 macro; // Open the A20 address line
Push AX
IN Al, 92H
OR Al, 00000010B
OUT 92H, Al
POP AX
ENDM
; ------------------------------------------------- ---------------------------
Close the A20 address line
; ------------------------------------------------- ---------------------------
DA20 Macro
Push AX
IN Al, 92H
And Al, 11111101B
OUT 92H, Al
POP AX
ENDM
; ------------------------------------------------- ---------------------------
Jump macro selector, offsetv; // jump macro
DB 0eah
DW offsetv
DW selector
ENDM
; ---------------------
Descriptor struc; // Descriptor structure
Limitl DW 0
Basel DW 0
Basem DB 0
Attributes dw 0
Baseh DB 0
Descriptor ENDS
; ---------------------
PDESC STRUC; // Pseudo Description
Limit DW 0
Base DD 0
PDESC ENDS
;
ATDW = 92H
ATCE = 98h
Atcer = 9ah
ATLDT = 82H
TIL = 04h
.386P
; ------------------------------------------------- -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
GDTSEG segment us16
GDT Label Byte
Dummy descriptor <>
Code Descriptor <0FFFFH,, ATCE,>
Code_sel = Code-GDT
; Descriptor of code segment codekseg
Codek Descriptor <0FFFFH,, ATCE,>
Codek_sel = Codek-GDT
General descriptor
Normal Descriptor <0FFFFH,, ATDW,>
NORMAL_SEL = Normal-GDT
; Partial descriptor
LDTable Descriptor <0FFFFH,, ATLDT,>
LDT_SEL = LDTable-GDT
GDTLEN = $ - GDT
VGDTR PDESC
GDTSEG Ends
; ------------------------------------------------- ------------------ Local Description Table
LDTSEG segment use6
LDT Label Byte
Data segment LDTSEG descriptor
Data Descriptor <0FFFFH,, ATDW,>
Data_sel = (Data-LDT) TIL
; Descriptor of the code segment vcode
Vcode Descriptor <0FFFFH,,, ATCE,>
vcode_sel = (vcode-ldt) til
; To display the data
VData Descriptor <0FFFFH,, ATDW,>
VDATA_SEL = (VDATA-LDT) TIL
; Data to display 2
VData1 Descriptor <0FFFFH,, ATDW,>
VDATA1_SEL = (VDATA1-LDT) TIL
;
Vbuf Descriptor <0fffh, 8000H, 0BH, ATDW,>
VBUF_SEL = (VBUF-LDT) TIL
LDTSEG ENDS
; ------------------------------------------------- ----
Code segment
Vdseg segment use6
YANG DB 'How Are you', 0
VDSEG Ends
; ------------------------------------------------- ----
vdseg1 segment us16
Hello DB 'Hello', 0
Hellolen = $ - Hello
VDSEG1 ENDS
; ------------------------------------------------- ----
vcseg segment us16
Assume cs: vcseg
vStart: MOV AX, VDATA_SEL
MOV DS, AX
MOV AX, VBUF_SEL
Mov ES, AX
Lea Si, YANG
XOR BX, BX
MOV CX, 11
Again: MOV Al, [Si]
MOV AH, 87H
MOV ES: [BX], AX
Add bx, 2
Inc Si
Loop again
Hello
MOV AX, VDATA1_SEL
MOV DS, AX
Lea Si, Hello
Mov CX, Hellolen
Again1: MOV Al, [Si]
MOV AH, 0F4H
MOV ES: [BX], AX
Add bx, 2
Inc Si
LOOP AGAIN1
Gojump: Jump
Vcseg Ends
; ------------------------------------------------- ----
Codekseg segment us16
Assume CS: Codekseg
Startk: MOV AX, LDT_SEL
LLDT AX
Jump
CodekSeg Ends
; ------------------------------------------------- ---- CSEG segment use6
Assume CS: CSEG, DS: GDTSEG
Start: MOV AX, GDTSEG
MOV DS, AX
MOV BX, 16
Mul bx
Add Ax, Offset GDT
ADC DX, 0
Mov Word Ptr vgdtr.base, AX
Mov Word Ptr Vgdtr.Base 2, DX
;
MOV AX, CSEG
Mul bx
Mov Word Ptr Code.Basel, AX
MOV BYTE PTR Code.basem, DL
Mov Byte Ptr Code.baseh, DH
;
Mov AX, Codekseg
Mul bx
Mov Word PTR Codek.Basel, AX
Mov Byte Ptr Codek.basem, DL
Mov Byte Ptr Codek.Baseh, DH
;
MOV AX, LDTSEG
Mul bx
Mov Word Ptr Ldtable.Basel, AX
MOV BYTE PTR LDTABLE.BASEM, DL
Mov Byte Ptr Ldtable.Baseh, DH
;
Push DS
MOV AX, LDTSEG
MOV DS, AX
Assume DS: ldtseg
MOV AX, VCSEG
Mul bx
Mov Word Ptr vcode.basel, AX
MOV BYTE PTR Vcode.basem, DL
Mov Byte Ptr vcode.baseh, DH
;
MOV AX, VDSEG
Mul bx
Mov Word Ptr vdata.Basel, AX
MOV BYTE PTR VDATA.BASEM, DL
Mov Byte Ptr vdata.baseh, DH
;
MOV AX, VDSEG1
Mul bx
Mov Word Ptr vdata1.basel, AX
MOV BYTE PTR VDATA1.BASEM, DL
Mov Byte Ptr vdata1.baseh, DH
POP DS
;
Assume DS: GDTSEG
LGDT QWORD PTR VGDTR
CLI
EA20; //
MOV EAX, CR0; // Enter protection mode
OR EAX, 1; //
MOV CR0, EAX; //
JUMP
Toreal: Mov AX, Normal_SEL
MOV DS, AX
MOV Eax, Cr0
And Eax, 0FFFFFEHMOV CR0, EAX
Jump
REAL: DA20
STI
MOV AH, 4CH
Int 21h
CSEG Ends
End Start
This code, we can use the previous map. To use LDT, we need to use MOV AX, GDTSEG, LLDT AX these two commands are sent to the local description table to the local descriptor table register. Do two After the command, we can use the paragraphs pointed to the table in the local descriptor. Huh,.
