$TITLE ('Initial Task')        #此处title为开始标识，$符号表示取地址

NAME    INIT                #为一段程序命名

init_stack  SEGMENT RW            #定义栈段 可读写 标识符为init_stack

DW    DUP(?)                #保留20个双字位置

tos         LABEL   WORD        #定义一个标号tos，类型为字

init_stack  ENDS            #栈段init_stack定义结束

init_data   SEGMENT RW PUBLIC        #定义数据段 公共 可读写 标识符为init_data

DW    DUP(?)                #段长度20个双字，即重复保留20个双字

init_data   ENDS            #数据段定义结束

init_code   SEGMENT ER PUBLIC        #代码段开始 可执行，只读，公共

ASSUME      DS:init_data        #设定此时的数据段为上面定义的数据段

nop

nop

nop

init_start:                #入口点标识符

; set up stack

mov ax, init_stack            #设置栈，将标识符init_stack放入ax

mov ss, ax                #设置栈段寄存器

mov esp, offset tos            #设置栈顶指针为tos的偏移，栈是高向低

mov a1,                #al中放1

blink:                    #标号blink

xor a1,                #al清0

out 0e4h,a1                #向0e4H端口输入al中值，此时al =

mov cx,3FFFh                #cx = 3FFFh

here:                    #标号here

dec cx                    #cx = cx -

jnz here                #ZF位不等0， 则跳转到here标号处

jmp SHORT blink                #跳到blink标号处

Hlt                    #停机

init_code   ends            #代码段结束

END init_start, SS:init_stack, DS:init_data    #入口点结束

$TITLE('Protected Mode Transition -- 386 initialization')

NAME  RESET

;*****************************************************************

; Upon reset the 386 starts executing at address 0FFFFFFF0H.  The

; upper 12 address bits remain high until a FAR call or jump is

; executed.

;

; Assume the following:

;

;

; -  a short jump at address 0FFFFFFF0H (placed there by the

;    system builder) causes execution to begin at START in segment

;    RESET_CODE.

;

;

; -  segment RESET_CODE is based at physical address 0FFFF0000H,

;    i.e.   at the start of the last  64K in the 4G address space.

;    Note that  this is the base of the CS register at reset.  If

;    you locate ROM code above  this  address,  you  will  need  to

;    figure out an adjustment factor to address things within this

;    segment.

;

;*****************************************************************

$EJECT ;

; Define addresses to locate GDT and IDT in RAM.

; These addresses are also used in the BLD386 file that defines

; the GDT and IDT. If you change these addresses, make sure you

; change the base addresses specified in the build file.

GDTbase         EQU    00001000H   ; physical address for GDT base

IDTbase         EQU    00000400H   ; physical address for IDT base

PUBLIC     GDT_EPROM

PUBLIC     IDT_EPROM

PUBLIC     START

DUMMY      segment rw      ; ONLY for ASM386 main module stack init

DW

DUMMY   ends

;*****************************************************************

;

; Note: RESET CODE must be USEl6 because the 386 initally executes

;       in real mode.

;

RESET_CODE segment er PUBLIC    USE16

ASSUME DS:nothing, ES:nothing

;

; 386 Descriptor template

DESC       STRUC

lim_0_15    DW                ; limit bits (0..15)

bas_0_15    DW                ; base bits (0..15)

bas_16_23   DB                ; base bits (16..23)

access      DB                ; access byte

gran        DB                ; granularity byte

bas_24_31   DB                ; base bits (24..31)

DESC       ENDS

; The following is the layout of the real GDT created by BLD386.

; It is located in EPROM and will be copied to RAM.

;

; GDT[O]      ...  NULL

; GDT[1]      ...  Alias for RAM GDT

; GDT[2]      ...  Alias for RAM IDT

; GDT[2]      ...  initial task TSS

; GDT[3]      ...  initial task TSS alias

; GDT[4]      ...  initial task LDT

; GDT[5]      ...  initial task LDT alias

;

; define entries in GDT and IDT.

GDT_ENTRIES    EQU

IDT_ENTRIES    EQU

; define some constants to index into the real GDT

GDT_ALIAS      EQU    *SIZE DESC

IDT_ALIAS      EQU    *SIZE DESC

INIT_TSS       EQU    *SIZE DESC

INIT_TSS_A     EQU    *SIZE DESC

INIT_LDT       EQU    *SIZE DESC

INIT_LDT_A     EQU    *SIZE DESC

;

; location of alias in INIT_LDT

INIT_LDT_ALIAS    EQU    *SIZE DESC

;

; access rights byte for DATA and TSS descriptors

DS_ACCESS   EQU   010010010B

TSS_ACCESS  EQU   010001001B

;

; This temporary GDT will be used to set up the real GDT in RAM.

Temp_GDT    LABEL   BYTE        ; tag for begin of scratch GDT

NULL_DES    DESC <>             ; NULL descriptor

; 32-Gigabyte data segment based at 0

FLAT_DES    DESC <0FFFFH,,,92h,0CFh,>

GDT_eprom     DP    ?           ; Builder places GDT address and limit

; in this 6 byte area.

IDT_eprom     DP    ?           ; Builder places IDT address and limit

; in this 6 byte area.

;

; Prepare operand for loadings GDTR and LDTR.

TGDT_pword     LABEL  PWORD                 ; for temp GDT

DW     end_Temp_GDT_Temp_GDT -

DD

GDT_pword      LABEL  PWORD                 ; for GDT in RAM

DW     GDT_ENTRIES * SIZE DESC -

DD     GDTbase

IDT_pword      LABEL   PWORD                ; for IDT in RAM

DW     IDT_ENTRIES * SIZE DESC -

DD     IDTbase

end_Temp_GDT   LABEL   BYTE

;

; Define equates for addressing convenience.

GDT_DES_FLAT        EQU DS:GDT_ALIAS +GDTbase

IDT_DES_FLAT        EQU DS:IDT_ALIAS +GDTbase

INIT_TSS_A_OFFSET   EQU DS:INIT_TSS_A

INIT_TSS_OFFSET     EQU DS:INIT_TSS

INIT_LDT_A_OFFSET   EQU DS:INIT_LDT_A

INIT_LDT_OFFSET     EQU DS:INIT_LDT

; define pointer for first task switch

ENTRY POINTER LABEL DWORD

DW , INIT_TSS

;******************************************************************

;

;   Jump from reset vector to here.

START:

CLI                ;disable interrupts

CLD                ;clear direction flag

LIDT    NULL_des   ;force shutdown on errors

;

;   move scratch GDT to RAM at physical 0

XOR DI,DI

MOV ES,DI           ;point ES:DI to physical location 0

MOV SI,OFFSET Temp_GDT

MOV CX,end_Temp_GDT-Temp_GDT        ;set byte count

INC CX

;

;   move table

REP MOVS BYTE PTR ES:[DI],BYTE PTR CS:[SI]

LGDT    tGDT_pword                ;load GDTR for Temp. GDT

;(located at 0)

;   switch to protected mode

MOV EAX,CR0                       ;get current CRO

MOV EAX,                         ;set PE bit

MOV CRO,EAX                       ;begin protected mode

;

;   clear prefetch queue

JMP SHORT flush

flush:

; set DS,ES,SS to address flat linear space (0 ... 4GB)

MOV BX,FLAT_DES-Temp_GDT

MOV US,BX

MOV ES,BX

MOV SS,BX

;

; initialize stack pointer to some (arbitrary) RAM location

MOV ESP, OFFSET end_Temp_GDT

;

; copy eprom GDT to RAM

MOV ESI,DWORD PTR GDT_eprom + ; get base of eprom GDT

; (put here by builder).

MOV EDI,GDTbase                ; point ES:EDI to GDT base in RAM.

MOV CX,WORD PTR gdt_eprom +   ; limit of eprom GDT

INC CX

SHR CX,                       ; easier to move words

CLD

REP MOVS   WORD PTR ES:[EDI],WORD PTR DS:[ESI]

;

; copy eprom IDT to RAM

;

MOV ESI,DWORD PTR IDT_eprom + ; get base of eprom IDT

; (put here by builder)

MOV EDI,IDTbase                ; point ES:EDI to IDT base in RAM.

MOV CX,WORD PTR idt_eprom +   ; limit of eprom IDT

INC CX

SHR CX,

CLD

REP MOVS   WORD PTR ES:[EDI],WORD PTR DS:[ESI]

; switch to RAM GDT and IDT

;

LIDT IDT_pword

LGDT GDT_pword

;

MOV BX,GDT_ALIAS               ; point DS to GDT alias

MOV DS,BX

;

; copy eprom TSS to RAM

;

MOV BX,INIT_TSS_A              ; INIT TSS A descriptor base

; has RAM location of INIT TSS.

MOV ES,BX                      ; ES points to TSS in RAM

MOV BX,INIT_TSS                ; get inital task selector

LAR DX,BX                      ; save access byte

MOV [BX].access,DS_ACCESS      ; set access as data segment

MOV FS,BX                      ; FS points to eprom TSS

XOR si,si                      ; FS:si points to eprom TSS

XOR di,di                      ; ES:di points to RAM TSS

MOV CX,[BX].lim_0_15           ; get count to move

INC CX

;

; move INIT_TSS to RAM.

REP MOVS BYTE PTR ES:[di],BYTE PTR FS:[si]

MOV [BX].access,DH             ; restore access byte

;

; change base of INIT TSS descriptor to point to RAM.

MOV AX,INIT_TSS_A_OFFSET.bas_0_15

MOV INIT_TSS_OFFSET.bas_0_15,AX

MOV AL,INIT_TSS_A_OFFSET.bas_16_23

MOV INIT_TSS_OFFSET.bas_16_23,AL

MOV AL,INIT_TSS_A_OFFSET.bas_24_31

MOV INIT_TSS_OFFSET.bas_24_31,AL

;

; change INIT TSS A to form a save area for TSS on first task

; switch. Use RAM at location 0.

MOV BX,INIT_TSS_A

MOV WORD PTR [BX].bas_0_15,

MOV [BX].bas_16_23,

MOV [BX].bas_24_31,

MOV [BX].access,TSS_ACCESS

MOV [BX].gran,O

LTR BX                         ; defines save area for TSS

;

; copy eprom LDT to RAM

MOV BX,INIT_LDT_A              ; INIT_LDT_A descriptor has

; base address in RAM for INIT_LDT.

MOV ES,BX                      ; ES points LDT location in RAM.

MOV AH,[BX].bas_24_31

MOV AL,[BX].bas_16_23

SHL EAX,

MOV AX,[BX].bas_0_15           ; save INIT_LDT base (ram) in EAX

MOV BX,INIT_LDT                ; get inital LDT selector

LAR DX,BX                      ; save access rights

MOV [BX].access,DS_ACCESS      ; set access as data segment

MOV FS,BX                      ; FS points to eprom LDT

XOR si,si                      ; FS:SI points to eprom LDT

XOR di,di                      ; ES:DI points to RAM LDT

MOV CX,[BX].lim_0_15           ; get count to move

INC CX

;

; move initial LDT to RAM

REP MOVS BYTE PTR ES:[di],BYTE PTR FS:[si]

MOV [BX].access,DH             ; restore access rights in

; INIT_LDT descriptor

;

; change base of alias (of INIT_LDT) to point to location in RAM.

MOV ES:[INIT_LDT_ALIAS].bas_0_15,AX

SHR EAX,

MOV ES:[INIT_LDT_ALIAS].bas_16_23,AL

MOV ES:[INIT_LDT_ALIAS].bas_24_31,AH

;

; now set the base value in INIT_LDT descriptor

MOV AX,INIT_LDT_A_OFFSET.bas_0_15

MOV INIT_LDT_OFFSET.bas_0_15,AX

MOV AL,INIT_LDT_A_OFFSET.bas_16_23

MOV INIT_LDT_OFFSET.bas_16_23,AL

MOV AL,INIT_LDT_A_OFFSET.bas_24_31

MOV INIT_LDT_OFFSET.bas_24_31,AL

;

; Now GDT, IDT, initial TSS and initial LDT are all set up.

;

; Start the first task!

'

JMP ENTRY_POINTER

RESET_CODE ends

END START, SS:DUMMY,DS:DUMMY