本文详细解析了一个简单的引导加载程序（Bootloader），帮助读者逐步理解操作系统的核心工作原理，从16位实模式环境设置、BIOS参数块解析到指令集和关键函数实现，提供了一个深入浅出的操作系统入门教程。
;*********************************************
; Boot1.asm
; - A Simple Bootloader
;
; Operating Systems Development Tutorial
;*********************************************
bits 16 ; we are in 16 bit real mode
org 0 ; we will set regisers later
start: jmp main ; jump to start of bootloader
;*********************************************
; BIOS Parameter Block
;*********************************************
; BPB Begins 3 bytes from start. We do a far jump, which is 3 bytes in size.
; If you use a short jump, add a "nop" after it to offset the 3rd byte.
bpbOEM db "My OS "
bpbBytesPerSector: DW 512
bpbSectorsPerCluster: DB 1
bpbReservedSectors: DW 1
bpbNumberOfFATs: DB 2
bpbRootEntries: DW 224
bpbTotalSectors: DW 2880
bpbMedia: DB 0xf0 ;; 0xF1
bpbSectorsPerFAT: DW 9
bpbSectorsPerTrack: DW 18
bpbHeadsPerCylinder: DW 2
bpbHiddenSectors: DD 0
bpbTotalSectorsBig: DD 0
bsDriveNumber: DB 0
bsUnused: DB 0
bsExtBootSignature: DB 0x29
bsSerialNumber: DD 0xa0a1a2a3
bsVolumeLabel: DB "MOS FLOPPY "
bsFileSystem: DB "FAT12 "
;************************************************;
; Prints a string
; DS=>SI: 0 terminated string
;************************************************;
Print:
lodsb ; load next byte from string from SI to AL
or al, al ; Does AL=0?
jz PrintDone ; Yep, null terminator found-bail out
mov ah, 0eh ; Nope-Print the character
int 10h
jmp Print ; Repeat until null terminator found
PrintDone:
ret ; we are done, so return
absoluteSector db 0x00
absoluteHead db 0x00
absoluteTrack db 0x00
;************************************************;
; Convert CHS to LBA
; LBA = (cluster - 2) * sectors per cluster
;************************************************;
ClusterLBA:
sub ax, 0x0002 ; zero base cluster number
xor cx, cx
mov cl, BYTE [bpbSectorsPerCluster] ; convert byte to word
mul cx
add ax, WORD [datasector] ; base data sector
ret
;************************************************;
; Convert LBA to CHS
; AX=>LBA Address to convert
;
; absolute sector = (logical sector / sectors per track) + 1
; absolute head = (logical sector / sectors per track) MOD number of heads
; absolute track = logical sector / (sectors per track * number of heads)
;
;************************************************;
LBACHS:
xor dx, dx ; prepare dx:ax for operation
div WORD [bpbSectorsPerTrack] ; calculate
inc dl ; adjust for sector 0
mov BYTE [absoluteSector], dl
xor dx, dx ; prepare dx:ax for operation
div WORD [bpbHeadsPerCylinder] ; calculate
mov BYTE [absoluteHead], dl
mov BYTE [absoluteTrack], al
ret
;************************************************;
; Reads a series of sectors
; CX=>Number of sectors to read
; AX=>Starting sector
; ES:BX=>Buffer to read to
;************************************************;
ReadSectors:
.MAIN
mov di, 0x0005 ; five retries for error
.SECTORLOOP
push ax
push bx
push cx
call LBACHS ; convert starting sector to CHS
mov ah, 0x02 ; BIOS read sector
mov al, 0x01 ; read one sector
mov ch, BYTE [absoluteTrack] ; track
mov cl, BYTE [absoluteSector] ; sector
mov dh, BYTE [absoluteHead] ; head
mov dl, BYTE [bsDriveNumber] ; drive
int 0x13 ; invoke BIOS
jnc .SUCCESS ; test for read error
xor ax, ax ; BIOS reset disk
int 0x13 ; invoke BIOS
dec di ; decrement error counter
pop cx
pop bx
pop ax
jnz .SECTORLOOP ; attempt to read again
int 0x18
.SUCCESS
mov si, msgProgress
call Print
pop cx
pop bx
pop ax
add bx, WORD [bpbBytesPerSector] ; queue next buffer
inc ax ; queue next sector
loop .MAIN ; read next sector
ret
;*********************************************
; Bootloader Entry Point
;*********************************************
main:
;----------------------------------------------------
; code located at 0000:7C00, adjust segment registers
;----------------------------------------------------
cli ; disable interrupts
mov ax, 0x07C0 ; setup registers to point to our segment
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
;----------------------------------------------------
; create stack
;----------------------------------------------------
mov ax, 0x0000 ; set the stack
mov ss, ax
mov sp, 0xFFFF
sti ; restore interrupts
;----------------------------------------------------
; Display loading message
;----------------------------------------------------
mov si, msgLoading
call Print
;----------------------------------------------------
; Load root directory table
;----------------------------------------------------
LOAD_ROOT:
; compute size of root directory and store in "cx"
xor cx, cx
xor dx, dx
mov ax, 0x0020 ; 32 byte directory entry
mul WORD [bpbRootEntries] ; total size of directory
div WORD [bpbBytesPerSector] ; sectors used by directory
xchg ax, cx
; compute location of root directory and store in "ax"
mov al, BYTE [bpbNumberOfFATs] ; number of FATs
mul WORD [bpbSectorsPerFAT] ; sectors used by FATs
add ax, WORD [bpbReservedSectors] ; adjust for bootsector
mov WORD [datasector], ax ; base of root directory
add WORD [datasector], cx
; read root directory into memory (7C00:0200)
mov bx, 0x0200 ; copy root dir above bootcode
call ReadSectors
;----------------------------------------------------
; Find stage 2
;----------------------------------------------------
; browse root directory for binary image
mov cx, WORD [bpbRootEntries] ; load loop counter
mov di, 0x0200 ; locate first root entry
.LOOP:
push cx
mov cx, 0x000B ; eleven character name
mov si, ImageName ; image name to find
push di
rep cmpsb ; test for entry match
pop di
je LOAD_FAT
pop cx
add di, 0x0020 ; queue next directory entry
loop .LOOP
jmp FAILURE
;----------------------------------------------------
; Load FAT
;----------------------------------------------------
LOAD_FAT:
; save starting cluster of boot image
mov dx, WORD [di + 0x001A]
mov WORD [cluster], dx ; file's first cluster
; compute size of FAT and store in "cx"
xor ax, ax
mov al, BYTE [bpbNumberOfFATs] ; number of FATs
mul WORD [bpbSectorsPerFAT] ; sectors used by FATs
mov cx, ax
; compute location of FAT and store in "ax"
mov ax, WORD [bpbReservedSectors] ; adjust for bootsector
; read FAT into memory (7C00:0200)
mov bx, 0x0200 ; copy FAT above bootcode
call ReadSectors
; read image file into memory (0050:0000)
mov ax, 0x0050
mov es, ax ; destination for image
mov bx, 0x0000 ; destination for image
push bx
;----------------------------------------------------
; Load Stage 2
;----------------------------------------------------
LOAD_IMAGE:
mov ax, WORD [cluster] ; cluster to read
pop bx ; buffer to read into
call ClusterLBA ; convert cluster to LBA
xor cx, cx
mov cl, BYTE [bpbSectorsPerCluster] ; sectors to read
call ReadSectors
push bx
; compute next cluster
mov ax, WORD [cluster] ; identify current cluster
mov cx, ax ; copy current cluster
mov dx, ax ; copy current cluster
shr dx, 0x0001 ; divide by two
add cx, dx ; sum for (3/2)
mov bx, 0x0200 ; location of FAT in memory
add bx, cx ; index into FAT
mov dx, WORD [bx] ; read two bytes from FAT
test ax, 0x0001
jnz .ODD_CLUSTER
.EVEN_CLUSTER:
and dx, 0000111111111111b ; take low twelve bits
jmp .DONE
.ODD_CLUSTER:
shr dx, 0x0004 ; take high twelve bits
.DONE:
mov WORD [cluster], dx ; store new cluster
cmp dx, 0x0FF0 ; test for end of file
jb LOAD_IMAGE
DONE:
mov si, msgCRLF
call Print
push WORD 0x0050
push WORD 0x0000
retf
FAILURE:
mov si, msgFailure
call Print
mov ah, 0x00
int 0x16 ; await keypress
int 0x19 ; warm boot computer
datasector dw 0x0000
cluster dw 0x0000
ImageName db "KRNLDR SYS"
msgLoading db 0x0D, 0x0A, "Loading Boot Image ", 0x00
msgCRLF db 0x0D, 0x0A, 0x00
msgProgress db ".", 0x00
msgFailure db 0x0D, 0x0A, "MISSING OR CURRUPT KRNLDR. Press Any Key to Reboot", 0x0D, 0x0A, 0x00
TIMES 510-($-$$) DB 0
DW 0xAA55
为了一步一步理解操作系统的原理奋斗吧！
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