硬盘引导记录MBR（Master Boot Record）是指硬盘之0面0道1扇区之内容，PC及其兼容机之ROM BIOS约定在上电及POST自检成功后，将其从硬盘读出，放置在内存0:7C00处，然后转去该地址执行。该段代码负责从代码尾部之4个分区表项中找出可以引导的项，读出其引导记录引导之。
MBR在相当长时间内都保持着1982年IBM设计IBM PC机时的代码原样，直到硬盘容量突破传统BIOS所能支持的最大容量8.4G之时，它才不得不加入新的INT13功能扩展代码，不过主要的功能还是没有改变。
; 硬盘主引导记录代码分析:
; 本段代码取自由WIN98SE之"FDISK /MBR"命令处理过的硬盘
;
; PC机之ROM在上电及POST自检成功后，将本段代码从硬盘之0面0道1扇区位置读出，
; 放置在0:7C00处，寄存器设置如下：
; CS=DS=ES=SS=0. IP=7C00h, SP=0400H
;
; 本段代码负责从代码尾部之4个分区表项中找出可以引导的项，读出其引导记录引导之。
;
; 流程如下：
;
; 1). 将代码从0:7C00移至0:600
; 2). 检查4个分区表项有效性:
; a).有否可引导分区？
; 无则转ROM BASIC（INT 18）
; b).多个引导分区？
; 是则显示'Invalid partition'后挂机
; c).可引导标志为0与80h之外的无效值?
; 是则显示'Invalid partition'后挂机
; 3). 找寻唯一有效引导分区项目,将之对应的引导记录读入0:7C00,
; a).读入方式有二种，
; 一般采用经典的INT13 AH=2号调用，
; 如果是0Eh系统ID，则使用另一种新型BIOS之INT13 AH=42号扩展功能
; b).如果读入操作错误(包括读入内容无效)就重复读10次， 如果系统ID为0B,0C,因为它们在原引导记录之后6个扇区位置 还有一个引导记录的备份，就从第6次开始读该备份
; c).仍然错误则转显示'Missing operating system'或 'Error loading operating system'后挂机
; 4). 转向有效的引导记录0:7C00
;
; 它载入引导记录至0:7C00，转向它时，寄存器设置如下： CS=DS=ES=SS=0. IP=7C00h, DI=SP=7C00H, SI=BP-->指向引导中的分区表项
; .386p
_data segment public
assume cs:_data, ds:_data
org 600h
mbr proc far
; The ROM in the IBM PC starts the boot process by performing a hardware initialization and a verification of all external devices. If all goes well, it will then load from the boot drive the sector from track 0, head 0, sector 1. This sector is placed at physical address 07C00h. The initial registers are set up as follows: CS=DS=ES=SS=0. IP=7C00h, SP=0400H, CLI.
汇编代码：
0000:0600 start: ; relocate to 0:0600
0000:0600 33 C0 xor ax,ax
0000:0602 8E D0 mov ss,ax
0000:0604 BC 7C00 mov sp,7C00h ; new stack at 0:7c00
0000:0607 FB sti ; interrupts ok now
0000:0608 50 push ax
0000:0609 07 pop es
0000:060A 50 push ax
0000:060B 1F pop ds ; ES:DS=0
0000:060C FC cld ; movsb direction: forward
0000:060D .BE 7C1B mov si,offset loc_restart - 600h + 7C00h
0000:0610 .BF 061B mov di,offset loc_restart
0000:0613 50 push ax
0000:0614 57 push di
0000:0615 B9 01E5 mov cx,offset code_end - offset loc_restart
0000:0618 F3/ A4 rep movsb ; move CX byte data from DS:SI to ES:DI
0000:061A CB retf ; return address = 0:061b = offset
loc_loc_restart
; look throught partition table
; for valid & activate entry
0000:061B loc_restart:
0000:061B .BE 07BE mov si,offset partition_tab
0000:061E B1 04 mov cl,4 ; number of table entrie
0000:0620 loc_nextpe:
0000:0620 38 2C cmp [si],ch ; is boot indicator <= 0(ch=0)?
0000:0622 7C 09 jl short loc_boot ; < 0, that is 80h, bootable entry found
0000:0624 75 15 jnz short loc_bad ; !=0 & !<0, that is invalid (0 & 80h only)
0000:0626 83 C6 10 add si,10h ; = 0, go partition next entry
0000:0629 E2 F5 loop loc_nextpe
; no more entries to lookup
0000:062B CD 18 int 18h ; no bootable entries - go to rom basic
0000:062D loc_boot: ; xref 0622
0000:062D 8B 14 mov dx,[si] ; head and drive to boot from
0000:062F 8B EE mov bp,si ; save table entry address to pass to partition boot record
0000:0631 loc_nextrpe: ; all remaining entries should begin with 0
0000:0631 83 C6 10 add si,10h ; next table entry
0000:0634 49 dec cx ; # entries left
0000:0635 74 16 jz short loc_tabok ; all entries look ok
0000:0637 38 2C cmp [si],ch ; other entries = 0 ?
0000:0639 74 F6 je loc_nextrpe ; yes, this one is ok
0000:063B loc_bad: ; found a invalid entry :
; A). from 0624: boot id !=0 and !=80h
; B). from 0639: multi entries with id=80h
0000:063B .BE 0710 mov si,offset msg1+1 ; 'Invalid partition'
0000:063E loc_halt: ; show msg then halt
0000:063E 4E dec si
0000:063F loc_msg: ; xref 064B, 06BA
0000:063F AC lodsb ; got a message char
0000:0640 3C 00 cmp al,0
0000:0642 74 FA je loc_halt ; no more char, then halt
0000:0644 BB 0007 mov bx,7
0000:0647 B4 0E mov ah,0Eh
0000:0649 CD 10 int 10h ; then display it: ah=functn 0Eh
; write char al, bl=attr, teletype mode
000:064B loc_msgh: ; xref 06BF
0000:064B EB F2 jmp short loc_msg ; do the entire message
; Tempory variable in heap:
; bp + 24h db ? ; boot record drive
; bp + 25h dw ? ; boot record sector (2nd copy of boot record)
0000:064D loc_tabok: ; xref 0635
0000:064D 89 46 25 mov [bp+25h],ax ; clear sector/cyl of 2nd copy of boot recordto 0
0000:0650 96 xchg si,ax ; si=0
0000:0651 8A 46 04 mov al,[bp+4] ; sys_id
0000:0654 B4 06 mov ah,6 ;
0000:0656 3C 0E cmp al,0Eh ; sys_id = 0Eh?
0000:0658 74 11 je short loc_check13ext ; yes
0000:065A B4 0B mov ah,0Bh
0000:065C 3C 0C cmp al,0Ch ; sys_id = 0Ch?
0000:065E 74 05 je short loc_sysid0B0C ; yes
0000:0660 3A C4 cmp al,ah ; sys_id = 0Bh?
0000:0662 75 2B jne short loc_int13old ; no
0000:0664 40 inc ax ; chang zf to jump over check13ext
0000:0665 loc_sysid0B0C: ; sys_id = 0Bh, 0Ch, got here
0000:0665 C6 46 25 06 mov byte ptr [bp+25h],6 ; set boot record sector = 6
0000:0669 75 24 jnz short loc_int13old
0000:066B loc_check13ext:
; Function 41 (Check Extensions Present); In : AH -Int 13 function Number
; BX -55AAh
; DL -Drive
; Out: AL -Internal Use, not preserved
; AH -21h, Major version of these extensions
; BX -AA55h
; CX -Interface support bit map as follows,
; 0 Extended access functions.
; 1 Drive Locking and Ejecting
; 2 EDD Support
0000:066B BB 55AA mov bx,55AAh ; 3-15 Reserved, must be 0
0000:066E 50 push ax ; Carry flag Clear if INT 13h, FN 41h supported
0000:066F B4 41 mov ah,41h ; Check Extensions Present notifies the caller that
0000:0671 CD 13 int 13h ; Extended drive support is preset.
0000:0673 58 pop ax ; support int13 ext?
0000:0674 72 16 jc short loc_no13ext ; no
0000:0676 .81 FB AA55 cmp bx,0AA55h
0000:067A 75 10 jne short loc_no13ext
0000:067C F6 C1 01 test cl,1
0000:067F 74 0B jz short loc_no13ext
; yes
0000:0681 8A E0 mov ah,al ; change code to jump over old int 13
0000:0683 88 56 24 mov [bp+24h],dl ; drive
0000:0686 C7 06 06A1 1EEB mov word ptr ds:[6A1h],1EEBh; "jump short 06C1"
0000:068C loc_no13ext:
0000:068C 88 66 04 mov [bp+4],ah ; return from Function 41 (Check Extensions Present)
0000:068F loc_int13old: ; 0Bh,0Ch,0Eh, got here
0000:068F BF 000A mov di,0Ah ; retry count
0000:0692 loc_readin: ; read in boot record, xref 06B6, 06D8
0000:0692 B8 0201 mov ax,201h ; func ah=2 :read, al=1 :sector
0000:0695 8B DC mov bx,sp ; es:bx=7C00h :buffer
0000:0697 33 C9 xor cx,cx
0000:0699 83 FF 05 cmp di,5 ; retry 5 times still error ?
0000:069C 7F 03 jg short loc_readbackup ; no
; yes, read 2nd copy of boot record
0000:069E 8B 4E 25 mov cx,[bp+25h] ; ch=cyl, cl=sector
; jmp short loc_int13ext
0000:06A1 loc_readbackup: ; xref 069C
0000:06A1 03 4E 02 add cx,[bp+2] ; start_sector (bits 0-5)
0000:06A4 CD 13 int 13h ; Disk dl=drive ? ah=func 02h
; read sectors to memory es:bx
; al=#,ch=cyl,cl=sectr,dh=head