Windows启动代码的一段汇编 看不懂了

[bobo_包子]

;/*
; *                      Microsoft Confidential
; *                      Copyright (C) Microsoft Corporation 1983 - 1991
; *                      All Rights Reserved.
; */
;       BOOT - IBM hard disk boot record             6/8/82
;
;
; This is the standard boot record that will be shipped on all hard disks. It contains:
;
; 1. Code to load (and give control to) the boot record for 1 of 4 possible
;    operating systems.
;
; 2. A partition table at the end of the boot record, followed by the required signature.
;
;

relocated_org   equ 0600h
buildtime_org   equ 0100h
org_delta       equ (relocated_org - buildtime_org)

_data   segment public
        assume  cs:_data,ds:_data

;
; /tiny programs start at 100h.
;

        org buildtime_org
start:

        cli             ;no interrupts for now
        xor ax,ax
        mov ss,ax
        mov sp,7c00h    ;new stack at 0:7c00
        mov si,sp       ;where this boot record starts - 0:7c00
        push ax
        pop es          ;seg regs the same
        push ax
        pop ds
        sti             ;interrupts ok now
        cld
        mov di,relocated_org ;where to relocate this boot record to
        mov cx,100h
        rep movsw       ;relocate to 0:0600
;       jmp entry2 + org_delta
        db   0eah
        dw   $+4+org_delta,0
entry2:
        mov si,(offset tab) + org_delta  ;partition table
        mov bl,4        ;number of table entries
next:
        cmp byte ptr[si],80h  ;is this a bootable entry?
        je boot         ;yes
        cmp byte ptr[si],0    ;no, is boot indicator zero?
        jne bad         ;no, it must be x"00" or x"80" to be valid
        add si,16       ;yes, go to next entry
        dec bl
        jnz next
        int 18h         ;no bootable entries - go to rom basic
boot:
        mov dx,[si]     ;head and drive to boot from
        mov cx,[si+2]   ;cyl, sector to boot from
        mov bp,si       ;save table entry address to pass to partition boot record
next1:
        add si,16       ;next table entry
        dec bl          ;# entries left
        jz tabok        ;all entries look ok
        cmp byte ptr[si],0    ;all remaining entries should begin with zero
        je next1        ;this one is ok
bad:
        mov si,(offset m1) + org_delta ;oops - found a non-zero entry - the table is bad
msg:
        lodsb           ;get a message character
        cmp al,0
        je  hold
        push si
        mov bx,7
        mov ah,14
        int 10h         ;and display it
        pop si
        jmp msg         ;do the entire message
;
hold:   jmp hold        ;spin here - nothing more to do
tabok:
        mov di,5        ;retry count
rdboot:
        mov bx,7c00h    ;where to read system boot record
        mov ax,0201h    ;read 1 sector
        push di
        int 13h         ;get the boot record
        pop di
        jnc goboot      ;successful - now give it control
        xor ax,ax       ;had an error, so
        int 13h         ;recalibrate
        dec di          ;reduce retry count
        jnz rdboot      ;if retry count above zero, go retry
        mov si,(offset m2) + org_delta ;all retries done - permanent error - point to message,
        jmp msg          ;go display message and loop
goboot:
        mov si,(offset m3) + org_delta ;prepare for invalid boot record
        mov di,07dfeh
        cmp word ptr [di],0aa55h ;does the boot record have the
                                   ;    required signature?
        jne msg         ;no, display invalid system boot record message
        mov si,bp       ;yes, pass partition table entry address
        db 0eah
        dw 7c00h,0

include x86mboot.msg

        org 2beh
tab:                    ;partition table
        dw 0,0          ;partition 1 begin
        dw 0,0          ;partition 1 end
        dw 0,0          ;partition 1 relative sector (low, high parts)
        dw 0,0          ;partition 1 # of sectors (low, high parts)
        dw 0,0          ;partition 2 begin
        dw 0,0          ;partition 2 end
        dw 0,0          ;partition 2 relative sector
        dw 0,0          ;partition 2 # of sectors
        dw 0,0          ;partition 3 begin
        dw 0,0          ;partition 3 end
        dw 0,0          ;partition 3 relative sector
        dw 0,0          ;partition 3 # of sectors
        dw 0,0          ;partition 4 begin
        dw 0,0          ;partition 4 end
        dw 0,0          ;partition 4 relative sector
        dw 0,0          ;partition 4 # of sectors
signa   db 55h,0aah     ;signature

_data   ends
        end  start

这里实在看不懂 看高人解答
jmp entry2 + org_delta
db 0eah
dw $+4+org_delta,0
entry2:
mov si,(offset tab) + org_delta

[shybrian]

这上面都注释了啊，就是跳转到entry2+org_delta
db 0eah相当于在这里写了个跳转至零，跳转的地址就是下面的dw $+4+org_delta,0
; jmp entry2 + org_delta
db 0eah
dw $+4+org_delta,0

[shybrian]

由于这段代码将boot代码移位，下一步就是访问分区表了
mov si,(offset tab) + org_delta
分区表的段内偏移加上移位的地址差也就是你现在分区表所在的位置。
我的理解是这样的。