高人帮忙解释下 不胜感激

[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

[bobo_包子]

jmp entry2 + org_delta
dw $+4+org_delta,0
mov si,(offset tab) + org_delta

这里都加上org_delta，让我很迷糊

[zara]

看下 org_delta 的定义，它是载入的实际偏移和编译时定义的偏移的差，所以是用来校正由于编译时的没有设置适当的 org 语句而致代码里对地址的直接引用的错误值的。你可用分别使用和不使用这个项创建引导代码，debug 载入到实际的偏移处（这时只是看看，可用暂时忽略段址），看看涉及到的指令上的目标地址的区别。