question about MBR of DOS

[david]

In the MBR of DOS, there is a few code that moving MBR from 0:7c00 to 0:600.

Code: Select all

    xor ax, ax
    ....
    mov sp, 7c00
    mov si, sp
    push ax
    pop es
    push ax
    pop ds
    ....
    mov di, 0600
    mov cx, 0100
    repnz
    movsw

i have a question , why use 'repnz' ? why not use 'rep' ?

[mathematician]

According to AMD's manual, repnz can't be used with movsw. So either the programmer has made a mistake, or your disassembler has.

[XCHG]

REPNZ will repeat the instruction while the Zero Flag is Not Zero (NZ). A Move instruction can not possibly set or clear the Zero Flag so that doesn't make sense. REP MOVSW is correct and I also use it in the BootStrap code that I have written for the MBR of the HDD:

Code: Select all

MOV     SI , ORIGINAL_MBR_SEGMENT_OFFSET                        
MOV     DI , RELOCATED_MBR_SEGMENT_OFFSET                       
CLD                                                             
MOV     CX , BYTESPERSECTOR / SIZEOF_CX                         
REP     MOVSW                                                   
JMP     LOADED_BOOT_SECTOR_STAGE1_SEGMENT_ADDRESS:\             
        RELOCATED_MBR_SEGMENT_OFFSET + LABEL_EIP(.RestofTheCode)

[david]

the reason maybe is that Microsoft don't want DOS run on AMD CPU, only run on Intel CPU.
Perhaps because Wintel...
i guess

My compter is Intel CPU .

[JAAman]

the reason maybe is that Microsoft don't want DOS run on AMD CPU, only run on Intel CPU.
Perhaps because Wintel...
i guess

My compter is Intel CPU .

no, AMD and Intel CPUs are almost 100% identical...

and MS definately (and quite specifically) does want it to run on AMD

your assembler must be mistaken... i cant seem to find my copy to check at the moment

[david]

method of get MBR in DOS(Virtual PC) :
1 debug
2 read MBR to memory buffer from Hard Disk use int 13h
3 use 'U' command disassemble the memory buffer(MBR)
then, the result is that i show you .
so, i don't think the method have problem .
i also think 'U' command is correct !

i don't know why the programmer who write MBR of DOS use 'repnz' .
because i always use 'rep' in this case .
so i want to know the reason!

maybe it doesn't make any sense.

[Tyler]

It doesn't. Here's the closet thing on the Web, already correctly disassembled and well documented.

http://mirror.href.com/thestarman/asm/mbr/STDMBR.htm

EDIT: There is a note at the bottom of the page that explains the reason it appears as REPNZ. So the page also includes your answer.

[david]

It doesn't. Here's the closet thing on the Web, already correctly disassembled and well documented.

http://mirror.href.com/thestarman/asm/mbr/STDMBR.htm

EDIT: There is a note at the bottom of the page that explains the reason it appears as REPNZ. So the page also includes your answer.

thank you very much!

[1234]

[post deleted]

[david]

REP MOVSW is correct and I also use it in the BootStrap code that I have written for the MBR of the HDD:

i know.

i am programming a routine , it is in NIC's option ROM, it will use PXE to downland a bootstrap from PXE server .

i think i must load the Onboard NIC driver , ready for UNDI. DHCP and PXE.

do you know how to load the Onboard NIC driver ?

thanks !