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Loading Boot Sector to buffer

Posted on 2002-04-21
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Last Modified: 2008-02-01
INT 13 - DISK - READ SECTOR(S) INTO MEMORY
 AH = 02h
 AL = number of sectors to read (must be nonzero)
 CH = low eight bits of cylinder number
 CL = sector number 1-63 (bits 0-5)
      high two bits of cylinder (bits 6-7, hard disk only)
 DH = head number
 DL = drive number (bit 7 set for hard disk)
 ES:BX -> data buffer
Return: CF set on error
     if AH = 11h (corrected ECC error), AL = burst length
 CF clear if successful
 AH = status (see #00234)
 AL = number of sectors transferred (only valid if CF set for some
       BIOSes)
Notes: errors on a floppy may be due to the motor failing to spin up quickly
   enough; the read should be retried at least three times, resetting
   the disk with AH=00h between attempts
 most BIOSes support "multitrack" reads, where the value in AL
   exceeds the number of sectors remaining on the track, in which
   case any additional sectors are read beginning at sector 1 on
   the following head in the same cylinder; the MSDOS CONFIG.SYS command
   MULTITRACK (or the Novell DOS DEBLOCK=) can be used to force DOS to
   split disk accesses which would wrap across a track boundary into two
   separate calls
 the IBM AT BIOS and many other BIOSes use only the low four bits of
   DH (head number) since the WD-1003 controller which is the standard
   AT controller (and the controller that IDE emulates) only supports
   16 heads
 AWARD AT BIOS and AMI 386sx BIOS have been extended to handle more
   than 1024 cylinders by placing bits 10 and 11 of the cylinder number
   into bits 6 and 7 of DH
 under Windows95, a volume must be locked (see INT 21/AX=440Dh/CX=084Bh)
   in order to perform direct accesses such as INT 13h reads and writes
 all versions of MS-DOS (including v7 [Win95]) have a bug which prevents
   booting on hard disks with 256 heads, so many modern BIOSes provide
   mappings with at most 255 heads
SeeAlso: AH=03h,AH=0Ah,AH=06h"V10DISK.SYS",AH=21h"PS/1",AH=42h"IBM"
SeeAlso: INT 21/AX=440Dh/CX=084Bh,INT 4D/AH=02h

According to the above, how do you setup the registers for loading boot sector (of Hardisk C:) to memory (GAS or GCC)?

Regards,
Jacky
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Question by:luckie
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Accepted Solution

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dimitry earned 400 total points
ID: 6958151
In order to read boot sector from C: with INT 13h you need to set next values:
AH = 02h
AL = 1
CH = 0
CL = 1
DH = 1
DL = 0x80
ES:BX = pointer to 512 bytes buffer

But if you want only to see the contents I suggest you to use DiskEditor, for example from:
http://www.sf-soft.de/winhex/index-m.html

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Expert Comment

by:DanRollins
ID: 6959266
I think it is likely that you will need to set DH to 0.

-- Dan
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LVL 11

Expert Comment

by:dimitry
ID: 6959289
If you want to read MBR then Dan is absolutely right:
DH should be 0. But if you want to read boot sector of first partition you need to set DH to 1.
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Author Comment

by:luckie
ID: 6959295
Ok. That will do... Thanks
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