Re: confused by the several lines of source code in mbr.S

["abutter gao" <abutter.gao%gmail.com@localhost>]

Yes, I read the mbr.S and even boot and bootxx source code just for
fun, and everything goes well.

I'm a newbie to NetBSD, so asked for some comments, though I really
thought it is a inessential bug,

2008/1/9, David Laight <david%l8s.co.uk@localhost>:
> On Tue, Jan 08, 2008 at 10:01:29PM +0800, abutter gao wrote:
> > First, I am confused by the following lines of code from the mbr.S in
> > i386/stand/mbr directory.
>
> If you understand the rest of the code - well done :-)
>
> > ---------------------------------- snip
> > --------------------------------------------------------
> > 216     movw    $lba_info, %si
> > 217     movb    $0x42, %ah
> > 218     pop     %dx                     /* recover drive # */
> > 219     push    %dx                     /* save drive */
> > 220     int     $0x13
> > 221     jc      wait_key                /* abort menu on read fail */
> > 222     cmpw    $MBR_MAGIC, LOADADDR + MBR_MAGIC_OFFSET
> > 223     movw    $nametab - LOADADDR + BOOTADDR, %bx
> > 224     je      next_extended
> > ---------------------------------- snip
> > --------------------------------------------------------
> >
> > Moreover, the BOOTADDR and LOADADDR are defined as below:
> >
> > ---------------------------------- snip
> > --------------------------------------------------------
> > 65 #define BOOTADDR     0x7c00
> > 66 #define LOADADDR     0x0600          /* address were are linked to */
> > ---------------------------------- snip
> > --------------------------------------------------------
> >
> > When we call INT 0x13, BIOS will transfer the PBR code to the address
> > BOOTADDR included in the lba_info struct, and then we should check
> > whether the PBR code is valid.
> >
> > The address LOADADDR + MBR_MAGIC_OFFSET in line 222 is 0x0600 +
> > MBR_MAGIC_OFFSET, which is just the magic number's address of the mbr
> > code but not the PBR code, and no process will modify the mbr's magic
> > number, so, there is alwasy a jump to next_extended.
> >
> > So, is there any other purpos or just a bug because of incaution?
>
> Certainly the intention is to look for the 0x55aa in the extended
> partition table in order to validate it.
> OTOH it does indeed look as the wrong location is checked.
>
> I'll either fix it, or delete the test - since no one has reported any
> problems with the code processing invalid sectors.
>
>        David
>
> --
> David Laight: david%l8s.co.uk@localhost
>