msdosfs header unification patch

[Bruce Evans]

On Fri, 13 Jul 2007, Bruce Evans wrote:

> On Fri, 13 Jul 2007, Brian Chu wrote:
> % ...
> % -		/* Check backup FSInfo?					XXX 
> */
> % +
> % +    /* Unnecessary to check the backup FSInfo because there isn't
> % +     * an physical backup copy of the FSInfo block.  There's only a
> % +     * backup of the FSInfo block number, which was checked (bsBPB) above.
> % +     */
> %  	}
>
> But there is a physical backup FSinfo.
>
> The normal layout is:
>
> 	main boot sector (sector 0)
> 	fsinfo (sector 1, but can be anywhere)
> 	second boot sector (next sector? -- no pointer to this)
> 	   fsisig4 checks/clobbers here
> 	backup main boot sector (sector 3, but can be anywhere)

Actually, it is sector 6.  Sectors 3-5 are unused.  Microsoft docs say not
to use any other layout (for portability, since some utilities hard-code 6)
...

> 	backup fsinfo (next sector? -- no pointer to this, since backup
> 	   main boot sector can't have a different pointer than main bs)
> 	backup second boot sector (next sector?)
> 	   backup fsisig4 would check/clobber here
>
> newfs_msdos omits the second boot sector and creates the following layout:
>
> 	main boot sector (sector 0)
> 	fsinfo (sector 1)
> 	backup main boot sector (sector 2)
> 	   fsisig4 checks/clobbers here
> 	backup fsinfo (sector 3)
> 	   backup fsisig4 would check/clobber outside of all boot sectors

... so this is a very unusual layout and shouldn't be produced by default.

Bruce