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Operating Systems Linux C++ Code to Access Linux Hard Disk Sectors (with a LoopBack Virtual Hard Disk) Post 302491808 by Corona688 on Friday 28th of January 2011 11:05:38 AM
Old 01-28-2011
Quote:
Originally Posted by shen747
Hi Corona688,

Thanks a lot for pointing that out. I've been doing some reading & now I feel going for the FAT-12(classical floppy disk) format file system would be the best idea as it was the 1st form of FAT introduced to the world & I guess therefore it would be much simpler & easier to implement.
It's not. I already explained why. But if you're willing to deal with the 12-bit strangeness, all right...
Quote:
I found this explaining how to write a file to a FAT-12 file system. But I'm still finding trouble finding a basic code to get me started on this.
If you don't want to write the code, you can just mount the filesystem and use it...

I think you're getting too far ahead of yourself. You can't write a file if you haven't even built things to deal with FAT tables, sectors, directory entries... Work from the ground up. Read up on the structure and try to find and use everything they tell you about.

Look at the boot block structure I gave you. Load the boot sector from your floppy into it and see if the values in it make any sense -- if not, either you loaded the wrong data or I made a mistake in that structure.

Once you get that looking okay, use the values in it to find the FAT table. Write a function to get a 12-bit values out of it(you can't just use an array because of the weird 12-bit size). Use hexdump to see what values you should be getting.

Once you can read the FAT table, start checking what things these FAT entries are pointing to. (If your FAT entries are scrambled somehow you could end up reading garbage that makes no sense, so make really sure they're right. Test a bunch of them.) Read in data from where they direct you and these will be file and directory entry structures. Their size might depend on values you read somewhere else. Try following a chain of them to get a complete file or directory listing.

Once you've done all that, the steps you see in that course information will make a whole lot more sense to you.

Last edited by Corona688; 01-28-2011 at 12:11 PM..
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NEWFS_MSDOS(8)						    BSD System Manager's Manual 					    NEWFS_MSDOS(8)

NAME
newfs_msdos -- construct a new MS-DOS (FAT) file system SYNOPSIS
newfs_msdos [-N] [-B boot] [-F FAT-type] [-I volid] [-O OEM] [-S sector-size] [-a FAT-size] [-b block-size] [-c cluster-size] [-e dirents] [-f format] [-h heads] [-i info] [-k backup] [-m media] [-n FATs] [-o hidden] [-r reserved] [-s total] [-u track-size] [-v volume-name] special [disktype] DESCRIPTION
The newfs_msdos utility creates a FAT12, FAT16, or FAT32 file system on device special, using disktab(5) entry disktype to determine geome- try, if required. The options are as follow: -N Don't create a file system: just print out parameters. -B boot Get bootstrap from file. -F FAT-type FAT type (one of 12, 16, or 32). -I volid Volume ID. -O OEM OEM string (up to 8 characters). The default is "BSD 4.4". -S sector-size Number of bytes per sector. Acceptable values are powers of 2 in the range 128 through 32768. -a FAT-size Number of sectors per FAT. -b block-size File system block size (bytes per cluster). This should resolve to an acceptable number of sectors per cluster (see below). -c cluster-size Sectors per cluster. Acceptable values are powers of 2 in the range 1 through 128. -e dirents Number of root directory entries (FAT12 and FAT16 only). -f format Specify a standard (floppy disk) format. The eight standard formats are (capacities in kilobytes): 160, 180, 320, 360, 640, 720, 1200, 1232, 1440, 2880. -h heads Number of drive heads. -i info Location of the file system info sector (FAT32 only). A value of 0xffff signifies no info sector. -k backup Location of the backup boot sector (FAT32 only). A value of 0xffff signifies no backup sector. -m media Media descriptor (acceptable range 0xf0 to 0xff). -n FATs Number of FATs. Acceptable values are 1 to 16 inclusive. The default is 2. -o hidden Number of hidden sectors. -r reserved Number of reserved sectors. -s total File system size. -u track-size Number of sectors per track. -v volume-name Volume name (filesystem name), up to 11 characters. The name should consist of only those characters permitted in regular DOS (8+3) filenames. NOTES
FAT file system parameters occupy a "Boot Sector BPB (BIOS Parameter Block)" in the first of the "reserved" sectors which precede the actual file system. For reference purposes, this structure is presented below. struct bsbpb { u_int16_t bps; /* [-S] bytes per sector */ u_int8_t spc; /* [-c] sectors per cluster */ u_int16_t res; /* [-r] reserved sectors */ u_int8_t nft; /* [-n] number of FATs */ u_int16_t rde; /* [-e] root directory entries */ u_int16_t sec; /* [-s] total sectors */ u_int8_t mid; /* [-m] media descriptor */ u_int16_t spf; /* [-a] sectors per FAT */ u_int16_t spt; /* [-u] sectors per track */ u_int16_t hds; /* [-h] drive heads */ u_int32_t hid; /* [-o] hidden sectors */ u_int32_t bsec; /* [-s] big total sectors */ }; /* FAT32 extensions */ struct bsxbpb { u_int32_t bspf; /* [-a] big sectors per FAT */ u_int16_t xflg; /* control flags */ u_int16_t vers; /* file system version */ u_int32_t rdcl; /* root directory start cluster */ u_int16_t infs; /* [-i] file system info sector */ u_int16_t bkbs; /* [-k] backup boot sector */ }; EXAMPLES
newfs_msdos /dev/disk0s1 Create a file system, using default parameters, on /dev/disk0s1. newfs_msdos -f 1440 -v foo fd0 Create a standard 1.44M file system, with volume name "foo", on /dev/fd0. SEE ALSO
fdisk(8), mount(8) DIAGNOSTICS
Exit status is 0 on success and 1 on error. HISTORY
The newfs_msdos command appeared in FreeBSD 3.0. AUTHORS
Robert Nordier <rnordier@FreeBSD.org>. BSD
July 6, 1998 BSD
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