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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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mkfs_pcfs(1M)															     mkfs_pcfs(1M)

NAME
mkfs_pcfs - construct a FAT file system SYNOPSIS
mkfs -F pcfs [generic_options] [-o FSType_specific_options] raw_device_file The pcfs-specific module of mkfs constructs a File Allocation Table (FAT) on removable media (diskette, JAZ disk, ZIP disk, PCMCIA card), a hard disk, or a file (see ). FATs are the standard MS-DOS and Windows file system format. Note that you can use fdformat(1) to construct a FAT file system only on a diskette or PCMCIA card. mkfs for pcfs determines an appropriate FAT size for the medium, then it installs an initial boot sector and an empty FAT. A sector size of 512 bytes is used. mkfs for pcfs can also install the initial file in the file system (see the pcfs-specific -o i option). This first file can optionally be marked as read-only, system, and/or hidden. If you want to construct a FAT with mkfs for pcfs on a medium that is not formatted, you must first perform a low-level format on the medium with fdformat(1) or format(1M). Non-diskette media must also be partitioned with the fdisk(1M) utility. Note that all existing data on the diskette or disk partition, if any, is destroyed when a new FAT is constructed. generic_options are supported by the generic mkfs command. See mkfs(1M) for a description of these options. raw_device_file indicates the device on which to write unless the -o N option has been specified, or if the -V or -m generic options are passed from the generic mkfs module. See mkfs(1M) for the list of supported generic options. The following options are supported: -o FSType_specific_options Specify pcfs file system-specific options in a comma-separated list with no intervening spaces. If invalid options are specified, a warning message is printed and the invalid options are ignored. b=label Label the media with volume label. The volume label is restricted to 11 uppercase characters. B=filename Install filename as the boot loader in the file system's boot sector. If you don't specify a boot loader, an MS-DOS boot loader is installed. The MS-DOS boot loader requires specific MS-DOS system files to make the diskette bootable. See NOTES for more information. fat=n The size of a FAT entry. Currently, 12, 16, and 32 are valid values. The default is 12 for diskettes, 16 for larger media. h Mark the first file installed as a hidden file. The -i option must also be specified. hidden=n Set the number of hidden sectors to n. This is the number of sectors on the physical disk preceding the start of the volume (which is the boot sector itself). This defaults to 0 for diskettes or a computed valued (based on the fdisk ta- ble) for disks. This option may be used only in conjunction with the nofdisk option. i=filename Install filename as the initial file in the new file system. The initial file's contents are guaranteed to occupy con- secutive clusters at the start of the files area. When creating bootable media, a boot program should be specified as the initial file. nofdisk Do not attempt to find an fdisk table on the medium. Instead rely on the size option for determining the partition size. By default, the created FAT is 16 bits and begins at the first sector of the device. This origination sector can be modified with the hidden option (-h). nsect=n The number of sectors per track on the disk. If not specified, the value is determined by using a dkio(7I) ioctl to get the disk geometry, or (for diskette) from the results of an FDIOGCHAR ioctl. ntrack=n The number of tracks per cylinder on the disk. If not specified, the value is determined by using a dkio(7I) ioctl to get the disk geometry, or (for diskette) from the results of an FDIOGCHAR ioctl. N No execution mode. Print normal output, but do not actually write the file system to the medium. This is most useful when used in conjunction with the verbose option. r Mark the first file installed as read-only. The -i option must also be specified. reserve=n Set the number of reserved sectors to n. This is the number of sectors in the volume, preceding the start of the first FAT, including the boot sector. The value should always be at least 1, and the default value is exactly 1. s Mark the first file installed as a system file. The -i option must also be specified. size=n The number of sectors in the file system. If not specified, the value is determined from the size of the partition given in the fdisk table or (for diskette) by way of computation using the FDIOGCHAR ioctl. spc=n The size of the allocation unit for space within the file system, expressed as a number of sectors. The default value depends on the FAT entry size and the size of the file system. v Verbose output. Describe, in detail, operations being performed. FILES
raw_device_file The device on which to build the FAT. The device name for a diskette must be specified as /dev/rdiskette0 for the first diskette drive, or /dev/rdiskette1 for a second diskette drive. For non-diskette media, a disk device name must be qualified with a suffix to indicate the proper partition. For example, in the name /dev/rdsk/c0t0d0p0:c, the :c suffix indicates that the first partition on the disk should receive the new FAT. For a file, raw_device_file is the block device name returned by lofiadm(1M). EXAMPLES
The media in these examples must be formatted before running mkfs for pcfs. See for more details. Example 1: Creating a FAT File System on a Diskette The following command creates a FAT file system on a diskette: mkfs -F pcfs /dev/rdiskette Example 2: Creating a FAT File System on a Disk The following command creates a FAT file system on the second fdisk partition of a disk attached to an based system: mkfs -F pcfs /dev/rdsk/c0d0p0:d Example 3: Creating a FAT File System on a ZIP Disk The following command creates a FAT file system on a ZIP disk located on a SPARC based system: mkfs -F pcfs /dev/rdsk/c0t4d0s2:c Example 4: Creating a FAT File System on a JAZ Disk The following command creates a FAT file system on a JAZ disk located on a SPARC based system and overrides the sectors/track and tracks/cylinder values obtained from the device's controller: mkfs -F pcfs -o nsect=32,ntrack=64 /dev/rdsk/c0t3d0s2:c See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Availability |SUNWesu | +-----------------------------+-----------------------------+ |Interface Stability |Stable | +-----------------------------+-----------------------------+ fdformat(1), fdisk(1M), format(1M), lofiadm(1M), mkfs(1M), attributes(5), fd(7D), dkio(7I), fdio(7I) NOTES
The default MS-DOS boot loader, which is installed by default if -o B is not specified, requires specific MS-DOS system files to make the diskette bootable. These MS-DOS files are not installed when you format a diskette with mkfs for pcfs, which makes a diskette formatted this way not bootable. Trying to boot from it on an based system will result in the following message: Non-System disk or disk error Replace and strike any key when ready You must format a diskette with the DOS format command to install the specific MS-DOS system files required by the default boot loader. You can use lofiadm to create a file that appears to a mkfs command (for example, mkfs_pcfs or mkfs_ufs) as a raw device. You can then use a mkfs command to create a file system on that device. See lofiadm(1M) for examples of creating a UFS and a PC (FAT) file system on a device created by lofiadm. 1 Dec 2003 mkfs_pcfs(1M)
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