Ms dos file system

Shop now. These systems are particularly simple because they were designed for write-once media. Among other things, for example, they have no provision for keeping track of free blocks because on a CD-ROM files cannot be freed or added after the disk has been manufactured. Virtually every CD-ROM currently on the market is compatible with this standard, sometimes with the extensions to be discussed below.

One of the goals of this standard was to make every CD-ROM readable on every computer, independent of the byte ordering used and independent of the operating system used. As a consequence, some limitations were placed on the file system to make it possible for the weakest operating systems then in use such as MS-DOS to read it. CD-ROMs do not have concentric cylinders the way magnetic disks do.

Instead there is a single continuous spiral containing the bits in a linear sequence although seeks across the spiral are possible. The bits along the spiral are divided into logical blocks also called logical sectors of bytes. Some of these are for preambles, error correction, and other overhead. The payload portion of each logical block is bytes. Often the position of a block along the spiral is quoted in minutes and seconds. Next comes one block containing the primary volume descriptor , which contains some general information about the CD-ROM.

Among this information are the system identifier 32 bytes , volume identifier 32 bytes , publisher identifier bytes , and data preparer identifier bytes. The manufacturer can fill in these fields in any desired way, except that only upper case letters, digits, and a very small number of punctuation marks may be used to ensure cross-platform compatibility. The primary volume descriptor also contains the names of three files, which may contain the abstract, copyright notice, and bibliographic information, respectively.

In addition, certain key numbers are also present, including the logical block size normally , but , , and larger powers of two are allowed in certain cases , the number of blocks on the CD-ROM, and the creation and expiration dates of the CD-ROM. Finally, the primary volume descriptor also contains a directory entry for the root directory, telling where to find it on the CD-ROM i.

From this directory, the rest of the file system can be located. In addition to the primary volume descriptor, a CD-ROM may contain a supplementary volume descriptor. It contains similar information to the primary, but that will not concern us here. The root directory, and all other directories for that matter, consists of a variable number of entries, the last of which contains a bit marking it as the final one.

The directory entries themselves are also variable length. Each directory entry consists of 10 to 12 fields, some of which are in ASCII and others of which are numerical fields in binary. The binary fields are encoded twice, once in little-endian format used on example. Thus a bit number uses 4 bytes and a bit number uses 8 bytes. The use of this redundant coding was necessary to avoid hurting anyone's feelings when the standard was developed. If the standard had dictated little endian, then people from companies with big-endian products would have felt like second-class citizens and would not have accepted the standard.

The format of an ISO directory entry is illustrated in Fig. Since directory entries have variable lengths, the first field is a byte telling how long the entry is. This byte is defined to have the high-order bit on the left to avoid any ambiguity. Directory entries may optionally have an extended attributes.

If this feature is used for a directory entry, the second byte tells how long the extended attributes are. Next comes the starting block of the file itself. Files are stored as contiguous runs of blocks, so a file's location is completely specified by the starting block and the size, which is contained in the next field.

The date and time that the CD-ROM was recorded is stored in the next field, with separate bytes for the year, month, day, hour, minute, second, and time zone. Years begin to count at , which means that CD-ROMs will suffer from a Y problem because the year following will be This problem could have been delayed by defining the origin of time to be the year the standard was adopted. Had that been done, the problem would have been postponed until Every 88 extra years helps.

Figure The ISO directory enty. The Flags field contains a few miscellaneous bits, including one to hide the entry in listings a feature copied from MS-DOS , one to distinguish an entry that is a file from an entry that is a directory, one to enable the use of the extended attributes, and one to mark the last entry in a directory. A few other bits are also present in this field but they will not concern us here.

The next field deals with interleaving pieces of files in a way that is not used in the simplest version of ISO , so we will not consider it further. The field marked L in Fig. Also, it is easier to create, maintain and update the system if it is done in the form of layers. Change in one layer specification does not affect the rest of the layers. However, the layers in MS-DOS are not that sharply defined and the layer specifications often bleed into each other.

These programs perform a particular function directly for the users. With this numer a chain reaction can be started, because each block does have the number of the following block etc till the end of the file. This set up is also the reason why it's not possible to implement links: you can't have two elements with the same number for the first block. If it were the situation each would have its owm time stamp, date and size, wich would lead to inconsistencies in the file system.

Assume that the lookup went ok. The directory-element will be copied to the system file table and the file posisiton is set to 0. The file descriptor is delivered to the caller, and you're done with OPEN. In addition, the boot record on FAT32 drives has been expanded to include a backup of critical data structures which ones exactly?

The root directory on a FAT32 drive is now an ordinary cluster chain, so it can be located anywhere on the drive. For this reason, the previous limitations on the number of root directory entries no longer exist. These features allow for dynamic resizing of FAT32 partitions. OS Tutorial. Next Topic Spooling in Operating System.

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