6.4.3 The MS-DOS File System
To a first approximation, MS-DOS is a bigger and better version of CP/M. It runs only on Intel platforms, does not support multiprogramming, and runs only in the PC's real mode (which was originally the only mode). The shell has more features and there are more system calls, but the basic function of the operating system is still loading programs, handling the keyboard and screen, and managing the file system. It is the latter functionality that interests us here.
The MS-DOS file system was patterned closely on the CP/M file system, including the use of 8 + 3 (upper case) character file names. The first version (MS-DOS 1.0) was even limited to a single directory, like CP/M. However, starting with MS-DOS 2.0, the file system functionality was greatly expanded. The biggest improvement was the inclusion of a hierarchical file system in which directories could be nested to an arbitrary depth. This meant that the root directory (which still had a fixed maximum size) could contain subdirectories, and these could contain further subdirectories ad infinitem. Links in the style of UNIX were not permitted, so the file system formed a tree starting at the root directory.
It is common for different application programs to start out by creating a subdirectory in the root directory and put all their files there (or in subdirectories thereof), so that different applications do not conflict. Since directories are themselves just stored as files, there are no limits on the number of directories or files that may be created. Unlike CP/M, however, there is no concept of different users in MS-DOS. Consequently, the logged in user has access to all files.
To read a file, an MS-DOS program must first make an open system call to get a handle for it. The open system call specifies a path, which may be either absolute or relative to the current working directory. The path is looked up component by component until the final directory is located and read into memory. It is then searched for the file to be opened.
Although MS-DOS directories are variable sized, like CP/M, they use a fixed-size 32-byte directory entry. The format of an MS-DOS directory entry is shown in Fig. 6-4. It contains the file name, attributes, creation date and time, starting block, and exact file size. File names shorter than 8 + 3 characters are left justified and padded with spaces on the right, in each field separately. The Attributes field is new and contains bits to indicate that a file is read-only, needs to be archived, is hidden, or is a system file. Read-only files cannot be written. This is to protect them from accidental damage. The archived bit has no actual operating system function (i.e., MS-DOS does not examine or set it). The intention is to allow user-level archive programs to clear it upon archiving a file and to have other programs set it when modifying a file. In this way, a backup program can just examine this attribute bit on every file to see which files to back up. The hidden bit can be set to prevent a file from appearing in directory listings. Its main use is to avoid confusing novice users with files they might not understand. Finally, the system bit also hides files. In addition, system files cannot accidentally be deleted using the del command. The main components of MS-DOS have this bit set.
Figure 6-4 The MS-DOS directory entry.
The directory entry also contains the date and time the file was created or last modified. The time is accurate only to ±2 sec because it is stored in a 2-byte field, which can store only 65,536 unique values (a day contains 86,400 unique seconds). The time field is subdivided into seconds (5 bits), minutes (6 bits), and hours (5 bits). The date counts in days using three subfields: day (5 bits), month (4 bits), and year-1980 (7 bits). With a 7-bit number for the year and time beginning in 1980, the highest expressible year is 2107. Thus MS-DOS has a built-in Y2108 problem. To avoid catastrophe, MS-DOS users should begin with Y2108 compliance as early as possible. If MS-DOS had used the combined date and time fields as a 32-bit seconds counter, it could have represented every second exactly and delayed the catastrophe until 2116.
Unlike CP/M, which does not store the exact file size, MS-DOS does. Since a 32-bit number is used for the file size, in theory files can be as large as 4 GB. However, other limits (described below) restrict the maximum file size to 2 GB or less. A surprising large part of the entry (10 bytes) is unused.
Another way in which MS-DOS differs from CP/M is that it does not store a file's disk addresses in its directory entry, probably because the designers realized that large hard disks (by then common on minicomputers) would some day reach the MS-DOS world. Instead, MS-DOS keeps track of file blocks via a file allocation table in main memory. The directory entry contains the number of the first file block. This number is used as an index into a 64K entry FAT in main memory. By following the chain, all the blocks can be found. The operation of the FAT is illustrated in Fig. 6-0.
The FAT file system comes in three versions for MS-DOS: FAT-12, FAT-16, and FAT-32, depending on how many bits a disk address contains. Actually, FAT-32 is something of a misnomer since only the low-order 28 bits of the disk addresses are used. It should have been called FAT-28, but powers of two sound so much neater.
For all FATs, the disk block can be set to some multiple of 512 bytes (possibly different for each partition), with the set of allowed block sizes (called cluster sizes by Microsoft) being different for each variant. The first version of MS-DOS used FAT-12 with 512-byte blocks, giving a maximum partition size of 212 ´ 512 bytes (actually only 4086 ´ 512 bytes because 10 of the disk addresses were used as special markers, such as end of file, bad block, etc. With these parameters, the maximum disk partition size was about 2 MB and the size of the FAT table in memory was 4096 entries of 2 bytes each. Using a 12-bit table entry would have been too slow.
This system worked well for floppy disks, but when hard disks came out, it became a problem. Microsoft solved the problem by allowing additional block sizes of 1 KB, 2 KB, and 4 KB. This change preserved the structure and size of the FAT-12 table, but allowed disk partitions of up to 16 MB.
Since MS-DOS supported four disk partitions per disk drive, the new FAT-12 file system worked up to 64-MB disks. Beyond that, something had to give. What happened was the introduction of FAT-16, with 16-bit disk pointers. Additionally, block sizes of 8 KB, 16 KB, and 32 KB were permitted. (32,768 is the largest power of two that can be represented in 16 bits.) The FAT-16 table now occupied 128 KB of main memory all the time, but with the larger memories by then available, it was widely used and rapidly replaced the FAT-12 file system. The largest disk partition that can be supported by FAT-16 is 2 GB (64K entries of 32 KB each) and the largest disk 8 GB, namely four partitions of 2 GB each.
For business letters, this limit is not a problem, but for storing digital video using the DV standard, a 2-GB file holds just over 9 minutes of video. As a consequence of the fact that a PC disk can support only four partitions, the largest video that can be stored on a disk is about 38 minutes, no matter how large the disk is. This limit also means that the largest video that can be edited on line is less than 19 minutes, since both input and output files are needed.
Starting with the second release of Windows 95, the FAT-32 file system, with its 28-bit disk addresses, was introduced and the version of MS-DOS underlying Windows 95 was adapted to support FAT-32. In this system, partitions could theoretically be 228 ´ 215 bytes, but they are actually limited to 2 TB (2048 GB) because internally the system keeps track of partition sizes in 512-byte sectors using a 32-bit number and 29 ´ 232 is 2 TB. The maximum partition size for various block sizes and all three FAT types is shown in Fig. 6-5.
Figure 6-5 Maximum partition size for different block sizes. The empty boxes represent forbidden combinations.
In addition to supporting larger disks, the FAT-32 file system has two other advantages over FAT-16. First, an 8-GB disk using FAT-32 can be a single partition. Using FAT-16 it has to be four partitions, which appears to the Windows user as the C:, D:, E:, and F: logical disk drives. It is up to the user to decide which file to place on which drive and keep track of what is where.
The other advantage of FAT-32 over FAT-16 is that for a given size disk partition, a smaller block size can be used. For example, for a 2-GB disk partition, FAT-16 must use 32-KB blocks, otherwise with only 64K available disk addresses, it cannot cover the whole partition. In contrast, FAT-32 can use, for example, 4-KB blocks for a 2-GB disk partition. The advantage of the smaller block size is that most files are much shorter than 32 KB. If the block size is 32 KB, a file of 10 bytes ties up 32 KB of disk space. If the average file is, say, 8 KB, then with a 32-KB block, ¾ of the disk will be wasted, not a terribly efficient way to use the disk. With an 8-KB file and a 4-KB block, there is no disk wastage, but the price paid is more RAM eaten up by the FAT. With a 4-KB block and a 2-GB disk partition, there are 512K blocks, so the FAT must have 512K entries in memory (occupying 2 MB of RAM).
MS-DOS uses the FAT to keep track of free disk blocks. Any block that is not currently allocated is marked with a special code. When MS-DOS needs a new disk block, it searches the FAT for an entry containing this code. Thus no bitmap or free list is required.