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Each file is stored in a directory, and uses a directory entry that describes its characteristics such as its name and size. The directory entry also contains a pointer to where the file is stored on disk. One of the characteristics stored for each file is a set of file attributes that give the operating system and application software more information about the file and how it is intended to be used.
The use of attributes is "voluntary". What this means is that any software program can look in the directory entry to discern the attributes of a file, and based on them, make intelligent decisions about how to treat the file. For example, a file management program's delete utility, seeing a file marked as a read-only system file, would be well-advised to at least warn the user before deleting it. However, it doesn't have to. Any programmer that knows what it is doing can override the attributes of a file, and certainly, the writers of viruses do this as a matter of course!
That said, most operating systems assign definite meanings to the attributes stored for files, and will alter their behavior according to what they see. If at a DOS prompt you type "DIR" to list the files in the directory, by default you will not see any files that have the "hidden" attribute set. You have to type "DIR /AH" to see the hidden files.
A file can have more than one attribute attached to it, although only certain combinations really make any sense. The attributes are stored in a single byte, with each bit of the byte representing a specific attribute (actually, only six bits are used of the eight in the byte). Each bit that is set to a one means that the file has that attribute turned on. (These are sometimes called attribute bits or attribute flags). This method is a common way that a bunch of "yes/no" parameters are stored in computers to save space. The following are the attributes and the bits they use in the attribute byte:
The attribute bits are summed to form the attribute byte. So, the attribute byte for a hidden, read-only directory would be 00010011, which is simply the codes for those three attributes from the table above, added together. Here is a more detailed description of what these attributes mean (or more accurately, how they are normally used). Note that each of the attributes below apply equally to files and directories (except for the directory attribute of course!):
Most of the attributes for files can be modified using the DOS ATTRIB command, or by looking at the file's properties through the Windows 95 Windows Explorer or other similar file navigation tools.