FMPC - Files

Hard Disk and Files

The computer's hard disk is also measured by the number of bytes that it holds. Modern systems have hard disks that hold between 80M and 4G (Gigabyte = 1 billion bytes). Information is stored on the disk physically in fixed sized hardware units called 'disk sectors', but the Mac OS (as does any other modern OS) provides convenient ways to work with the information stored on the disk in terms of variable sized collections of bytes of information called 'files'. Most computer systems allow grouping files into 'directories'. Directories themselves may hold directories, which may hold directories, etc. In the Mac OS, this feature is also implemented, but in keeping with the Mac OS "Desktop metaphor", the term "folder" is used in place of "directory".

There is no inherent difference between the act of loading a 'program' or loading 'data' (this is especially true in the Mac OS since the code itself is stored as a resource). Each file in the Mac OS supported file system has some information (File Type, and Creator) added to it to allow the OS to know how to interact with it, and this information is used by the OS to know how to react when the user double clicks on the file's icon.

Most OS's offer the ability to read/write information in one of two ways to any individual file, either sequentially or by random access. Sequential access files require the user to read (or write) each byte of the file that precedes any specific byte to 'get to' the desired byte. Random access files provide a higher level of interaction with the underlying disk access hardware, and allow the user to specify any location in the file for a subsequent read/write operation to occur. The Mac OS provides calls for both of these access methods.

Some OS's offer even more sophisticated file access methods. Although this is heresy to some, the Mac OS does provide a built in data base system in the form of the Resource Manager. This data base system allows the programmer to conveniently store and retrieve up to 65,536 separate instances of over 4 billion different types of information. In practice, no one would approach those limits in an existing file, but they are theoretically possible. Apple has predefined some of the 4-Billion-plus possible resource types to hold the information needed to create many of the Mac OS interface components (such as windows, menus, and controls). Routines built into the Mac OS and Toolbox can use the information contained in these types of resources to create and manage these components.

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This Page Maintained by: radar pangaean * * * Original Author: radar pangaean
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