At the end of each file’s data section is a FCB that contains specific information about the data file. The FCB starts immediately after the last frame in the data section of the file. These frames contain information about the file, including index pointers and their compiled a (algebraic)-processing codes.
Important flags are also contained in the FCB, as are fields indicating that the file is open, closed or in the process of being deleted, and so on. This protects the system from deleting a file while another process is using that file.
The FCB is used for internal housekeeping and is not meant to be edited.
Items are added to a file using an algorithm to calculate a location within the file, called hashing, where the item is to be inserted. These spots are called groups. A group can hold more than one item depending on the size of the group and the size of the item.
As the file fills up, several items can come to rest in the same group and the space in that group is completely used up. At this point, additional frames are retrieved from the overflow table and appended to that group.
The initial allocation of disk space is called primary file space, and items overflow into secondary file space. The speed of a system is reduced if the system has to traverse a lot of secondary file space to find an item. Files can be resized to make this process more efficient.
Transporting a file to another system with a different frame size can affect the system’s performance.
See Also
Data Input and Output Specifications