Size of Files

Computer hard disks can hold much, much more information that main memory of a computer. A typical desktop computer has one hard disk that can hold 100 billion bytes. A byte is roughly the amount of memory needed to hold one character. Usually a disk is divided into tens of thousands of files. The main memory of a desktop computer is usually only about one billion bytes.

Main frame computers have much larger capacity than desktop computers. Their main memory may be several billion bytes, and their hard disks may hold many trillions of bytes.

The disadvantage of disk files is that access to the data in them is much slower than access to data in main memory (data in arrays and variables). Usually programs are written to read in data from files, process that data using main memory and variables, then finally write the results back to a file.