Memory is hardware that is used by computer to load the operating system and run programs. It is buildup of RAM chip that has different memory modules. The amount of main memory in a computer is limited to the amount of RAM that has installed. Generally memory sizes are 256 MB, 512 MB, & 1 GB, because of computer has limited amount of RAM. When too many programs are simultaneously it is possible to run a program out of memory. This is the concept where virtual memory comes. Virtual memory enhance the available memory of a computer has by enlarging the address space or place in memory where data can be stored. Hard disk is used for additional memory allocation .However, since secondary storage is much slower than the RAM, program which is in Virtual Memory must be mapped back to virtual memory in order to be used. The process of mapping data and forth between the hard disc and RAM takes longer than accessing it directly from the memory. It means virtual memory is increased, the more it will slow your computer down. While virtual memory enables your computer to run more than one program it could, otherwise it is the best way to having as main memory as possible.