### TechNote

Number: 04505-01
Date: April 1994

## Physical memory and virtual memory

Physical memory comprises all the writable memory locations in the computer, most of which are on memory modules. Portions of the physical memory are reserved for the operating system, for the Initial Machine Load (IML) image that is created during the Power-On Self Test (POST) process, and for the video data that is displayed. System memory is the part of physical memory that is available for data that is used when the computer runs programs.

Virtual memory is memory that appears to be allocated to application programs. The operating system uses a portion of the hard disk as virtual memory, and swaps data between the hard disk and physical memory. Virtual memory enables multitasking. If your computer needs to run several programs simultaneously, and the memory that all these programs require exceeds the amount of physical memory available, the operating system allocates virtual memory to meet the total memory requirements of each program, and then manages the available physical memory to meet the actual memory requirements at each point in time. Therefore, the amount of virtual memory that is allocated can be much greater than the amount of physical memory that is installed in the computer.

The maximum amount of physical memory that you can install in your computer and the maximum amount of virtual memory that the operating system can allocate are determined by the type of microprocessor in your computer. The width of the microprocessor address interface limits the amount of physical and virtual memory that the microprocessor can address. A microprocessor with a 24-bit address interface can address up to 16Mb of physical memory and 1Gb of virtual memory, in protected mode. A microprocessor with a 32- bit address interface can address up to 4Gb of physical memory and 64Tb (that is, approximately 64 000 000 000 000 bytes) of virtual memory, in protected mode. For information on protected mode, refer to TechNote 04507.