Protected mode is a mode of program operation in a computer with an Intel-based microprocessor in which the program is restricted to addressing a specific contiguous area of 640 kilobytes. Intel's original PC microprocessor, the 8088, provided a one megabyte (1 Mbyte) random access memory (RAM). The memory was divided into several areas for basic input/output system data, signals from your display, and other system information. The remainder or 640 kilobytes of contiguous space was left for the operating system and application programs. The 8088 ensured that any instruction issued by a program running in protected mode would not be able to address space outside of this contiguous 640 kilobytes. Typically, much operating system code and almost all application programs run in protected mode to ensure that essential data is not unintentionally overwritten.
Real mode is program operation in which an instruction can address any space within the 1 megabyte of RAM. Typically, a program running in real mode is one that needs to get to and use or update system data and can be trusted to know how to do this. Such a program is usually part of the operating system or a special application subsystem.
As new microprocessors (such as the 80386) with larger RAM followed the 8088, DOS continued to preserve the 640 kilobyte addressing limitation so that newly-written application programs could continue to run on both the old as well as new microprocessors. Several companies developed DOS "extenders" that allowed DOS applications to be freed from the 640K constraint by inserting memory management code into the application. Microsoft developed the DOS Protected Mode Interface to go with a DOS extender included with Windows 3.0 (which was itself a DOS application). Microsoft later gave the standard to an industry organization, the DPMI Committee.
Today's personal computers, using microprocessors that succeeded the 8088, typically contain eight or more megabytes of RAM. Today's operating systems (including the latest DOS versions) come with extended memory management that frees the programmer from the original addressing constraints.
Besides the DPMI standard interface, two other standard extended memory management interfaces exist. Extended Memory Specification (XMS) is a program added to more recent versions of DOS and Windows when the system is loaded. It's called HIMEM.SYS. Another extended memory manager is the Virtual Control Program Interface (VCPI). A copy of the DPMI Specification is viewable at the Tenberry Web site.