Although Windows primarily exists to run new programs specifically designed for the environment, Windows can also run many non-Windows MS-DOS programs. The Windows User's Guide refers to these as ”standard applications,“ but many Windows programmers call them ”old applications“ or ”old apps.“
These MS-DOS programs can be divided into two broad categories: Well-behaved applications (or ”good old apps“) are those that use the MS-DOS and PC ROM BIOS (basic input/output system) software interrupts to read the keyboard and write to the video display. These programs can generally run in a window.
”Bad apps“ are those that write directly to the video display, use graphics, or take control of the hardware keyboard interrupt. The term ”bad“ here refers not to the quality of the program—many of the best programs written for the PC are bad apps when it comes to Windows—but to the way in which the program uses the hardware of the PC. When running on a 286-based machine, there is simply no way Windows can allow such a program to be windowed or multitasked. However, Windows can use the ”virtual 86“ mode of the 386 microprocessor to window and multitask even bad applications.