3.11 Windows 3.1 uses the services of DOS, the DOS device drivers, and the BIOS to access disk files and some hardware devices. Because DOS and BIOS only operate on buffers below a megabyte, this typically involves moving data between Windows memory above a megabyte and the DOS 640K area. Windows then has to return to the ordinary DOS mode of execution in order to make the DOS or BIOS call using the INT instruction.
Results have to be reflected back to the application program running in Windows memory.
Windows does not use DOS services for the display, mouse, or keyboard. Its use of these devices is much more sophisticated than DOS drivers can support. An application program needs to read a file on disk. Under Windows 3.1, the request would be passed on to DOS through the INT 21 interface. If the file was on a local hard disk, DOS would interpret the FAT directory, locate the file, and read the sectors off disk using BIOS calls.