However, the WIN16 interface was never designed to permit isolation. When a WIN16 program writes something on the screen, it doesn't call another process, address space, or subsystem. It calls a subroutine that helps the program to display the data directly on the display. Nothing except good manners prevents a program from writing on a part of the screen that belongs to another program. Screen savers depend on this.
Every program is authorized.
BIOS and DOS calls are useful, but optional. DOS is a set of services that the program calls to do something it could do itself, but does not want to bother with. WIN16 is explicitly designed as a set of subroutines that cooperating programs call to coordinate their direct use of system hardware resources. But WIN16 assumes that the programs all run in a common memory and share the control blocks that determine where the window are on the screen and which window is on top. Running all the programs together with a common set of control blocks saves memory and speeds up processing.