16-bit Windows Modes:

16-bit versions of Windows (which basically means Windows 3.x and below) had
three modes which you could run them in: Real Mode, Standard Mode, and (get
ready) 386 Enhanced Mode. You could pick one by adding the appropriate switch
to the "win" command, like this:

win /r: Real Mode.
win /s: Standard Mode.
win /3: 386 Enhanced Mode.

Real Mode basically uses the same memory model as DOS. It is intended for
two purposes: 1. To provide maximum backward compatibility with Windows
applications for previous versions of Windows. 2. It is the only mode usable
for computers with less than 1 MB of RAM. Real Mode uses only the normal 640K
of conventional memory, and will not directly access extended memory.
Standard Mode (which, as the name implies, was meant to be the "normal"
operating mode for Windows) allows usage of extended memory, and also lets
you run more than one non-Windows application. Standard Mode requires at
least a 286 CPU, and 1 MB of RAM. 386 Enhanced Mode is the only mode which
32-bit versions of Windows (Win95 and up) will run in. It allows use of
virtual memory (which in turn allows the use of more memory than the computer
actually has, achieved using a disk-based swap file as virtual RAM). And,
thanks to the protected-mode capabilities of the 386, this mode also allows
multitasking among non-Windows applications by creating a "virtual machine"
for each one. (Other modes will suspend all other applications while a
non-Windows application is active.) It also lets you run non-Windows
applications in a window. (Other modes let you run non-Windows applications
in full-screen mode only.) Obviously, 386 Enhanced Mode requires at least a
386 CPU; It also requires at least 1 MB of RAM (though 2 MB is strongly
recommended).