The network driver (as well as Windows and non-Windows applications) may require network support software to actually carry out network requests. In many cases, a network driver carries out a request by calling corresponding functions in MS-DOS, the NetBIOS, or in real-mode network device drivers or terminate-and-stay-resident (TSR) programs. Since the network driver runs in protected mode, extra measures are required to call this real-mode software. Furthermore, since Windows can switch between applications while network requests are being processed, measures are required to prevent loss and corruption of data as a result of a switch.
The following sections describe network support in standard- and 386 enhanced-mode Windows.