Direct IO Access

In order to achieve better performance, application programs sometimes access IO ports directly (rather than going through the OS services). As a side effect, this renders the program much more platform design-dependent. In addition, the OS is left outside the loop, so it doesn't always “know” the current state of an IO device. In a single-task OS environment this usually will not cause problems because the OS only starts one application program at a time and lets it run to completion before starting another. Because an application program can manipulate IO ports directly, application programs (and the OS) cannot make any assumptions about the current state of an IO device when they begin execution, but must always initialize all of the device's IO registers to a known state during each session.