How did Windows 3.1’s virtual machine manager get the information to show in the text-mode Alt+Tab switcher?

Raymond Chen

When I told the life story of the Switch­To­This­Window function, some people were curious about how the virtual machine manager knew about the current windows and their z-order, so that it could show them in the fake text-mode Alt+Tab switcher.

The virtual machine manager has a friend, namely the Windows program that provided the graphical user interface for the virtual machine, named WINOLDAP. It got that name because it is the program which provides the Windows user interface for old (MS-DOS) applications. If the MS-DOS virtual machine is running windowed, then this program’s main window draws a graphical depiction of the contents of the virtual machine’s screen. If the MS-DOS virtual machine is running full-screen, then this program’s main window doesn’t need to draw anything, but it is still there, ready to help out the virtual machine manager.

If you press the Alt+Tab hotkey while in a full-screen MS-DOS virtual machine, the virtual machine manager receives the hotkey and tries to emulate the graphical Alt+Tab user interface.

The text-mode Alt+Tab code asks the virtual machine scheduler to switch execution context to the Windows virtual machine, and then call it back once this has occurred.

In 16-bit Windows, the Post­Message function is special in that it is one of the few Windows function that could be called from a hardware interrupt handler. The callback function (now running in the context of the Windows virtual machine) saves the current virtual machine context, and then creates a new execution context that calls the Post­Message function, which from the virtual machine client’s point of view looks like a hardware interrupt in the sense that the function is being called out of the blue, from an unknown context. After the call to Post­Message function returns, the callback function restores the original context and allows execution to continue normally.

The message is posted to the WINOLDAP window, and when it receives the message, it gathers information about the next window in the Alt+Tab list, as well as information about the user’s color preferences. It then issues a kernel call into the text-mode Alt+Tab code, passing this information along.

The text-mode Alt+Tab code uses the color information to render a text-mode version of the Alt+Tab user interface, showing the name of the window being proposed as the switch destination.

Program Manager

If the user completes the Alt+Tab sequence, and the destination is a full-screen MS-DOS virtual machine, then the text-mode Alt+Tab switcher performs a direct virtual machine switch to the target. Performing the switch directly avoids bouncing through the Windows desktop. It then returns control back to WINOLDAP with a return value that means “You’re all done, do nothing more.”

If the destination is not a full-screen MS-DOS virtual machine, then the text-mode Alt+Tab code returns control back to WINOLDAP with a return value that means “Switch to this window and return,” and in response, WINOLDAP calls the Switch­To­This­Window function to tell the window manager to switch to the window in the style of Alt+Tab.

If the user continues the Alt+Tab sequence, either by tabbing forward or holding the Shift key to tab backward, then the text-mode Alt+Tab code returns control back to WINOLDAP with a return value that means “Get information about the next/previous window in the Alt+Tab order and call me back.”

The communication between the text-mode Alt+Tab code and WINOLDAP basically takes the form of a captive thread, where the WINOLDAP function issues a kernel call which doesn’t return until the user decides what they want to do next with the Alt+Tab sequence.

It’s a rather complicated dance (all written in assembly language, which was the fashion at the time) to accomplish something that looks so simple and obvious to the end user.