{"id":105979,"date":"2021-11-29T07:00:00","date_gmt":"2021-11-29T15:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=105979"},"modified":"2021-11-29T05:43:52","modified_gmt":"2021-11-29T13:43:52","slug":"20211129-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20211129-00\/?p=105979","title":{"rendered":"How did Windows 3.1’s virtual machine manager get the information to show in the text-mode Alt<\/KBD>+Tab<\/KBD> switcher?"},"content":{"rendered":"

When I told the life story of the Switch\u00adTo\u00adThis\u00adWindow<\/code> function<\/a>, 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<\/kbd>+Tab<\/kbd> switcher.<\/p>\n

The virtual machine manager has a friend, namely the Windows program that provided the graphical user interface for the virtual machine, named WINOLDAP<\/code>. 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.<\/p>\n

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

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

In 16-bit Windows, the Post\u00adMessage<\/code> 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 Post\u00adMessage<\/code> 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 Post\u00adMessage<\/code> function returns, the callback function restores the original context and allows execution to continue normally.<\/p>\n

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

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

\n
Program Manager<\/div>\n<\/div>\n

If the user completes the Alt<\/kbd>+Tab<\/kbd> sequence, and the destination is a full-screen MS-DOS virtual machine, then the text-mode Alt<\/kbd>+Tab<\/kbd> 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<\/code> with a return value that means “You’re all done, do nothing more.”<\/p>\n

If the destination is not a full-screen MS-DOS virtual machine, then the text-mode Alt<\/kbd>+Tab<\/kbd> code returns control back to WINOLDAP<\/code> with a return value that means “Switch to this window and return,” and in response, WINOLDAP<\/code> calls the Switch\u00adTo\u00adThis\u00adWindow<\/code> function to tell the window manager to switch to the window in the style of Alt<\/kbd>+Tab<\/kbd>.<\/p>\n

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

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

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.<\/p>\n","protected":false},"excerpt":{"rendered":"

Getting by with a little help from a friend.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[2],"class_list":["post-105979","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-history"],"acf":[],"blog_post_summary":"

Getting by with a little help from a friend.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/105979","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=105979"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/105979\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=105979"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=105979"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=105979"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}