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Blit isn't content with just being a graphical user interface with multiple windows and right-click context menus. The windows can also overlap, and the system employs, true to its UNIX base, full multitasking. The video below demonstrates how you could edit code in a text editor, switch to a different windowed terminal and execute make, and play Asteroid in another overlapping window while the code compiled. Further along the video, it shows an application for integrated circuit design with a debugger running in a separate window.

It ran on a Motorola MC68000 processor in 1982, but would be ported to the Western Electric WE32000 microprocessor so it could be used on commercially available machines like the AT&T 5620 starting in 1984. It was created by Rob Pike and Bart Locanthi, and detailed in this paper on the subject. For instance, this is how windows (or layers, as they call them) were implemented:

Interestingly enough, the goal behind creating Blit was to make graphical user interfaces run on less powerful, cheaper hardware than the Alto. "The original idea behind the development of the Blit hardware was to provide a graphics machine with about the power of the Xerox Alto, but using 1981 technology (large address space microprocessors, 64K RAMs and programmed array logic) to keep size, complexity and particularly cost much lower," the paper notes. This may remind you of some other projects developed at around the same timeframe - you know, independent groups coming to the same conclusions because they're working in the same constraints set by available hard and software.

The Blit was connected to a timeshared host through RS-232, but this wasn't a big problem due to its relatively powerful hardware. It was also small and "portable", and could even be used on 1200 baud connections from the engineers' own homes; not as smoothly as on higher-speed connections, of course, but "a Blit at 1200 baud is much nicer than a regular terminal at 1200 baud".

Despite being a relatively early graphical user interface, it already sports some interesting ideas. For instance, inactive windows receive a dotted texture to indicate they are not the currently selected window. Furthermore, Blit doesn't have focus-follows-mouse:

The conventions around mouse operations are also incredibly fascinating. Blit uses a three-button mouse, set to the following conventions:

The mouse cursor is modal; it will change its appearance depending on the actions one can perform. "For example, when the user selects New on the mpxterm menu, the cursor switches to an outlined rectangle with an arrow, indicating that the user should define the size of the layer to be created by sweeping the screen area out with the mouse," the paper explains, "Similarly, a user who has selected the Exit menu entry is warned by a skull-and-crossbones cursor that confirmation is required before that potentially dangerous operation will be executed."

There's a whole boatload of interesting stuff in the paper about Blit, and it's definitely recommended reading - easy to read, too. There's also a detailed FAQ on the AT&T 5620, the commercially available machine from 1984 which ran Blit.

This is once again more proof that the industry heavily studied, implemented, and experimented on graphical user interfaces in the '70s and early '80s. It also shows that, unlike what some want you to believe, it wasn't just one company that saw the value in bringing the graphical user interface to the masses - even UNIX people at Bell Labs saw its potential and ran with it. You wouldn't believe it from reading about entitled corporations competing in courtrooms, but it almost seems like this is how the technology industry has always worked.

Truly fascinating stuff, and you owe it to yourself to dive into it.