Originally Posted by Fred the Duck
I am not sayng the Lisa was first, only that the Amiga was not. Though Amiga scored first in the pre-emptive arena. I believe the OS Xerox used was called SmallTalk.
There were at least four programming languages for the Xerox Alto, Smalltalk was a verbose-ish object-oriented language.
The Alto was about as multi-tasking as the first Mac: It could do certain things like talk to I/O and screen while running code (due to its hardware), and you could have more than one app open, but when you switched apps, the former app would stop executing.
True multi-tasking is in book defined as being able to run several apps at one time; preferably as many as the memory can allow for. Two would do it for me - that's "multi" all right. It would also have to do that on a single processor - having one CPU for each task is just a matter of hardware configuration. To be a multi-tasking _OS_ also requires an environment the tasks use to let the user operate the computer.
The Alto was a revolution because it had these things:
1. WIMP (Windows, Icons, Mouse and Pointer)
2. Object-oriented programming (AND os!)
3. WYSIWYG (portrait screen, hires bitmap)
4. Networking (yes, Ethernet!)
It was never a personal computer though, since it wasn't commercially available - no person could buy it, only institutions. (I remember researching the First Personal Computer and coming up with a Very early French computer, but I can't remember its name right now... bah.)
I know IBM mainframes served dozens of users running their programs concurrently, before the Alto. But I don't know if more than one program was run on one CPU - what those mainframes looked like inside is unknown to me. And it probably wasn't an OS per se; probably just a main program switching time slots between more than one user.
Some site lists UNIX as the first multi-tasking OS - in 1969. Wouldn't surprise me.