[myrandomcomment]

The 68K lacked an MMU, so cooperative multitasking was really the only way to do it. Same reason MacOS and AmigaOS were cooperative multitasking.

[vidarh]

The only thing you need to achieve pre-emptive multitasking is interrupts and the ability to cleanly save the current CPU state.

The 68k lacked the ability to resume with full state intact after a bus fault, which made an off-chip MMU painful (but there was one - the MC68451[1]), but this doesn't affect the ability to do pre-emptive multitasking at all.

AmigaOS famously did have preemptive multitasking - we used it to mock PC and Mac users with for years.

[1] https://en.wikipedia.org/wiki/Motorola_68451 Note that to do full virtual memory with a 68k, Motorola proposed using a second 68k to handle page faults due to a design flaw:

https://retrocomputingforum.com/t/correcting-errors-by-dupli...

[steve1977]

There were also alternatives to the Atari ST TOS which feature preemptive multitasking, like

https://en.m.wikipedia.org/wiki/MagiC

and

https://en.m.wikipedia.org/wiki/MiNT

[monocasa]

You can preemptively schedule without an MMU just fine, just like there's nothing stopping multiple threads in the same address space from being preemptively scheduled.

[spogbiper]

Microware OS9 implemented preemptive multitasking on the motorola 6809 without an MMU back in 1980. You don't have memory protection without an MMU, but you can have preemptive multitasking.

[mikepavone]

As another commenter pointed out, you can do pre-emptive multitasking just fine without an MMU. And as it turns out AmigaOS had just that. All you need for pre-emptive multitasking is a suitable interrupt source to use for task switching.

What it did not have was memory protection or virtual memory. You do need an MMU for those.

[mben]

AmigaOS had preemptive multitasking.