[Someone]

I remember reading about an OS where processes weren’t basic building blocks. Instead it had a syscall to create an address space and to create a thread in an address space.

Create a thread in your own address space, and your process becomes multi-threaded. Create an address space, load some code in it, and create a thread there, and you fork/exec-ed.

In my memory, that OS was MACH, but Google doesn’t confirm that for me.