> 25. Processes
> The Lisp Machine supports multi-processing; several computations can be executed "concurrently" by placing each in a separate process. A process is like a processor, simulated by software. Each process has its own "program counter", its own stack of function calls and its own special-variable binding environment in which to execute its computation. (This is implemented with stack groups, see chapter 12, page 163.)
These computers could send/receive network traffic, handle the GUI, read from disk, while the user would have processes for each application and window.
On my mid 1980s Symbolics 3640 I could compile a Lisp application from scratch in a Lisp REPL for half an hour, while I was using Zmacs to edit code. The practical limitations were CPU speed, memory size, disk speed and the Garbage collector. A full GC would halt everything else. Better incremental GCs helped a lot.
In GNU Emacs last I tried I could not use two IELM repls executing Lisp code at the same time. One would block the rest of GNU Emacs Lisp code.
The Lisp Machine only had one processor for the Lisp Machine, and it did absolutely support multi-processing and multi-threading.
A good explanation can be found in the Lisp Machine manual:
https://tumbleweed.nu/r/lm-3/uv/chinual.html#Processes
https://tumbleweed.nu/r/lm-3/uv/chinual.html#Stack-Groups