From the 1981 4th edition Lisp Machine Manual, page 428ff > 25. Processes > The Lisp Machine supports multi-processing; several computations can be executed "

From the 1981 4th edition Lisp Machine Manual, page 428ff > 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.