The discussion asks what kinds of Lisp programs can run when the entire system - including its runtime stack and cons pool - must fit in 64K bytes. It sets that limit against early machines: the MIT IBM 704 had 32K 36-bit words, with Lisp using 12K words and leaving space for just under 20,000 cons cells. Many 704s had far less memory and could not run Lisp. A 1964 PDP-1 implementation could run with 4096 18-bit words; its roughly 2000-word system left room for about 2000 cells. Historical Lisp examples show how programs were expressed and entered on those machines.
The practical focus is Kilo LISP, a portable ANSI C implementation small enough to compile as a tiny-model DOS program within the 64K limit. Its system takes about 13K bytes, leaving room for 8192 cons cells, each five bytes. A short association-list function illustrates the style of code it can support. By comparison, Kilo LISP offers fewer cells than a well-equipped IBM 704 but about four times as many as a minimal PDP-1. Although those original machines were extremely slow by modern standards, execution time is set aside: the central concern is how much Lisp functionality and data fit in a very small memory, with subsequent examples intended to convey the experience of programming under that constraint.
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