Editing Lisp machine (section)

===Initial development===

In 1973, [[Richard Greenblatt (programmer)|Richard Greenblatt]] and [[Tom Knight (scientist)|Thomas Knight]],<!-- I can't seem to get it straight which of the two had primacy in development here. To be safe, I include both, since both seem to have done substantial work regardless of primacy. --> programmers at [[Massachusetts Institute of Technology]] (MIT) [[MIT Artificial Intelligence Laboratory|Artificial Intelligence Laboratory]] (AI Lab), began what would become the MIT Lisp Machine Project when they first began building a computer hardwired to run certain basic Lisp operations, rather than run them in software, in a 24-bit [[tagged architecture]]. The machine also did incremental (or ''Arena'') [[garbage collection (computer science)|garbage collection]].{{citation needed|date=May 2013}} More specifically, since Lisp variables are typed at runtime rather than compile time, a simple addition of two variables could take five times as long on conventional hardware, due to test and branch instructions. Lisp Machines ran the tests in parallel with the more conventional single instruction additions. If the simultaneous tests failed, then the result was discarded and recomputed; this meant in many cases a speed increase by several factors. This simultaneous checking approach was used as well in testing the bounds of arrays when referenced, and other memory management necessities (not merely garbage collection or arrays).

Type checking was further improved and automated when the conventional byte word of 32&nbsp;bits was lengthened to 36&nbsp;bits for [[Symbolics]] 3600-model Lisp machines<ref>{{cite journal |title=Architecture of the Symbolics 3600 |journal=ACM SIGARCH Computer Architecture News |volume=13 |issue=3| pages=76–83 |first=David A. |last=Moon |author-link=David A. Moon |doi=10.1145/327070.327133 |publisher=Portal.acm.org |year=1985 |s2cid=17431528}}</ref> and eventually to 40&nbsp;bits or more (usually, the excess bits not accounted for by the following were used for [[error correction and detection|error-correcting codes]]). The first group of extra bits were used to hold type data, making the machine a [[tagged architecture]], and the remaining bits were used to implement [[CDR coding]] (wherein the usual linked list elements are compressed to occupy roughly half the space), aiding garbage collection by reportedly an order of magnitude. A further improvement was two microcode instructions which specifically supported Lisp [[subroutine|functions]], reducing the cost of calling a function to as little as 20 clock cycles, in some Symbolics implementations.

The first machine was called the CONS machine (named after the list construction operator <code>[[cons]]</code> in Lisp). Often it was affectionately referred to as the ''Knight machine'', perhaps since [[Tom Knight (scientist)|Knight]] wrote his master's thesis on the subject; it was extremely well received.{{Citation needed|date=March 2007}} It was subsequently improved into a version called CADR (a pun; in Lisp, the <code>[[cAR and CDR|cadr]]</code> function, which returns the second item of a list, is pronounced {{IPA|/ˈkeɪ.dəɹ/}} or {{IPA|/ˈkɑ.dəɹ/}}, as some pronounce the word "cadre") which was based on essentially the same architecture. About 25 of what were essentially prototype CADRs were sold within and without MIT for ~$50,000; it quickly became the favorite machine for hacking{{snd}} many of the most favored software tools were quickly ported to it (e.g. [[Emacs]] was ported from [[Incompatible Timesharing System|ITS]] in 1975{{Disputed inline|date=March 2019}}). It was so well received at an AI conference held at MIT in 1978 that [[Defense Advanced Research Projects Agency]] (DARPA) began funding its development.