Lisp Machine - Technical Overview

Technical Overview

Initially the Lisp Machines were designed as personal workstations for software development in Lisp. They were used by a single person and offered no multi-user mode. The Lisp Machines provided a large, black and white, bitmap display, keyboard and mouse, network adapter, local hard disks, more than 1 MB RAM, serial interfaces, and a local bus for extension cards. Color graphics cards, tape drives, and laser printers were optional.

The processor did not run Lisp directly, but was a stack machine with instructions optimized for compiled Lisp. The early Lisp Machines used microcode to provide the instruction set. For several operations type checking and dispatching was done in hardware at runtime. There was for example only a single addition operation that could be used with various numeric types (integer, float, rational and complex numbers). The result was a very compact compiled representation of Lisp code.

The following example uses a function that counts the number of elements of a list for which a predicate returns 'true'.

(defun example-count (predicate list) (let ((count 0)) (dolist (i list count) (when (funcall predicate i) (incf count)))))

The disassembled machine code for above function (for the Ivory microprocessor from Symbolics):

Command: (disassemble (compile #'example-count)) 0 ENTRY: 2 REQUIRED, 0 OPTIONAL ;Creating PREDICATE and LIST 2 PUSH 0 ;Creating COUNT 3 PUSH FP|3 ;LIST 4 PUSH NIL ;Creating I 5 BRANCH 15 6 SET-TO-CDR-PUSH-CAR FP|5 7 SET-SP-TO-ADDRESS-SAVE-TOS SP|-1 10 START-CALL FP|2 ;PREDICATE 11 PUSH FP|6 ;I 12 FINISH-CALL-1-VALUE 13 BRANCH-FALSE 15 14 INCREMENT FP|4 ;COUNT 15 ENDP FP|5 16 BRANCH-FALSE 6 17 SET-SP-TO-ADDRESS SP|-2 20 RETURN-SINGLE-STACK

The operating system used virtual memory to provide a large address space. Memory management was done with garbage collection. All code shared a single address space. All data objects were stored with a tag in memory, so that the type could be determined at runtime. Multiple execution threads were supported and called 'processes' – all processes were running in the single address space.

All operating system software was written in Lisp. Xerox used InterLisp. Symbolics, LMI and TI used Lisp Machine Lisp (descendant of MacLisp). With the appearance of Common Lisp, Common Lisp was supported on the Lisp Machines and some system software was ported to Common Lisp or later written in Common Lisp.

Some later Lisp Machines (like the TI MicroExplorer, the Symbolics MacIvory or the Symbolics UX400/1200) were no longer complete workstations, but boards designed to be embedded in host computers: Apple Macintosh II and SUN 3 or 4.

Some Lisp Machines, like the Symbolics XL1200, had extensive graphics capabilities using special graphics boards. These Lisp Machines were used in domains like medical image processing, 3D animation and CAD.