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Steele and Sussman == Introduction ==

An idea which has increasingly gained attention is that computer architectures should reflect specific language structures to be supported. This is an old idea; one can see features in the machines of the 1960’s intended to support COBOL, FORTRAN, ALGOL, and PL/I. More recently research has heen conducted into architectures to support string or array processing as in SNOBOL or APL.

An older and by now well-accepted idea is that of the stored-program computer. In such a computer the program and the data reside in the same memory; that is, the program is itself data which can be manipulated as any other data by the processor. It is this idea which allows the implementation of such powerful and incestuous software as program editors, compilers, interpreters, linking loaders, debugging systems, etc.

One of the great failings of most high-level languages is that they have abandoned this idea. It is extremely difficult, for example, for a PL/I (PASCAL, FORTRAN, COBOL …) program to manipulate PL/I (PASCAL, FORTRAN, COBOL …) programs.

On the other hand, many of these high-level languages have introduced other powerful ideas not present in standard machine languages. Among these are (1) recursively defined, nestad data structures; and (2) the use of functional composition to allow programs to contain expressions as well as (or instead of) statements. The LISP language in fact has both of these features. It is unusual among high—leve1 languages in that it also explicitly supports the stored-program idea: LISP programs are represented in a standardized way as recursively defined, nested LISP data structures. By contrast with some APL implementations, for example, which allow programs to be represented as arrays of characters, LISP also reflects the structure of program expressions in the structure of the data which represents the program. (An array of APL characters must be parsed to determine the logical structure of the APL expressions represented by the array. Similar remarks apply to SNOBOL statements represented as SNOBOL strings.)

It is for this reason that LISP is often referred to as a high-level machine language. As with standard stared-program machine languages, programs and data are made of the same stuff. In a standard machine, however, the stuff is a homogeneous, linear (ordered) vector of fixed-size bit fields; a program is represented as an ordered sequence of bit fields (instructions) within the overall vector. In LISP, the stuff is a heterogeneous, unordered set or records linked to form lists, trees, and graphs; a program is represented as a tree (a parse tree or expression tree ) of linked records (a subset of the overall set of records). Standard machines usually exploit the linear nature of the stuff through such mechanisms as indexing by additive offset and linearly advancing program counters. A computer based on LISP can similarly exploit tree structures. The counterpart of indexing is component selection; the counterpart of linear instruction execution is evaluation of expressions by recursive tree-walk.