Section 7 ½ Language-Based Computers 487

The HP 9845A

Desk-top calculators have evolved over the years from "function per key" systems with limited I/O to desk-top computers with a general-purpose programming language and extensive I/O (see Chaps. 48 and 49 and Part 4, Sec. 3). The HP 9845A is a sophisticated single-language, single-user computing system (see Figs. 1 and 2) that supports Type 2(b) interactive interpretation. Since the user environment is well defined, it is possible to customize techniques found in more open-ended computer structures such as timesharing systems.

The HP 9845A has three levels of abstraction. The highest level is a BASIC machine which deals with abstractions such as processes, run queues, symbol tables, and internal forms for language statements. These internal forms allow for more efficient execution than source language interpreters, since the statement