Improving the performance of UCSD Pascal via microprogramming on the PDP-11/60

Abstract

UCSD Pascal is implemented as a machine-independent virtual machine. This virtual machine is usually interpreted on a host computer by machine language instructions, which, in turn, are often interpreted by microcode. Performance of a virtual machine is generally a function of the number of levels of interpretation required to perform a computation. By bypassing one or more of these layers, it should be possible to enhance performance.To test this hypothesis, a Pascal interpreter was written for the PDP-11/60. After analysis of the Pascal machine, changes were made that interpreted selected portions of the virtual machine directly in microcode.This paper describes the microcode which was added, and discusses the improvements which resulted.