80-Bus News

Introduction

References have been made in 80-Bus News (and its predecessor) to “UCSD” and in the last issue of 1982, S. Monger mentioned that I had implemented it for the Nascom then confessed not knowing what it is. Working on the assumption that many other readers will not know what it is I am writing this short article to explain the origins, subsequent development and current state of the UCSD p-system and its implementation for 80-Bus computers. You may also find it worth your while to read a series of three articles in PCW (July – September, 1982) and an article in the first issue of Computer Answers (Nov/​Dec 1982).

What is UCSD?

Many people know of UCSD Pascal. It was the first full-scale Pascal implemented on micro-computers (and was adopted by Apple as “Apple Pascal”). The original Pascal compiler was then used to develop a complete operating system for microcomputers that is not just a rival to CP/M but contains many features which CP/M users can only dream of. Subsequently other compilers (Fortran, BASIC, Lisp, APL, Modula-2 and the new INMOS language Occam) have been added and the only remaining attachment to Pascal is now historical (and the fact that the use of Pascal naturally influenced that history).

I was first introduced to UCSD two years ago and was so impressed that, after a year of uselessly wishing that someone would buy me some disk drives or sell an automatic cassette deck with UCSD as its operating system, when I finally got my disk drives, I decided to forget CP/M and go straight for UCSD on my Nascom and have been actively using it now for nearly a year.

The major and, I believe, unique feature of the system is its use of “p-code” or pseudo-code. P-code was originally specified as a pseudo-machine code to run on a hypothetical stack-oriented processor. This was to facilitate the compilation of Pascal in a single pass. (Subsequently a real microprocessor, the Western Digital Microengine, was developed to run p-code as its native code.) This p-code is subsequently executed by a p-code interpreter which is coded in the native code of the microprocessor being used. This use, of p-code gives an incredible portability to UCSD software. As long as a p-code interpreter exists, any program compiled into p-code (including the whole of the UCSD operating system) will run on any microprocessor! Except in special circumstances, the same programs that run under UCSD on an Apple, IBM PC, Sirius, Sage II, etc. will also run on a Nascom or Gemini without change – and vice versa.

This sort of portability is impossible under CP/M without resorting to expensive “Z80 cards”. (And just look at the mess CP/M has got into in upgrading to 16-bit: they have even had to produce their OWN Z80 card for the IBM PC to allow users to run their old software!)

Of course this portability has been bought at a price. P-code, because it is interpreted, runs slower than native code. Supporters of CP/M-based Pascal compilers have been quick to point out that the Pascal Benchmarks published in PCW run quicker under Z80 compilers than p-code compilers. Of course, the importance of execution speed is tremendously exaggerated by these critics. (During the last year I have not once been adversely affected by slow execution of p-code.) However, even if it were important, such critics are now out of date. There now exists a Native Code Generator which can translate a p-code program, wholly or partly, into Z80 native code. The resultant code file can then be saved and treated as any other p-code file – except that during execution, those passages which are time-critical will now run at native code speed. With UCSD,