UCSD Pascal - UCSD Pascal and The P-System

UCSD Pascal and The P-System

UCSD Pascal was developed at The University of California, San Diego Institute for Information Systems in 1978 to provide students with a common operating system that could run on any of the then available microcomputers as well as campus DEC PDP-11 minicomputers. UCSD p-System (Version IV, supplied by SofTech) was one of three operating systems (along with PC-DOS and CP/M-86) that IBM offered for its original IBM PC; but the p-System never sold very well for the IBM PC, mainly because of a lack of applications and because it was more expensive than the other choices. Previously, IBM had offered the UCSD p-System as an option for Displaywriter, an 8086-based dedicated word processing machine (not to be confused with IBM's DisplayWrite word processing software.) (The Displaywriter's native operating system had been developed completely internally and was not opened for end-user programming.)

Notable extensions to standard Pascal include separately compilable Units and a String type. Both of these extensions influenced the design of the Ada language. Some intrinsics were provided to accelerate string processing (e.g. scanning in an array for a particular search pattern); other language extensions were provided to allow the UCSD p-System to be self-compiling and self-hosted.

UCSD Pascal was based on a p-code machine architecture. Its contribution to these early virtual machines was to extend p-code away from its roots as a compiler intermediate language into a full execution environment. The UCSD Pascal p-Machine was optimized for the new small microcomputers with addressing restricted to 16-bit (only 64KB of memory). James Gosling cites UCSD Pascal as a key influence (along with the Smalltalk virtual machine) on the design of the Java virtual machine.

UCSD p-System achieved machine independence by defining a virtual machine, called the p-Machine (or pseudo-machine, which many users began to call the "Pascal-machine" like the OS—although UCSD documentation always used "pseudo-machine") with its own instruction set called p-code (or pseudo-code). Urs Ammann, a student of Niklaus Wirth, originally presented a p-code in his PhD thesis, from which the UCSD implementation was derived, the Zurich Pascal-P implementation. The UCSD implementation changed the Zurich implementation to be "byte oriented". The UCSD p-code was optimized for execution of the Pascal programming language. Each hardware platform then only needed a p-code interpreter program written for it to port the entire p-System and all the tools to run on it. Later versions also included additional languages that compiled to the p-code base. For example, TeleSoft (also located in San Diego) offered an early Ada development environment that used p-code and was therefore able to run on a number of hardware platforms including the Motorola 68000, the System/370, and the Pascal MicroEngine.

UCSD p-System shares some concepts with the more current Java platform. Both use a virtual machine to hide operating system and hardware differences, and both use programs written to that virtual machine to provide cross-platform support. Likewise both systems allow the virtual machine to be used either as the complete operating system of the target computer or to run in a "box" under another operating system.

The UCSD Pascal compiler was distributed as part of a portable operating system, the p-System.