Editing UCSD Pascal (section)

==The p-System==
In 1977, the University of California, San Diego (UCSD) Institute for Information Systems developed UCSD Pascal to provide students with a common environment that could run on any of the then available [[microcomputer]]s as well as campus [[Digital Equipment Corporation|DEC]] [[PDP-11]] [[minicomputer]]s. The operating system became known as UCSD p-System.

There were three operating systems that [[IBM]] offered for its original [[IBM Personal Computer|IBM PC]]. The first was UCSD p-System, with [[IBM PC DOS]] and [[CP/M-86]] as the other two.<ref name="williams198201">{{cite news | url=https://archive.org/stream/byte-magazine-1982-01/1982_01_BYTE_07-01_The_IBM_Personal_Computer#page/n37/mode/2up | title=A Closer Look at the IBM Personal Computer | work=BYTE | date=January 1982 | access-date=19 October 2013 | author=Williams, Gregg | pages=36}}</ref> Vendor [[SofTech|SofTech Microsystems]]<ref>{{Cite web|url = http://bitsavers.trailing-edge.com/pdf/softech/Softech_IV_AppleII_Mar83.pdf|title = SOFTECH MICROSYSTEMS UCSD p-SYSTEM VERSION IV FOR THE APPLE II COMPUTER}}</ref> emphasized p-System's application portability, with virtual machines for 20 CPUs as of the IBM PC's release. It predicted that users would be able to use applications they purchased on future computers running p-System;<ref name="edlinbunnell19820203">{{cite news | url=https://books.google.com/books?id=w_OhaFDePS4C&pg=PA42 | title=IBM's New Personal Computer: Taking the Measure / Part One | work=PC Magazine | date=February–March 1982 | access-date=20 October 2013 |author1=Edlin, Jim |author2=Bunnell, David | pages=42}}</ref> advertisements called it "the Universal Operating System".<ref name="byte198208">{{cite news | url=https://archive.org/stream/byte-magazine-1982-08/1982_08_BYTE_07-08_Logo#page/n193/mode/2up | title=This person develops applications for the entire market, including the Z-80, 8080, 8086/8088, 6502, LSI-11/PDP-11, 9900, and the M68000 with the best there is. | work=BYTE | date=August 1982 | access-date=19 October 2013 | author=Advertisement | pages=191, 193–195}}</ref>

''[[PC Magazine]]'' denounced UCSD p-System on the IBM PC, stating in a review of [[Context MBA]], written in the language, that it "simply does not produce good code".<ref name="zachmann198306">{{cite news | url=https://books.google.com/books?id=14Kfbrc6cbAC&pg=PA123 | title=Context MBA: Half A Step In The Right Direction | work=PC Magazine | date=June 1983 | access-date=21 October 2013 | author=Zachmann, Mark S. | pages=123}}</ref> The p-System did not sell very well for the IBM PC, 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 [[IBM Displaywriter System|IBM Displaywriter]], an [[Intel 8086|8086]]-based dedicated word processing machine. (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 ''[[Unit (Software Development)|Units]]'' and a ''String'' type. 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-hosting (compilers)|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.{{clarify|date=October 2010}} The UCSD Pascal p-Machine was optimized for the new small microcomputers with addressing restricted to 16-bit (only 64 KB of memory). [[James Gosling]] cites UCSD Pascal as a key influence (along with the [[Smalltalk]] virtual machine) on the design of the [[Java (programming language)|Java]] virtual machine.<ref>{{cite web
 |url        = http://queue.acm.org/detail.cfm?id=1017013
 |title      = A Conversation with James Gosling 
 |last       = Allman
 |first      = Eric
 |date       = 1 July 2004
 |work       = [[ACM Queue]]
 |publisher  = [[Association for Computing Machinery]]
 |access-date = 24 December 2012
}}</ref>

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&mdash;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,<ref>{{Cite journal | last1 = Ammann | first1 = U. | title = On code generation in a PASCAL compiler | doi = 10.1002/spe.4380070311 | journal = Software: Practice and Experience | volume = 7 | issue = 3 | pages = 391–423 | year = 1977 | hdl = 20.500.11850/68668 | s2cid = 2143405 | hdl-access = free }}</ref> 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, Apple Computer offered a Fortran Compiler<ref>{{Cite web|url = http://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Software/Languages/Apple%20II%20FORTRAN/Manuals/Apple%20FORTRAN%20Language%20Reference%20Manual.pdf|title = Apple Fortran Language Reference Manual}}</ref> (written by Silicon Valley Software, Sunnyvale California) producing p-code that ran on the Apple version of the p-system. Later, TeleSoft (also located in [[San Diego]]) offered an early [[Ada (programming language)|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 later [[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.