IEEE Spectrum July, 2011 - 48

Ammann wrote a Pascal compiler and output operations, allowing you, for
Urs Ammann, a student of Wirth's at the
Swiss Federal Institute of Technology that generated what he called "p-code" example, to access data in the middle of a
Zurich, had recently devised the per- for a p-machine, which was used to bring file without having to read everything that
fect solution: Create a "p-machine" (the Pascal to several different mainframes. came before it first. The group also added
something called units, which
p stands for pseudo), which
were modules of code, each
is to say a virtual machine,
with a well-defined interface
in software.
to other system components.
To appreciate what such
Units were a step toward moda virtual machine is, you
ern object-oriented design, in
need to understand somewhich the building blocks of a
thing about how computer
program are cohesive, reusable
programming is done. The
software modules that packcentral processing unit of a
age data and code together.
computer carries out very
In addition, the UCSD
simple instructions-add
p-System included a fi le hanone number to another,
dler, a debugger, and a text
that sort of thing. These
editor for writing programs-
instructions are encoded
a l l d e ve lop e d i n P a s c a l
and passed to the procesand compiled into p-code.
sor in the form of binary
Graduate student Richard
numbers, a sequence of 1s
Kaufmann wrote the editor,
and 0s.
one of the earliest to offer a
Wr it i ng a pr o g r a m
full screen of text and a curdirectly in machine code
sor that could be moved anyis tedious in the extreme.
where on the page using key
People generally program
commands. These software
in higher-level languages,
tools included drop-down
whose instructions are
menus, which appeared again
considerably easier to
much later in Apple's Lisa
understand: "If A equals
and Macintosh computers.
B , execute com ma nd
Although the initial UCSD
C," for instance. Special
p-System was developed for a
software translates such
PDP-11 minicomputer, some of
high-level instructions
Bowles's students also impleinto the machine code
mented a p-machine for microthat the processor actucomputers that used Zilog's
ally runs. That special
Z80 microprocessor. The first
software is typically a
test of the system's portability,
compiler that is specific
carried out soon after that chip
to the lang uage you're
was introduced in July 1976,
using to write the proRAINBOW COALITION: Ken Bowles teamed up with Apple computer,
was an ambitious one: running
gram and the machine which sold ucSD pascal (rebranded as Apple pascal) for its Apple ii
the p-code for Kaufmann's text
you want it to run on. A microcomputer, as evidenced by this colorful syntax chart for the language.
editor on a Z80. It operated
compiler that works on
flawlessly on the first attempt.
a Windows computer, for
example, will not compile your pro- Bowles recognized the beauty of this The UCSD p-System was later ported
approach, and his UCSD team adopted to work with other chips, including the
gram to run on an iPad.
The UCSD p-System included a com- this strategy for use on micro- and MOS 6502 (of Apple II fame), the Motorola
piler that translated high-level instruc- minicomputers. UCSD's p-machine was 68000 (the original Macintosh chip), and
tions into a machine code of sorts-but not slimmed down in some ways compared the Intel 8086 family of microprocessors
one that would run on any computer of the with Ammann's to meet the limitations (used in the first IBM PCs).
day. The code was designed for a machine of the many small computers of the time.
that didn't exist. A processor could in prin- By early 1976, Bowles's students had proowles and his students hoped
ciple be built to run this flavor of machine grammed the fi rst UCSD p-machine in
the p-System would revolutioncode, but that wasn't the idea. Rather, the assembly language on a PDP-11.
ize how someone got an education.
To Wirth's relatively bare-bones defi- But for that to happen, they needed to get
strategy was to write another piece of software that could translate this code on the nition of Pascal, the UCSD group added their software out to the world, which
fly into the current machine's native oper- variables that could represent charac- was no small trick in the pre-Web era.
ations. With such software, your com- ter strings (up to that point, the available
Their first plan was to distribute the
puter-no matter what kind of processor kinds of variables could hold only num- software themselves. Starting in 1978,
it had at its heart-could become, at least bers or at most one character each). The UCSD students copied the p- System
researchers included more powerful input onto floppy disks and sent them to anyvirtually, a p-machine.
48

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JEF RASKIN

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Table of Contents for the Digital Edition of IEEE Spectrum July, 2011