Special issue on
specialized
computer architecture simulators that see the present and may hold the future
Authors: Yurcik William (Bill) 
Published: October 2002

Welcome to this special issue of the
Journal of Educational
Resources in Computing
(JERiC), which is focused on the topic
of
specialized
computer architecture simulators, and is part
of back-to-back JERiC issues describing the state-of-the-art in
computer architecture simulators for educational purposes. JERiC is
a unique forum, in that the simulation software corresponding to
each of the articles in this issue is available for download and
execution.

The guest editorial in the previous issue of JERiC described how
these back-to-back special issues came to be dedicated to computer
architecture simulators. In this issue I would like to focus on
promoting creativity as an important part of being an educator.
While the general von Neumann architecture still predominates as it
has for the last halfcentury, there are clearly new, advanced
technologies on the horizon. While part of our role as educators is
to help students master existing technology, we must also nurture
the seeds of creativity that may someday break the current
paradigms. While it is unclear what the future may hold -- perhaps
biological, molecular, or quantum computing -- some clues can be
found in current advanced, specialized research. Just as it is
recursive to teach how a computer works by simulating a virtual
computer on a computer, we can also learn from past and present
specialized areas to begin developing the future.
In this issue we present interactive simulation tools for
teaching the specialized areas of computer architecture. The
articles span from digital logic simulators (Logisim) to historic
machine simulators (Atari-6502, PDP-8), to a real machine simulator
(DEC Alpha/KScalar), to an embedded system (LegoSim), to parallel
computer architectures (High Performance Computer Architecture and
Algorithm Simulator). Although these simulators are clearly useful
for teaching general computer architecture, they are perhaps most
valuable for specialized topics in advanced or elective
courses.
This issue begins with Logisim, a visual simulator for building
and testing digital logic circuits. While general computer
architecture survey courses have abstracted away from lower-level
digital logic circuits, this teaching tool is invaluable for
computer engineering or electrical engineering courses that focus
specifically on logic design. Logisim can be used hierarchically,
and is powerful; the author notes that he has successfully created
a simple 8-bit CPU in the system.
The use of computer simulators provides a number of secondary
benefits -- the most pertinent is highlighted in the next article
on the PDP-8 Emulator. Simulators make it feasible to interact with
computers that no longer exist physically (except possibly in a
museum). Thus, obsolescence is no longer the dominant obstacle in
teaching computer architecture. An extinct machine is oftentimes
the best example of an architectural concept or may have the best
educational documentation. A perfect example of an historic
architecture to study is the PDP-8. The PDP-8 was a breakthrough
12-bit machine in 1965, to which many of our modern architectural
features can be traced. Through this emulator, students can now
interact with a virtual PDP-8, instead of simply reading about it
in a textbook.
The next two articles are related because they both supplement
seminal textbooks in the field by Hennessy and Patterson [1996] and
Patterson and Hennessy [1997]. While the DLX, MIPS, and SPIM series
of simulators are well documented, SLOOP-SMOK and KScalar are new
simulators that complement these established tools. The SLOOP-SMOK
toolkit emulates an Atari 6502 (a good choice for student
motivation) integrated with a machine organization simulator. The
KScalar simulator simulates the DEC Alpha with a wide range of
possible scalar performance enhancements such as pipelining,
caching, and speculative execution.
LegoSim is an important contribution because it focuses on the
simulation of an embedded system. Embedded systems, defined as a
component in a larger system that relies on its own processor, have
been around for decades. However, it was not until the early 1980s,
when 16-bit processors performing sophisticated specialized
applications began to appear in automobiles, that designers
realized the embedded system's almost limitless possibilities for
ubiquitous everyday products [Wolf 2002]. However, at present,
almost all of our educational focus is still on general-purpose
computers, even though the number of specialized embedded systems
currently dwarfs traditional computing machines by orders of
magnitude (a fact that we have ignored in part), and this gap is
increasing. A recent study by the National Academy of Science
documents this development [Computer Science and Telecommunications
Board 2001]. It is my hope that LegoSim will be the first in a long
line of teaching tools for this rapidly expanding area of
study.
The final article in this issue simulates the tradeoffs of
efficiency and performance inherent in parallel architectures. The
High-Performance Computer Architecture and Algorithm Simulator
allows students to design and interact with different forms of
parallelism such as multiple pipelines, multiple processors,
clustered computer systems, and distributed computing systems.
While the promise of parallel computing has yet to be realized on a
commercial scale (to be fair, parallel programming is at least
equally to blame), research continues, and students who learn by
using this tool may make the difference.
The underlying thread weaving through all these articles is the
interaction with the details of computer architecture: how a
computer processes bits at the lowest level (LogiSim), why
computers developed the way they did (KScalar, PDP-8, SLOOP-SMOK),
the future capabilities of embedded processors (LegoSim), and how
to make computers work together (High-Performance Computer
Architecture and Algorithm Simulator). In this issue it is our
intent that the specialized computer architecture simulators
described here provide powerful teaching tools to facilitate and
enhance student creativity for developing the next paradigms of
computer architecture.
Finally, I have very much enjoyed serving JERiC and ACM in my
capacity as guest editor, and I would like to repeat some
acknowledgements from my previous guest editorial, lest anyone
think I assembled this issue myself. I would like to thank JERiC
Co-Editors-in- Chief Lillian (Boots) Cassel and Ed Fox, Deborah
Knox of the Computer Science Teaching Center
(http://www.cstc.org/), and Jono Hardjowirogo of the ACM Press. I
would also like to especially thank the many authors and
peer-reviewers with whom I have traded email (about 60 people);
they have been responsive, patient, and accommodating. This issue
would not have been possible without them.
A special word to the authors of the articles and simulators we
declined: acceptance decisions were particularly difficult (60% of
submissions were declined). But I encourage you to persevere - this
is truly a growth area and your contributions are needed. Again, I
W. Yurcik hope you enjoy what we have assembled and that it
inspires many ideas for teaching and research!