HERA, the Haverford Educational RISC Architecture, is a single architecture that can span the needs of a wide variety of upper-level undergraduate applied CS courses. It is powerful enough to introduce assembly language programming in Principles of Programming Languages and serve as a target for compilers in Compiler Design, yet simple enough to be built as a student project in Principles of Computer Organization (using, for example, Jeffery P. Hansen's tkgate simulator). It has been used to support hardware/software co-design labs in our course on Operating Systems.
The use of a single simplified architecture and a familiar development environment minimizes the overhead of introducing new architectures or simulation tools, giving us more time to focus on the material we want to discuss. Students also seem to find a certain satisfaction from building, in their undergraduate courses, an entire system from the level of single-bit AND, OR, and NOT gates up through a high-level language. The HERA system was presented (by Dave W) in a poster session (not reviewed) at SIGCSE '05 and a paper (reviewed) at CCSCE '06; more details about the educational uses of HERA can be found in the CCSCE paper.
The current specification of HERA is available in either Full detail or as a Quick Reference; A printed copy can be purchased purchased as a paperback book (the quick reference is printed in a small font on the back cover) from lulu.com. (Current and some older versions can be reached by changing "-current" in the URL to be the version number with underscores instead of periods, e.g., HERA2_4_0.pdf for Version 2.4.0) As of 2020, the specification has been relatively stable for some years, and meets our current curricular needs; I thus expect future revisions to consist of minor clarifications to the documentation, and bug fixes for tools (the HERA-C simulator, the standard library for tiger, and the Hassem assembler). There is also an online list of updates and errata describing various older versions.
The HERA-C simulator is designed to make most features of HERA assembly language available in a standard C or C++ development environment. It consists of a collection of macros that are named after HERA instructions, so that a HERA program can be compiled and debugged in a C/C++ environment, like this. AA recent version is available here(email Dave W. if you want something more current).
The "Hassem" assembler is currently in "beta-test" stage and can be obtained by email request to Dave W.
I would like to thank all the students who worked with earlier version of this system for their patience, and especially Todd Miller (HC '01) for his work on HERA-C. The documentation for Version 2 of HERA was written with TeXmacs.
Documents describing the Version 1.0 HERA processor (developed and used from 2000 through the spring of 2003) are still available in postscript, PDF, and badly-formatted HTML. This web page, and the HERA specification and tools, are maintained by Dave W.
A much simpler student-buildable computer is described in Chapters 4 and 5 of Nisan and Schocken's The Elements of Computing Systems: Building a Modern Computer from First Principles (MIT Press, 2005, apparently out of print but downloadable via the link above). The full textbook covers material about programming languages and operating systems as well as hardware/architecture, perhaps a one-semester analog of our use of HERA across four semesters. My quick glance at this material suggests that the processor design is much simpler and based on register/memory operations rather than register/register; this simplicity allows one-book treatment of more topics and also allows implementation on novel hardware technologies.
Yale Pratt's Introduction to Computing Systems: From Bits and Gates to C and Beyond similarly combines material from several of our courses into a single-course form, with an architecture that is considerably simpler than HERA.
The RISC-V project addresses the other end of the scale: a processor architecture that is much more sophisticated than that of HERA, suitable for both study in courses that explore considerable detail beyond the Haverford curriculum, or manufacture and use in actual systems.