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Custom Processor Using an FPGA for Undergraduate Computer Architecture Courses

Abstract

The field programmable gate array (FPGA) and modern computer aided design tools provide new opportunities in teaching computer architecture. This paper presents a simple yet nontrivial Von Neumann style computer architecture and corresponding implementation suitable for an undergraduate course in computer architecture. The processor architecture itself is worthy of study, including such features as subroutines, stack relative addressing, interrupts, and conditional branching. The processor is able to pre-fetch with some instructions and provides performance comparable to traditional small microprocessors such as the Motorola/Freescale 68HC11. The architecture and implementation documents are written so that several options are possible for introducing nod4 into the classroom curriculum. In particular, students may investigate the nod4 processor or implement the processor themselves. It is also possible to present the processor architecture entirely without the implementation.

Introduction

This paper presents a simple yet nontrivial Von Neumann style computer architecture and corresponding implementation that undergraduate students may implement as a soft-core processor. Engineers are continually called upon to make decisions regarding what is appropriate for a given application. The grand vision serves as a north-star to inspire and help the designer in making decisions regarding a given architecture. The nod4 processor is designed to be a tool for teaching introductory computer architecture principles to undergraduates. The nod4 motto is, “simple yet nontrivial.” It is classic accumulator based Von Neumann style architecture. The design strives for clarity and is transparent so nothing is hidden from the student. It has an 8 bit address bus and primarily supports unsigned 8 bit integer math.

Relevant references include Mano and Kime1 as well as Tanenbaum2. To implement nod4 the target technology is the field programmable gate array (FPGA). Other than switches, light emitting diodes, and the clock oscillator, the nod4 processor system is implemented entirely in a FPGA. Students are provided with VHDL modules used to make schematic symbols. In this way students use register level or higher schematics. The development tools include a simulator for examining the system cycle by cycle behavior.

Providing support to software is an important concern to processor design. Compiler generated machine code makes use of only a few addressing modes and is generally supported by certain processor hardware features. The nod4 architecture has a stack, uses subroutines, and includes stack relative addressing which helps in passing parameters. Other than the possibility of a very simple executive, we have no interest in supporting a formal operating system. At the very least, to perform a context switch requires direct access to the processor stack.

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Hill, J. (2007, June), Custom Processor Using An Fpga For Undergraduate Computer Architecture Courses Paper presented at 2007 Annual Conference & Exposition, Honolulu, Hawaii. 10.18260/1-2--1665