5. What von Neumann Knew: Computer Architecture

The term Computer Architecture may refer to the entire hardware level of the computer. However, it is often used to refer to the design and implementation of the digital processor part of the computer hardware, and we focus on the computer processor architecture in this chapter.

The central processing unit (CPU, or processor) is the part of the computer that executes program instructions on program data. Program instructions and data are stored in the computer’s Random Access Memory (RAM). A particular digital processor implements a specific Instruction Set Architecture (ISA), which defines the set of instructions and their binary encoding, the set of CPU registers, and the effects of executing instructions on the state of the processor. There are many different ISAs, including: SPARC, IA32, MIPS, ARM, ARC, PowerPC, and x86 ISAs (which include IA32 and x86-64). Microarchitectures define the circuitry of an implementation of a specific ISA. Microarchitecture implementations of the same ISA can differ as long as they implement the ISA definition. For example, Intel and AMD produce different microprocessor implementations of IA32 ISA.

Some ISAs define a Reduced Instruction Set Computer (RISC), and others define a Complex Instruction Set Computer (CISC). RISC ISAs have a small set of basic instructions that execute quickly; each instruction executes in about a single processor clock cycle, and compilers combine sequences of several basic RISC instructions to implement higher-level functionality. In contrast, CISC ISA’s instructions provide higher-level functionality than RISC instructions. CISC architectures also define a larger set of instructions than RISC, support more complicated addressing modes (ways to express the memory locations of program data), and support variable length instructions. A single CISC instruction may perform a sequence of low-level functionality and may take several processor clock cycles to execute. This same functionality would require multiple instructions on a RISC architecture.

The History of RISC versus CISC

In the early 1980s, researchers at Berkeley and Stanford Universities developed RISC through the Berkeley RISC project and the Stanford MIPS project. David Paterson of Berkeley and John Hennessy of Stanford won the 2017 Turing Award1 (the highest award in computing) for their work developing RISC architectures.

At the time of its development, the RISC architecture was a radical departure from the commonly held view that ISAs needed to be increasingly complex to achieve high performance. "The RISC approach differed from the prevailing complex instruction set computer (CISC) computers of the time in that it required a small set of simple and general instructions (functions a computer must perform), requiring fewer transistors than complex instruction sets and reducing the amount of work a computer must perform."2

CISC ISAs express programs in fewer instructions than RISC, often resulting in smaller program executables. On systems with small main memory, the size of the program executable is an important factor in the program’s performance, since a large executable leaves less RAM space available for other parts of a running program’s memory space. Microarchitectures based on CISC are also typically specialized to efficiently execute the CISC variable length and higher-functionality instructions. Specialized circuitry for executing more complex instructions may result in more efficient execution of specific higher-level functionality, but at the cost of requiring more complexity for all instruction execution.

In comparing RISC to CISC, RISC programs contain more total instructions to execute, but each instruction executes much more efficiently than most CISC instructions, and RISC allow for simpler microarchitecture designs than CISC. CISC programs contain fewer instructions, and CISC microarchitectures are designed to execute more complicated instructions efficiently, but they require more complex microarchitecture designs and faster clock rates. In general, RISC processors result in more efficient design and better performance. As computer memory sizes have increased over time, the size of the program executable is less important to a program’s performance. CISC, however, has been the dominant ISA due in large part to it being implemented by and supported by industry.

Today, CISC remains the dominant ISA for desktop and many server-class computers. For example, Intel’s x86 ISAs are CISC-based. RISC ISAs are more commonly seen in high-end servers (e.g. SPARC) and in mobile devices (e.g. ARM) due to their low power requirements. A particular microarchitecture implementation of a RISC or CISC ISA may incorporate both RISC and CISC design under the covers. For example, most CISC processors use microcode to encode some CISC instructions in a more RISC-like instruction set that the underlying processor executes, and some modern RISC instruction sets contain a few more complex instructions or addressing modes than the initial MIPS and Berkeley RISC instruction sets.

All modern processors, regardless of their ISA, adhere to the von Neumann Architecture model. The general-purpose design of the von Neumann architecture allows it to execute any type of program. It uses a stored-program model, meaning that the program instructions reside in computer memory along with program data, and both are inputs to the processor.

This chapter introduces the von Neumann architecture and the ancestry and components that underpin modern computer architecture. We build an example digital processor (CPU) based on the von Neumann architecture model, design a CPU from digital circuits that are constructed from logic gate building blocks, and demonstrate how the CPU executes program instructions.

References

  1. ACM A. M. Turing Award Winners. https://amturing.acm.org/

  2. "Pioneers of Modern Computer Architecture Receive ACM A.M. Turing Award", ACM Media Center Notice, March 2018. https://www.acm.org/media-center/2018/march/turing-award-2017