RISC architecture

RISC architecture
Konstantin Lanzet (with permission) · CC BY-SA 3.0 · source
Name	RISC architecture
Designer	John Cocke, David Patterson, John L. Hennessy
Bits	32-bit, 64-bit
Introduced	1980s
Design	RISC
Type	Register-Register
Encoding	Fixed
Endianness	Bi-endian

RISC architecture. Reduced Instruction Set Computer architecture is a design philosophy for central processing units that emphasizes a small, highly optimized set of simple instructions. This approach contrasts with the complex instruction set computing paradigm and emerged from research at institutions like IBM and the University of California, Berkeley. The design principles, championed by researchers such as David Patterson and John L. Hennessy, have profoundly influenced modern computing, from embedded systems to supercomputers.

Overview

The core concept of this design philosophy is to execute simple instructions rapidly, typically within a single clock cycle, by leveraging a pipeline (computing). This requires instructions to be of uniform length and to use a load/store architecture, where only specific instructions access computer memory. This regularity simplifies the control unit and enables more advanced microarchitecture techniques. The approach was a direct reaction to the increasing complexity of Cray-1 and VAX designs, aiming for greater efficiency and performance through hardware simplicity.

Design principles

Key principles include a limited number of instruction formats and addressing modes, enforcing a strict separation between operations that perform arithmetic logic unit functions and those that handle memory access. All operations, except explicit load and store commands, are performed on processor registers. This design heavily relies on compiler technology, such as optimizing compilers developed for the GNU Compiler Collection, to schedule instructions efficiently and maximize instruction-level parallelism. The use of a large register file helps reduce the frequency of slower memory accesses.

Historical development

The foundational ideas were explored in the 1970s through projects like the IBM 801 minicomputer, led by John Cocke at the Thomas J. Watson Research Center. The term itself was coined by David Patterson during the Berkeley RISC project, which produced designs like the RISC-I microprocessor. Concurrently, the Stanford MIPS project under John L. Hennessy advanced similar concepts. These academic efforts directly led to commercial ventures, including MIPS Computer Systems and the founding of Sun Microsystems, which utilized the SPARC processor family.

Comparison with CISC

The primary alternative, Complex Instruction Set Computing, is exemplified by the x86 architecture used in Intel and Advanced Micro Devices processors. CISC designs incorporate multi-cycle, variable-length instructions that can perform operations directly on memory, contrasting with the fixed-length, register-centric approach. While early debates, often called the "RISC versus CISC" wars, were heated, modern implementations like those from Apple Inc. for the Apple silicon or ARM Holdings for the ARM architecture have blurred the lines, with many CISC processors using internal RISC-like microcode cores.

Implementations and examples

Prominent commercial implementations began with the Acorn Computers ARM architecture, which later became dominant in mobile devices and embedded systems. The PowerPC architecture, developed by the AIM alliance between Apple Inc., IBM, and Motorola, powered Macintosh computers for years. Other significant examples include the MIPS architecture, the SPARC processors by Sun Microsystems, and the RISC-V open-standard instruction set architecture. Modern high-performance designs from Qualcomm and Apple Inc. for system on a chip products continue this legacy.

Impact and influence

The philosophy revolutionized the semiconductor industry, enabling the development of efficient, low-power processors critical for the proliferation of smartphones and tablet computers. Its principles are foundational to dominant architectures like ARM architecture, which powers devices from Samsung Electronics and Google. The design approach also influenced the evolution of x86 processors from Intel and the development of graphics processing units from Nvidia and Advanced Micro Devices. The open-standard RISC-V initiative, supported by organizations like the Linux Foundation, continues to extend its influence across computing sectors.

Category:Computer architecture Category:Central processing unit