Why Aren't RISC CPU Architectures Run at Much Higher Frequencies Than CISC?

The question of why RISC (Reduced Instruction Set Computer) CPUs aren't run at much higher frequencies than CISC (Complex Instruction Set Computer) CPUs is an intriguing one, rooted in intricate details of how these processors operate. This article explores the reasons behind this phenomenon, focusing on the limitations posed by clock rates, cache and memory access, and the evolution of CPU architectures.

The Limitations of Clock Rates

The primary limitation to increasing clock rates in CPUs is the speed at which data can be retrieved from the L1 cache and the speed at which operations (such as addition) can be performed. If a faster clock rate results in more cycles being required to access memory or complete addition operations, it won't necessarily result in faster performance. This concept is illustrated by the challenge faced by the Pentium 4 (P4) architecture, which had difficulties in achieving higher performance due to its memory and addition operations taking more cycles than expected.

Modern RISC and CISC Architectures

Modern RISC and CISC architectures have evolved significantly, breaking the traditional boundaries of these terms. Contemporary RISC processors incorporate complex instructions such as integer multiplication, addition, and multiple load/store operations. These operations can often execute in a single cycle, thanks to the advanced design of modern RISC cores. Similarly, CISC architectures have adapted by splitting their internal instructions into micro-operations (micro-ops) that are essentially RISC-like in nature, often executing in a single cycle as well.

The Historical Context and Performance Disparities

Back in the early days of RISC and CISC, performance disparities were more pronounced. There was a rule of thumb that the proper clock frequency for a computer is roughly 10 gate delays. For example, a computer built with 10 nanosecond TTL gates would yield a 10 MHz machine, while a machine with 1 nanosecond ECL gates would produce a 100 MHz Cray 1 computer.

This historical context highlights that the technology itself, not the architecture, often sets the clock speed. In the 1990s, when RISC processors were first gaining prominence, gates were expensive to use, making RISC architectures more cost-effective and enabling them to perform more instructions per clock cycle. However, these differences have diminished over time as CISC architectures have learned to translate their complex instructions into RISC-like micro-operations, allowing them to often execute in a single cycle, similar to RISC processors.

Conclusion

The evolution of RISC and CISC architectures has blurred the lines between the two, resulting in a landscape where both types of processors can operate at comparable frequencies. The key factors influencing this have been the complexity of instructions, how memory and operations are handled, and the underlying technology of the gates used to build the CPUs. As technology continues to advance, the distinctions between RISC and CISC are becoming increasingly less significant, with both types of architectures converging in terms of performance and clock speeds.

Keywords: RISC CPU, CISC CPU, clock frequency, CPU architecture, memory access