CISC vs. RISC

Before getting into the nuances of CISC and RISC, we must have a general idea of what CISC and RISC are. Simply put, they are the two types of computer, each processing information differently. But how do the work, and why are they different?

CISC

In the beginning, there was CISC, which stands for Complex Instruction Set Computers. RISC, Reduced Instruction Set Computers did exist, but were less affordable and feasible and were thus considered inferior to CISC because of these faults. Thus, for a while, CISC was the primary type of computer.

As hinted at by the name, each CISC instruction given to the computer did quit a bit. With lack of memory in computers at the time, this worked well: less time worrying about little things saved in memory.

As computers increased speed in 1980's, people failed to realize that CISC computers were holding speeds down. After all, CISC had always been used, and computers were getting faster.

Yet, as RISC computers, which did less with each instruction, began to come out of the closet, it was realized that CISC had many liabilities. Specialized, complex instructions allowed less freedom in programming and memory allocation.

The Advent of RISC

As computers quickly evolved, changes were in the favor of RISC. These included the following:

Caches to speed instruction fetches
Dramatic memory size increases/cost decreases
Better pipelining
Advanced optimizing compilers

It became evident that compilers preferred simple instructions, and less time was spent coding in assembly language, thus the bucolic code inherent to CISC was no longer necessary. Machine code was now for the machine, not the programmer

Caches decreased the time it took to get to memory. Thus the program length increase that results from the use of RISC, with all its simpler instructions, was now acceptable.

Another problem that became evident for CISC was its difficulty to keep up with improvements in pipelining. As myriad pipelines were used in parallel, instructions were passed more quickly, CISC instructions made it difficult to use pipelines because of differing lengths of instructions. For example, when instructions are being shuffled between several pipelines, if each varies in length, then instructions will soon come out of order, the next necessary in instruction stuck in a pipeline behind a very long instruction. RISC avoided this problem by keeping every instruction at the same length, making it easier for instructions to be pipelined in parallel.

Because of these factors, it became evident that RISC was the way to go. While CISC still exists, it looks more and more like RISC

Characteristics of RISC Computers

While not all RISC machines are the same, they do have many similar qualities.

As mentioned, instructions are of a uniform length.

Instruction sets were also streamlined to contain important and efficient instructions, refraining from instructions that copied higher level languages or were simply interesting.

The method by which memory was addressed was simplified. Any complex references were split up into several instructions.

The number of registers was increased and used to hold frequently-used variables. Thus RISC processors have at least 16 and as many as 64 registers.

Finally, delayed branches were created. In other words, when a branch in the code the computer was processing occurred (that is, there is an unanticipated call to another section of the program that necessitates another call to memory), instead of stalling the pipeline to retrieve the instructions that were branched to, the computer added a delaying instruction behind the branch that either did something useful while the other instructions were called, or at worst did nothing at all.

So What?

Well, now that we have looked at the most basic tenets of RISC computers, you're probably wondering what it has to do with HPC. RISC is central to HPC because it is the way a computers works with instructions to execute them to go more quickly and is designed, and refined, to go faster, which is necessary for huge and complex applications. From the earliest days of CISC to the present, refinement in design has allowed computers to do larger and larger things faster.

Of course, there is much more to HPC than RISC.

List of computer changes taken from Dowd

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