CPU Architecture: RISC vs CISC

You probably have both RISC (Reduced Instruction Set: phone, tablet etc.) and CISC (Complex Instruction Set: laptop, desktop) computers.

What's an Instruction?

Instructions are small programs implemented in the CPU hardware. These instructions do things like moving data, comparing numbers, adding, multiplying etc.

Note: Apple Silicon

Since 2020, Apple has moved from CISC to RISC architecture for its laptops and desktops. So, if you have an Apple computer, it may contain a RISC CPU (labeled M1, M2, etc...)

CISC CPUs are traditionally produced by companies like Intel and AMD.
RISC CPUs are traditionally designed by companies like ARM, or are increasingly open-source (RISC-V).
CISC CPUs need lots of transistors to represent the many complex instructions available.
In contrast, RISC CPUs have fewer, simpler instructions, and don’t need many transistors. Because they don't need as many transistors, RISC CPUs are simpler, physically smaller, and consume less power.
In addition to having fewer instructions, RISC CPUs have short fixed-length instructions. Short instructions run faster (that is, you can run more instructions per clock cycle). RISC's fixed-length instructions take the guesswork out of predicting how many clock cycles each instruction needs, which reduces overhead for the CPU.
By comparison, CISC instructions are variable length, so the processor can't predict how many clock cycles an instruction needs until it evaluates that instruction, which takes time.
So, RISC fixed-length instructions require less overhead to process and this speeds everything up.
A disadvantage of RISC is that it requires more of these short instructions to write a program and you need more RAM to hold more instructions.

The surprising implications of small size and simplicity

RISC designs are simple and use fewer transistors, so the chips use less energy, don't get as hot, and can thus be smaller. Surprisingly, even though electrons travel through these circuits at the speed of light, you can get a lot more done if the components are closer together! So, the smaller the chip, the faster the chip.

SOC (System on a Chip): RISC processors are so small that it is possible to build a single chip that has the CPU, memory, input-output, network and other interfaces. This is called a SOC and is what you have in your phone or tablet.

Caution! CISC architecture is better supported

There is a long history of designing computer software (like Word, Excel, Matlab etc.) for CISC architecture. Much of this software did not initially work on Apple computers after Apple shifted to RISC. The situation for Apple silicon continues to improve, but, if you choose RISC architecture, make sure the programs you want to run are compatible!

Watch this 3-minute video comparing RISC and CISC designs.

Hardware 4 (10 pts): Configuration