Explainer
An instruction set defines the operations software can ask a processor to perform. CISC designs include instructions that can express relatively complex work, and a processor may implement some of them through smaller internal operations or microcode. The instruction seen by a programmer and the work performed inside the chip are related, but they are not the same layer.
The key ideas
- An instruction set is the software-visible interface.
- Decoding is part of processor operation.
- Microcode can implement complex instruction flows.
Explore the subject
Original recording: TikTok, 2020-12-03.
Video transcript
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Automatically transcribed from the original recording. Small transcription errors may remain.
Intel and AMD make SysC processors, but what are they and how do they work? Well, SysC stands for Complex Instruction Set Computer. And in the late 70s, memory was expensive as fuck. And thus, a workaround came on by creating really powerful CPU instructions, which could do many different things. This changed a game for programmers who were already dealing with crappy compilers. And now, they could more easily code by utilizing single instructions to do exactly what they wanted. However, things started to get complicated after a while as programmers needed decoders for all these instructions.
One way the industry solved this was with microcode. For each machine code instruction in the instruction set architecture, you would have a little program stored in special memory known as read-only memory, or ROM for short, which is inside the CPU, made up of much smaller instructions called microcode. Because ROM's utilization was much cheaper at the time than random access memory or RAM, you had a great market workaround.
