CISC and RISC

In short: CISC processors have many, partly complex instructions, RISC processors few, simple and uniform ones. Today the difference blurs.

In more detail: x86 (Intel, AMD) counts as CISC, ARM and RISC-V as RISC. Modern x86 chips break their instructions into RISC-like micro-operations internally.

In Depth

• CISC (“complex instruction set computer”): One instruction can read memory, compute and write back. Advantage: compact code. Disadvantage: complex decoder, variable instruction length.
• RISC (“reduced instruction set computer”): Only load and store touch memory (load/store architecture), instructions mostly have fixed length. Advantage: simple pipelining, frugal.
• Today: x86 decodes into micro-ops, ARM cores grow many special instructions. Implementation, manufacturing and software decide, not the label.
• Consequence: ARM dominates phones and is arriving on laptops and servers. x86 holds the desktop and server installed base.
• Software: Programs must be compiled per instruction set. Emulation and binary translation (Rosetta) bridge gaps.

See also: CPU structure