- RISC (Reduced Instruction Set Computer)
Ex) ARM processor
RISC only has basic instructions and this is possible because
Most code only requires a few instructions - read/write memory, do arithmetic, jump, boolean logic, not much more.
- CISC (Complex Instruction Set Computer)
Ex) x86/x64 processors
Although almost all code can be represented by the basic instructions in RISC, certain patterns of instructions are common - for instance, "write this byte to memory then look at the immediately following byte".
CISC processors offer combo-instructions (previous example being STOSB) that handle these common instruction patterns super-efficiently.
Howerver, support for these combo-instructions requires extra hardware - and that hardware costs electricity.
- Comparision between RISC & CISC with example processor
| SoC | ARM AM3359 | Atom Z650 |
| # CORES | 1 | 1 |
| CLOCK SPEED | 720MHz | 1.2GHz |
| L1 CACHE | 64KB | 56KB |
| L2 CACHE | 256KB | 512KB |
| POWER CONSUMPTION | 0.7W | 3W |
Power consumption: RISC < CISC
ATOM can consume about 2x the electricity of the ARM processor, even at the same processor clock speed.
Because CISC needs bigger hardware than RISC
Program operating speed: RISC < CISC maybe
- Because a few set of instructions in RSIC can be hadled by one instruction in CISC