What do RISC and CISC stand for and what are the differences in practice?

Answer 1

RISC stands for Reduced Instruction Set Computer CISC stands for Complex Instruction Set Computer RISC processors have a simpler set of machine instructions than CISC processors, but are intended to be more efficient and flexible in processing programs and data. Because of the reduced instruction set, RISC processors demand more effort on the part of the compiler (not the end user) and use longer machine language programs. RISC processors are more flexible in the sense that they are not as preconfigured in how they process programs and data. CISC microprocessors actually preceded RISC microprocessors and were derived from microcontroller based mini-computers such as the HP2100. See the Wikipedia entries on "Complex Instruction Set Computer" and "Reduced Instruction Set Computer" for more.

Answer 2

RISC is Reduced Instruction Set Computer. CISC is Complex Instruction Set Computer.

RISC has fewer instructions and a simpler architecture. The tradeoff is performance, because the RISC is often so much faster than a CISC that the end result is better performance.

The IA-32 architecture presently used in most PC's is a CISC. The IBM 9113-550 is an example of a RISC.

CISC stands for Complex Instruction Set Computer.

CISC chips are relatively slow per instruction,but use fewer instructions. Optimized for the programmer.

RISC stands for Reduced Instruction Set Computer.

A fewer, simpler and faster instructions would be better, than the large, complex and slower CISC instructions. However, more instructions are needed to accomplish a task.

It's easier to write powerful optimized compilers, since fewer instructions exist.

Consider multiplying two numbers, then adding the same two numbers. A CISC machine will get the two values from memory, perform the multiplication, then store the result. It would then have to re-retrieve the same two values from memory and perform the addition and store the result. A RISC machine would pull the two values from memory, perform the multiplication, then store the result, but an optimizer would see that the values are already local to the processor and would skip re-retrieving these values from memory thus saving a number of internal clock cycles, and the more expensive memory access cycles (in which the processor would have to idle).

CISC stands for Complex Instruction Set Computer.

CISC chips are relatively slow per instruction,but use little instructions

RISC stands for Reduced Instruction Set Computer.

A fewer, simpler and faster instructions would be better, than the large, complex and slower CISC instructions. However, more instructions are needed to accomplish a task.

It's easier to write powerful optimised compilers, since fewer instructions exist.

CISC RISC Emphasis on hardware Emphasis on software Includes multi-clock

complex instructions Single-clock,

reduced instruction only Memory-to-memory:

"LOAD" and "STORE"

incorporated in instructions Register to register:

"LOAD" and "STORE"

are independent instructions Small code sizes,

high cycles per second Low cycles per second,

large code sizes Transistors used for storing

complex instructions Spends more transistors

on memory registers