What's the difference between SIMD and SSE?

Question

I am confused, what's the difference between SIMD and SSE, SSE2, SSE3, AVX etc?

According to my knowledge and research, SIMD is architecture which allows for a Single Instruction to operate on multiple data and SSE, AVX are instruction sets which implement a SIMD architecture.

And also is there a difference between vector sizes of each architecture like SSE has 128 bits and AVX has 256 bits? If the underlying SIMD architecture is the same (I think), then how do different ISAs have different vector sizes?

I'm not sure if this is true, can someone explain to me in detail what actually happens?

Answer 1

The Wikipedia page (http://en.m.wikipedia.org/wiki/SIMD) does a good job of explaining SIMD, and the instruction sets that implement it.

Single instruction, multiple data (SIMD), is a class of parallel computers in Flynn's taxonomy. It describes computers with multiple processing elements that perform the same operation on multiple data points simultaneously.

SIMD is the 'concept', SSE/AVX are implementations of the concept. All SIMD instruction sets are just that, a set of instructions that the CPU can execute on multiple data points. As long as the CPU supports executing the instructions, then it is feasible for multiple SIMD instruction sets to coexist, regardless of data size.