ARM Cortex-A8: Whats the difference between VFP and NEON

Question

In ARM Cortex-A8 processor, I understand what NEON is, it is an SIMD co-processor.

But is VFP(Vector Floating Point) unit, which is also a co-processor, works as a S

Answer 1

Architecturally, VFP (it wasn't called Vector Floating Point for nothing) indeed has a provision for operating on a floating-point vector in a single instruction. I don't think it ever actually executes multiples operations simultaneously (like true SIMD), but it could save some code size. However, if you read the ARM Architecture Reference Manual in the Shark help (as I describe in my introduction to NEON, link 1 in the question), you'll see at section A2.6 that the vector feature of VFP is deprecated in ARMv7 (which is what the Cortex A8 implements), and software should use Advanced SIMD for floating-point vector operations.

Worse yet, in the Cortex A8 implementation, VFP is implemented with a VFP Lite execution unit (read lite as occupying a smaller silicon surface, not as having less features), which means that it's actually slower than on the ARM11, for instance! Fortunately, most single-precision VFP instructions get executed by the NEON unit, but I'm not sure vector VFP operations do; and even if they do, they certainly execute slower than with NEON instructions.

Hope that clears thing up!