What are Radeon stream processors and SIMD engines?

Question

Top GeForce video cards have 480 GUDA cores (60 multicores by 8 stream each). They are like general purpose CPUs, but with specialized instructions, registers and memory working modes, and are oriented to float-point processing.

Top Radeon video cards usually have less speed than GeForce cards (at two times less price!). However, the official specs state that they have 1408 stream processors and 22-24 SIMD engines (by 16 streams each).

What are stream processors, and what does SIMD engine mean? Is a SIMD engine like CUDA cores? Are stream processors exactly the same as ALUs?

Answer 1

From some AMD presentation:

Each SIMD unit contains 16 stream cores, and each stream core houses five processing elements. Thus, each compute unit in the ATI Radeon HD 5870 has 80 (16 × 5) processing elements.

Analogue for CUDA core is Stream core, and SIMD unit is analogue to nVidia's Streaming Multiprocessor.

The difference between CUDA core and Stream core is that CUDA core consists of one processor, though is capable of performing only one operation at a time, and Stream core has 5 processors and can simultaneously perform up to 4 'simple' operations (like addition/multiplication), but only for vector types (like int4, float4). For scalar types 4 of this 5 processors wait while one does all the work.

The main difference in performance is not due to number of cores, but rather due to how this cores are made. nVidia's GPUs are better for floating point operations (so they are used in most scientific applications), when ATI's GPUs are unsurpassed in field of integer and bitwise operations, and are used for cryptography.