SIMD 代表Single Instruction Multiple Data实际上是 Flynn 分类中的一类并行计算机。它概述了具有多个处理元件的计算机，可以同时对多个数据点执行相同的操作。
并且，粒度是将系统分解为各种小部分的概念，我们可以说系统本身或系统的描述/观察。当一个更大的实体被细分为各个部分时，它实际上是相关的。例如，一个情节被分成几码以获得比仅仅说一个情节更精细的粒度。
SIMD(单指令多数据)可以分为多种类型，但 SIMD 的 2 种主要和最重要的类型是：
(i) 细粒度 SIMD：
这些实际上是处理更小的组件的详细描述，这些组件实际上由更大的组件组成。
(ii) 粗粒度 SIMD：
这些系统由更少的组件组成，这些组件显然比原始组件多，但比细粒度 SIMD 小得多，但组件的大小比系统的细粒度子组件大得多(高/更多)。
细粒度和粗粒度 SIMD 架构的区别：
| S.No. | Fine-Grained SIMD | Coarse-Grained SIMD |
|---|---|---|
| 1. | Fine Grain SIMD have less computation time then the coarse grain architecture. | Coarse Grain SIMD have more computation time then the Fine grain architecture. |
| 2. | Here, programs are broken into large number of small tasks. | Here, programs are broken into small number of large task. |
| 3 | Fine Grain SIMD have much higher level of parallelism then Coarse grain SIMD. | Coarse grain SIMD have lower level of parallelism then Fine Grain SIMD. |
| 4. | Here, Grain Size is over 1000 instructions. | Here, Grain Size in range of 2-500 instructions. |
| 5. | Here, the size of subcomponents is much smaller than the Coarse grained. | Here, the size of subcomponents is more than the Fine-Grained. |
| 6. | Here, two types of parallelism can be obtained – a) Instruction Level Parallelism b) Loop Level Parallelism |
Here, these two types of parallelism can be obtained – a) Sub-program b) Program Level Parallelism |
| 7. | In Fine Grain SIMD, Load Balancing is proper. | In Coarse Grain SIMD, Load Balancing is improper. |
| 8. | Here Parallelism can be detected using compiler. | Here Parallelism can’t be detected using compiler. |
| 9. | Fine Grain SIMD is a much costlier process than the Coarse Grain SIMD. | Coarse Grain SIMD is much cheaper than the Fine Grain SIMD. |
| 10. | Fine Grain is the concept of future multi-threaded architectures to be used in the future also. | Coarse Grain is in one of the earlier concepts of single-threaded architectures. |
| 11. | The Detailed description is further divided into many small subcomponents and makes the processes less complex from the original one and from the coarse-grained also. | The Detailed description is divided into large subcomponents and makes the processes less complex than the original one but more complex than Fine-Grained. |
| 12. | Examples – Connection Machine (CM-2), J-Machine, etc. |
Examples – CRAY Y, etc. |