Introduction to SIMD Architectures

SIMD(Single-Instruction Stream Multiple-Data Stream) architectures are essential in the parallel world of computers. Their ability to manipulate large vectors and matrices in minimal time has created a phenomenal demand in such areas as weather data and cancer radiation research. The power behind this type of architecture can be seen when the number of processor elements is equivalent to the size of your vector. In this situation, componentwise addition and multiplication of vector elements can be done simultaneously. Even when the size of the vector is larger than the number of processors elements available, the speedup, compared to a sequential algorithm, is immense. There are two types of SIMD architectures we will be discussing. The first is the True SIMD followed by the Pipelined SIMD. Each has its own advantages and disadvantages but their common attribute is superior ability to manipulate vectors.


True SIMD: Distributed Memory

The True SIMD architecture contains a single contol unit(CU) with multiple processor elements(PE) acting as arithmetic units(AU). In this situation, the arithmetic units are slaves to the control unit. The AU's cannot fetch or interpret any instructions. They are merely a unit which has capabilities of addition, subtraction, multiplication, and division. Each AU has access only to its own memory. In this sense, if a AU needs the information contained in a different AU, it must put in a request to the CU and the CU must manage the transferring of information. The advantage of this type of architecture is in the ease of adding more memory and AU's to the computer. The disadvantage can be found in the time wasted by the CU managing all memory exchanges.


True SIMD: Shared Memory

Another True SIMD architecture, is designed with a configurable association between the PE's and the memory modules(M). In this architecture, the local memories that were attached to each AU as above are replaced by memory modules. These M's are shared by all the PE's through an alignment network or switching unit. This allows for the individual PE's to share their memory without accessing the control unit. This type of architecture is certainly superior to the above, but a disadvantage is inherited in the difficulty of adding memory.


Pipelined SIMD

Pipelined SIMD architecture is composed of a pipeline of arithmetic units with shared memory. The pipeline takes different streams of instructions and performs all the operations of an arithmetic unit. The pipeline is a first in first out type of procedure. The size of the pipelines are relative. To take advantage of the pipeline, the data to be evaluated must be stored in different memory modules so the pipeline can be fed with this information as fast as possible. The advantages to this architecture can be found in the speed and efficiency of data processing assuming the above stipulation is met.