What are Vector-Access Memory Schemes in Computer Architecture?

The flow of vector operands between the main memory and vector registers is generally pipelined with various access paths. In this section, we specify vector operands and describe three vector-access schemes from interleaved memory modules allowing overlapped memory accesses.

Vector Operand Specifications − Vector operands can have arbitrary lengths. Vector elements are not essentially saved in contiguous memory areas. For example, the entries in a matrix may be stored in row-major or in column-major. Each row, column, or diagonal of the matrix can be used as a vector.

When row elements are stored in contiguous locations with a unit stride, the column elements must be stored with a stride of n, where n is the matrix order. Similarly, the diagonal elements are also separated by a stride of n +1.

To access a vector in memory, one must specify its base address, stride, and length. Since each vector register has a fixed number of component registers, only a segment of the vector can be loaded into the vector register in a fixed number of cycles. Long vectors must be segmented and processed one segment at a time.

C-Access Memory Organization − The m-way low-order interleaved memory structure allows m memory words to be accessed together in an overlapped structure. This concurrent access has been known as C-access.

The access cycles in various memory modules are staggered. The low-order bits choose the modules, and the high-order 6 bits select the word within every module, where 7n = 2° and a + b = n is the address length.

S-Access Memory Organization − The low-order interleaved memory can be rearranged to enable simultaneous access or S-access. In this method, all memory modules are created simultaneously in a synchronized method.

C/S-Access Memory Organization − A memory organization in which the C-access and S-access are combined is called C/S-access. This scheme where n access buses are applied with m interleaved memory modules connected to every bus.

The m modules on each bus arc m-way interleaved to enable C-access. The n buses work in parallel to enable S-access. In each memory cycle, at most m • n-words are fetched if the n buses are completely used with pipelined memory accesses.

The C/S-access memory is suitable for use in vector multiprocessor configurations. It provides parallel pipelined access to a vector data set with high bandwidth. A particular vector cache design is required within each processor to maintain smooth data movement between the memory and multiple vector processors.

Updated on: 30-Jul-2021

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