Logic and arithmetic computation-in-memory accelerators: Based on memristor devices

Conventional computing systems involve physically separated storing and processing units. To perform the processing, data is shuttled from the storing unit to the processing unit followed by the actual processing, and the processed data is shuttled back into the storing unit. Unfortunately, this data shuffling contributes significantly to the overall latency and energy consumption of the system. Computation-in-memory (CIM) offers a promising alternative that can process the data within the storing unit, thereby, alleviating the need for data shuffling. This can potentially lead to high energy efficiency and high throughput computation. In addition, emerging non-volatile memristors provide an excellent storing element that can inherently perform the computation in CIM while also retaining the data value.
Memristor-based CIM has been vastly explored to perform certain data-intensive computational tasks for numerous applications related to artificial intelligence (AI), Big Data, and data encryption while realizing high-performing, low-power micro-architectural solutions. This thesis first identifies the existing key challenges related to emerging memory technology and CIM memory array, and the circuit design of periphery logic to achieve low-power and high-performing CIM units. Thereafter, it presents several micro-architectural solutions to build CIM accelerators that can perform logic and arithmetic operations in an efficient manner.

Identifying the challenges of memristor-based CIM: The thesis briefly summarizes the key aspects of a memristor-based CIM architecture that can perform certain logic and arithmetic operations. First, the key components of a typical CIM architecture are presented, highlighting the design considerations to build these components. An account of computational accuracy and efficiency share of each of these components is presented supported by a comprehensive literature survey while highlighting the underlying concepts of each component. This is followed by identifying the key challenges in achieving stringent efficiency metrics required by the targeted applications while dealing with non-idealities. Finally, the limitations of the state-of-the-art solutions that target these challenges are highlighted, thus, providing the motivation to develop outperforming solutions that are presented in the thesis.

Developing micro-architectural solutions for efficient CIM-based logic accelerators: This part of the thesis presents several logic accelerators that can perform (N)OR, (N)AND, and XOR operations in a fast and energy-efficient manner. \textbf{FIVE} different solutions target different aspects of performing CIM-based logic, whereby, scaling the number of operands per cycle and the memory size, and expanding to different types of operations that can be supported while addressing the non-idealities. State-of-the-art solutions are comprehensively outperformed by our proposed solutions in terms of energy efficiency, performance, and the maximum number of operands that can be accurately performed in a single cycle.

Developing micro-architectural solutions for efficient CIM-based arithmetic accelerators: This part of the thesis presents several compact arithmetic accelerators that can perform multiply-and-accumulate (MAC) operations with low energy consumption. \textbf{THREE} different analog-to-digital converter (ADC) topologies are presented with two of them demonstrated with a chip prototype. Optimizing the memory structure is also explored to ensure accurate generation of analog output in the CIM memory array and to optimize the A/D conversion. A comparison of the proposed solutions with the state-of-the-art is presented, highlighting the promise of the developed micro-architectures.