A speculative gather system for Cool Mega-Array

Cool Mega Array (CMA) is a low power reconfigurable processor array for battery driven mobile devices. A prototype chip CMA-1 consists of a 8 × 8 PE (Processing Element) array and a micro-controller for controlling data alignment. Because the PE array of CMA is built with a combinatorial circuit, it does not have a signal which tells that operation in the PE array was completed. A propagate delay of the whole PE array corresponding to the operation time was estimated by using the data path and mapping information in the design stage of the application. The timing information for gathering the data was specified in the microcode of the controller. However, since this timing is fixed, it cannot treat the variation of environment temperature and voltage scaling for the PE array. Here, a speculative gather system is proposed which sets the timing of collecting operation results from the PE array dynamically. By collecting results twice and comparing them, it guarantees the correctness of the operation results and adjusts the gather timing automatically. The speculative gather system is implemented in the CMA, and evaluation results appear that the performance is improved by 25.3% on average with the overhead of 0.5% in area and 3.1% in power consumption.