Multi Fluxon Storage and its Implications for Microprocessor Design

Abstract

One of the main challenges for single flux quantum (SFQ) technology to successfully implement a microprocessor is to have a compact and robust on-chip memory that can be used for register files and cache memory. In this context, we designed high capacity destructive readout (HC-DRO) cells. The design is based on our insight that each cell can store more than one SFQ pulse with the same area footprint of a regular DRO cell. In our current design, we demonstrate a single HC-DRO cell that can store up to three SFQ pulses thereby enabling us to store the equivalent of two bits of memory in a single cell. We have designed a register file (RF) architecture for a microprocessor and used the HC-DRO cells to reduce the Josephson junction count required for its implementation. The RF architecture presented in this work can also be used for implementing cache memories. We also demonstrate the benefits of doubling the storage density with HC-DRO cells by designing a 2-bit branch predictor circuit block for a 5-stage pipeline microprocessor that dramatically improves the cycles per instruction metric of an SFQ based microprocessor. All the design details are presented in the paper by verifying the design concepts through JSIM simulations and Verilog simulations.