Modular multi-ported SRAM-based memories

Abstract

Multi-ported RAMs are essential for high-performance parallel computation systems. VLIW and vector processors, CGRAs, DSPs, CMPs and other processing systems often rely upon multi-ported memories for parallel access, hence higher performance. Although memories with a large number of read and write ports are important, their high implementation cost means they are used sparingly in designs. As a result, FPGA vendors only provide dual-ported block RAMs to handle the majority of usage patterns. In this paper, a novel and modular approach is proposed to construct multi-ported memories out of basic dual-ported RAM blocks. Like other multi-ported RAM designs, each write port uses a different RAM bank and each read port uses bank replication. The main contribution of this work is an optimization that merges the previous live-value-table (LVT) and XOR approaches into a common design that uses a generalized, simpler structure we call an invalidation-based live-value-table (I-LVT). Like a regular LVT, the I-LVT determines the correct bank to read from, but it differs in how updates to the table are made; the LVT approach requires multiple write ports, often leading to an area-intensive register-based implementation, while the XOR approach uses wider memories to accommodate the XOR-ed data and suffers from lower clock speeds. Two specific I-LVT implementations are proposed and evaluated, binary and one-hot coding. The I-LVT approach is especially suitable for larger multi-ported RAMs because the table is implemented only in SRAM cells. The I-LVT method gives higher performance while occupying less block RAMs than earlier approaches: for several configurations, the suggested method reduces the block RAM usage by over 44% and improves clock speed by over 76%. To assist others, we are releasing our fully parameterized Verilog implementation as an open source hardware library. The library has been extensively tested using ModelSim and Altera's Quartus tools.