02/21/2003 CART 1 On-chip MRAM as a High-Bandwidth, Low-Latency Replacement for DRAM Physical Memories Rajagopalan Desikan, Charles R. Lefurgy, Stephen W. Keckler, and Doug Burger Computer Architecture and Technology Lab University of Texas at Austin

02/21/2003 CART 2 Motivation Latency to off-chip memory hundreds of cycles Off-chip memory bandwidth becoming a performance limiting factor MRAM – Emerging memory technology with high bandwidth and low latency Goal of our work - To determine if the performance advantage of MRAM in high performance computing is worth more investment and research MRAM has the potential to provide low latency, high bandwidth

02/21/2003 CART 3 Outline MRAM Memory Description MRAM Memory Hierarchy Results Conclusions

02/21/2003 CART 4 MRAM Cell Magnetoresistive random access memory (MRAM) uses the magnetic tunnel junction (MTJ) to store information MRAM cell composed of a diode and an MTJ stack MTJ stack consists of two ferromagnetic layers separated by a thin dielectric barrier Polarization of one layer fixed, other used for information storage Bit Line Diode MTJ Stack Word Line Pt Co/F e Ni/F e Al 2 O 3 Co/F e Ni/F e Mn/ Fe Pt W Read/Write Current

02/21/2003 CART 5 MRAM Bank Design MRAM cells located at the intersection of each word and bit line Read – Connect current sources to bit lines and selected wordline is pulled low Writes – Polarity of current in the bit lines decides value stored MRAM banks accessed using vias

02/21/2003 CART 6 MRAM Bank Modeling Modified CACTI-3.0 to develop an area and timing tool to model MRAM banks Independently accessible composed of sub- banks Important features –Active area consumed –Delay due to vertical wires –MRAM capacity for a given die size and cell size –Support for multiple layers with sharing SIA 2001 roadmap at 90 nm technology

02/21/2003 CART 7 Chip-Level Architecture

02/21/2003 CART 8 MRAM Design Issues Number of Banks –More banks : Low latency, higher concurrency, higher network traversal time, higher miss rates Cache Line Size –Larger line size : More spatial locality, higher latency Page Placement Policy –Random –Round-robin –Least loaded

02/21/2003 CART 9 Methodology Simulated Processor –Alpha pipeline modified for 8 wide issue –3.8 GHz (10 FO4 inverters per stage) Base SDRAM System –Distributed L2 cache Base MRAM system –Distributed MRAM banks and reduced capacity distributed L2 cache Benchmarks –Memory intensive SPEC CPU2000, Scientific, Speech

02/21/2003 CART 10 Page Placement Policy IPC for 100 banks with different page placement policies Cost Least-Loaded = (L2 Hit Rate * L2 Hit Latency) + (L2 Miss Rate * MRAM Bank Latency) + Current Network Latency to Bank

02/21/2003 CART 11 MRAM Sensitivity MRAM Latency Sensitivity SDRAM Latency : 30 ns

02/21/2003 CART 12 Conclusions Developed an architectural model for exploiting an emerging memory technology, MRAM Analyzed the contribution to performance of the different components in our MRAM system MRAM system performs 15 % than conventional SDRAM