Abstract
The existing charge-based memories like DRAM and flash are reaching their scaling limits. Phase change memory (PCM) is one of the most promising emerging memory technologies due to its good scalability and low leakage power. Multi-level cell (MLC) operation in PCM, which stores two or more bits in a single cell, is necessary to achieve a high storage density. However, a reduced resistance range in multiple storage levels of MLC PCM, introduces a lot of soft errors because of the resistance drift phenomenon. The poor reliability of MLC PCM requires strong error correction codes (ECC) which could severely degrade the storage density and performance. In this paper, we propose a cost-effective reliable MLC PCM architecture for improving MLC PCM reliability, storage density and performance. The proposed architecture exploits the data-dependent nature of resistance drift problem to reduce the ECC overhead. A simple state mapping is used to generate virtual data, which is half of the actual data size. ECC parity bits are generated based on virtual data bits instead of actual message bits, thus resulting in a reduced number of cells for parity bits. This improves the reliability and storage density of MLC PCM. The performance is also improved by minimizing the ECC overhead. Simulation results show an improvement of about 10 4 times in reliability and 10.9% in storage density compared to a conventional MLC PCM which uses a typical error correction scheme. Performance and energy efficiency are also improved up to 13.7% and 10%, respectively, by the proposed architecture.
Highlights
Morden computer systems utilize large memory for computing and storing data and the required memory size continues to increase steadily
EVALUATION The proposed method minimizes the additional cells required for storing parity bits and this results in improvement of both reliability and storage density of Multi-level cell (MLC) Phase change memory (PCM)
MLC PCM would suffer from reduced reliability due to the resistance drift phenomenon which can cause soft errors
Summary
Morden computer systems utilize large memory for computing and storing data and the required memory size continues to increase steadily. A relatively small amplitude and long duration current pulse, with a temperature above the crystallization temperature (around 300â—¦C) but below the melting temperature, changes the material into crystalline state This state represents logic 1 and the operation is the SET operation. Different encoding ideas have been proposed which can considerably enhance the reliability [8], [14], [15] These architectural and encoding methods cannot remove the impact of resistance drift completely and require a multipleerror correction scheme like Bose-Chaudhuri-Hocquenghem (BCH) code to achieve a soft error rate (SER) similar to that of DRAM or flash. The reduced number of cells for parity bits improves both the reliability and storage density of MLC PCM.
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