Write-once memory (WOM) code for efficient FLASH memory utilization

Prof. Assaf Schuster | Computer Science
Prof. Eitan Yaakobi | Computer Science


Information and Computer Science

The Technology

Flash based solid state disks (SSD) are widely used thanks to their short read and write latencies and increasing throughput. However, once flash cells are written upon, they must be erased before they can be rewritten. These comparatively slow erasures, along with the additional overheads they incur, significantly slow down pending read and write operations. In addition, flash cells have a limited lifetime, measured as the number of erasures a block can endure before its reliability deteriorates below an acceptable level.
A promising technique for reducing block erasures is to use write-once memory (WOM) codes. WOM codes alter the logical data before it is physically written, thus allowing the reuse of cells for multiple writes. They ensure that, on every consecutive write, zeroes may be overwritten with ones, but not vice versa. They can be applied to flash memories, which impose the above constraints: the bit value of each cell is only allowed to increase, not decrease, unless the entire block is erased.


  • Provides the first practical design for using WOM codes to extend flash media lifetime without reducing the device’s usable capacity.
  • Extends flash media lifetime while, in addition, achieving significant performance improvement – I/O response times are reduced.
  • As opposed to most previous approaches, our design does not require any hardware modification of existing flash architectures.

Applications and Opportunities

  • Flash based product: including USB devices, SSDs, smart-phones, cameras etc., to reduce erase operations and extend the media’s lifetime and improve its performance.
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