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Destructive read

Also called: destructive readout, read-restore, restore after read, write-back after read

A read that disturbs the stored value, as in a DRAM cell whose capacitor is drained by reading. The chip must write the value back after each read.

In most memories, reading a cell just looks at it. A DRAM read is different: it's destructive.

To read a DRAM cell, the chip connects its tiny capacitor to a long bit line. The capacitor's charge spreads out onto the bit line, which a sense amplifier detects as a 0 or 1. But the charge is now shared, and the capacitor no longer holds a full-strength value. The act of reading has wiped the cell.

The fix is automatic: right after sensing, the chip drives the value back into the cell, recharging (or discharging) it fully. In fact, a whole row is sensed and restored together, which is the same operation the chip uses for refresh.

So from outside the chip, reading looks harmless: the same address returns the same word next time. The cost is time: the restore step makes each DRAM access cycle longer.

SRAM reads are non-destructive, because the latch actively drives its own value. ROM and flash reads are non-destructive too.

Worked example

Example

Reading the same DRAM word twice

A DRAM word holds 1010. The CPU reads it twice.

  1. 1.

    First read: the row's capacitors share their charge with the bit lines. The sense amplifiers detect 1010.

  2. 2.

    The chip immediately writes 1010 back, restoring full charge.

  3. 3.

    Second read: the cells hold 1010 again, so it returns 1010.

  4. 4.

    Without the restore step, the second read could return garbage.

Common mistakes

  • Thinking a DRAM read permanently erases the data. The chip writes it back automatically.

  • Thinking SRAM reads are destructive too. An SRAM latch keeps driving its value during a read.

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Destructive read is taught in Memory.

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