Write-only memory

Write-only memory (WOM) is the antithesis of read-only memory (ROM). By definition, a WOM is a memory device which can be written but never read. Since there is no practical use for a memory circuit from which data cannot be retrieved, the concept is most often used as a joke or a euphemism for a failed memory device.

Signetics unintentionally published write-only memory literature as the result of an inside practical joke which is frequently referenced within the electronics industry,[1][2] a staple of software engineering lexicons,[3] and included in collections of the best hoaxes.[4]

Contents

The Signetics original

Out of frustration with the long and seemingly useless chain of approvals required of component specifications during which no actual checking seemed to occur, an engineer at Signetics once created a specification for a write-only memory and included it with a bunch of other specifications to be approved. This inclusion came to the attention of Signetics management only when regular customers started calling and asking for pricing information. Signetics published a corrected edition of the data book and requested the return of the 'erroneous' literature.

Later, in 1972, Signetics bought a double-page spread in the April issue of Electronics and used the specification as an April Fool's Day joke. Instead of the more conventional characteristic curves, the 25120 "fully encoded, 9046 x N, Random Access, write-only-memory" data sheet included meaningless diagrams of "bit capacity vs. Temp.", "Iff vs. Vff", "Number of pins remaining vs. number of socket insertions", and "AQL[5][6] vs. selling price". The 25120 required a 6.3 VAC Vff (vacuum tube filament) supply, a +10 Vcc (double the Vcc of standard TTL logic of the day), and Vdd of zero volts (i.e. ground), ±2%.[7][8]

Bit buckets

In computers, there are many practical uses for a destination to write data which does not retain it. A common term among engineers is bit bucket, and many computer operating systems provide a pseudo-device which accepts input but does nothing with it; that is, it ignores or discards the data. This is a useful destination for directing unwanted output, such as expected or uninteresting error messages. Unix and Linux provide /dev/null, CP/M and Microsoft's MSDOS and Windows provide NUL:, and OpenVMS provide NL: and other variants. Most operating systems permit opening the null device for input, though attempts to read from it always receive an end-of-file indication.

Virtual bit buckets

Occasionally, a digital engineer will design a circuit with registers which can be written to but not read, or which respond to a read with information which was not last written. Such designs are a source of difficulty for device driver authors.[9]

See also

References

  1. ^ Robert A. Pease, Staff Scientist. "The origin of the WOM - the "Write Only Memory"". National Semiconductor. http://www.national.com/rap/Story/WOMorigin.html. Retrieved 2011-11-28. 
  2. ^ "Are you fooled?". China Daily. 2011-04-02. http://www.chinadaily.com.cn/2011-04/02/content_12268630.htm. Retrieved 2011-11-28. 
  3. ^ "write-only memory". Jargon File. http://www.retrologic.com/jargon/W/write-only-memory.html. Retrieved 2011-11-28. 
  4. ^ April Fools' Day, including: Kremvax, Udo Of Aachen, Ip Over Avian Carriers, Alternative 3, San Serriffe, Write-only Memory, Spaghetti Tree Hoax, Sidd Finch, Hyper Text Coffee Pot Control Protocol, Google's Hoaxes, Comic Strip Switcheroo, Evil Bit. Hephaestus Books. August 25, 2011. ISBN 978-1242757389. http://www.amazon.com/April-Fools-Day-including-Alternative/dp/1242757384. Retrieved 2011-11-28. 
  5. ^ "AQL - Acceptable Quality Level". Six Sigma SPC. http://www.sixsigmaspc.com/dictionary/AQL-acceptablequalitylevel.html. 
  6. ^ "AQL". Dictionary.com. http://dictionary.reference.com/browse/AQL. 
  7. ^ Signetics (1972). "Signetics 25120 Fully Encoded, 9046xN, Random Access Write-Only-Memory". http://www2.vmi.edu/Faculty/squirejc/Research/IC_Datasheets/digital_cmos/Write%20Only%20Memory.pdf. Retrieved 2011-11-03. 
  8. ^ "write-only memory". The Jargon File. http://www.catb.org/~esr/jargon/html/W/write-only-memory.html. 
  9. ^ Tim Roberts (August 17, 2004). "If every hardware engineer just understood that … write-only registers make debugging almost impossible, our job would be a lot easier". Microsoft. http://www.microsoft.com/whdc/resources/MVP/xtremeMVP_hw.mspx#ETB. Retrieved 2011-11-03. 
This article includes material from the Jargon File.