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Write Only Memory (WOM) is the logical converse of Read Only Memory (ROM). It is generally only proposed as a joke since there is no practical use for such a thing; since write-only memory cannot, by definition, be read; any information that is written to it will simply be lost.

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' ones.

Later, in 1972, Signetics bought a double-page spread in the April issue of Electronics and used the spec 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 diagrams of "bit capacity vs. Temp.", "Iff vs. Vff", "Number of pins remaining vs. number of socket insertions", and "AQL vs. selling price". The 25120 required a 6.3 VAC V_ff (vacuum tube filament) supply, a +10V V_cc (double the Vcc of standard TTL logic of the day), and V_dd of 0V (ie. ground), ±2%.

Although write only memory has no practical purpose in the general sense, there are hardware implementations of write-only registers in CPUs and I/O devices. [1] [2].

• Write-only language
• FINO — First In, Never Out scheduling algorithm
• /dev/null

• Official Signetics 25120 Data Sheet (PDF).

This article includes material from the Jargon File.