Write-only register

In information technology, a write-only memory (WOM) is a memory location or register that can be written to but not read. In addition to its literal meaning, the term may be applied to a situation when the data written by one circuit can be read only by other circuitry. The most common occurrence of the latter situation is when a writes data to a write-only register of hardware the processor is controlling. The hardware can read the instruction but the processor cannot. This can lead to problems in producing device drivers for the hardware.

Write-only memories also find applications in security and cryptography as a means of preventing data being intercepted as it is being decrypted.

In 1972, WOM, an antithesis of read-only memory (ROM), was introduced as an inside practical joke perpetrated by Signetics. However, it was soon recognized that this concept actually describes certain functionalities in microprocessor systems.

The most frequent occurrences of write-only memories are where the memory locations are registers or an integrated circuit being used to control, or pass information to, hardware outside the processor. A central processing unit (CPU) can write to these locations, and thus control the hardware, but cannot read back the information and discover the current state of the hardware. The memory is write-only relative only to the CPU's point of view. Such arrangements are common on small embedded microcontroller systems to save costs and can cause difficulties for authors of device driver software. When the current hardware state is unknown to the CPU it can only put it into a known state by sending a new command, which may result in the state changing. To alleviate this difficulty, the content of the WOM may be mirrored by the CPU in the regular memory. However, this strategy is only reliable if the register is read-only from the perspective of the hardware.

An example of this kind of use concerned access to memory on early PCs. The original PCs used 8086 or 8088 processors which had the ability to address only 1 MB of memory. A large part of this was occupied by the BIOS resulting in only 640 kB of addressable RAM being available. The memory requirement of many applications soon surpassed this figure. One method of overcoming the limitation was by the use of bank-switched RAM. A number of banks of RAM are provided, but are not permanently assigned to a memory location. In a particular implementation, a special board is added to a computer, which controls the allocation of banks to memory space. The control board has its own memory chip. A computer can write data into this chip, but only the control board itself can read the chip, hence it is dubbed "write only" memory.

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