NOR is a term from Boolean logic. It’s closely related but fundamentally different from the NAND gate and NAND logic because both are functionally complete operations. They can express all other Boolean operators, such as AND, OR, or NOT. The NOR gate itself relies on two or more inputs. There are only two possible outputs – they are usually expressed as 0 and 1 or High and Low. These values represent the true/false states in Boolean logic.
In the case of the NOR gate, the resulting output is always 0 unless all inputs given are 0 – then the result is 1. So, if the NOR gate receives a 0 and a one as an input, the output is 0. If it gets 1 and 1, the output is also 0. Only when all inputs are 0 is the output 1. This also applies if there are more than two inputs. Although rare, that is possible – and in that case, 1 is output only when all three inputs, for example, are 0.
Most commonly, NOR is used in the context of a NOR gate – a physical type of logic gate used to build memory modules, among other things. NOR and NAND gates are popular architecture types in various PC and electronics parts. Almost any sort of semiconductor structure can be built entirely from NOR gates since they are, as mentioned, functionally complete.
The most common type of NOR gate is the standard 2-input one – however, as many as eight inputs to one gate exist in commercially available devices. There are 2-, 3-, 4-, 5- and 8-input gates, and those that feature more complicated setups, such as triple 3-input NOR gates.
Use of NOR Gates
NOR-based memory isn’t widespread in computers or smartphones. The reason for this is the cost. NAND flash offers far superior storage density and does so at a cheaper price point. Several other vital differentiators also play a part. NOR flash is slow at writing data. This would make a NOR-flash-based SSD a problematic sell outside specific low-write use cases. NOR flash does have some advantages, though. It’s fast when reading data. Data can also be read in individual bits. NOR flash has a low power draw and is quite resistant in challenging environments. NOR flash is also directly addressable, meaning that code stored in it can be executed directly rather than needing to be copied to RAM first.
Depending on the writing requirement, writes can even be performed on the bit level in NOR flash. Updating a bit from 1 to 0 can be done per bit. However, erasing from 0 to 1 must be done at the block level. It’s the slow erasure process that makes NOR flash slowly at writing.
NOR-based memory does still have use cases. Being slow to write limits use cases to ones with low numbers of expected write operations. While archival storage would fit this bill, the high cost per unit of memory makes this a non-starter for a use-case with high-capacity requirements. NOR is used in microcontrollers and to store things like firmware or BIOS. These use cases typically don’t require much storage, so the price or size isn’t much of an issue. Updates are expected to be a regular occurrence but may be needed occasionally. Direct addressability is a substantial benefit, too, as the code can be directly executed without using RAM.
Theoretical NOR
With the low use of NOR flash memory, many references to NOR may refer to the simple boolean function. It was first described in 1880 by Charles Pierce and was shown to offer functional completeness. This means it is possible to use combinations of only NOR gates to create all other boolean logic gates. NAND is the only other boolean logic gate with which this can be done.
NOR Diagram and Truth Table
NOR stands for Not-OR and is the logical inverse of the boolean OR function. An OR gate is true if one of the inputs is true. A NOR gate is true if neither one nor the other input is true.
Tip: Neither the NOR nor the OR function should be confused with XOR. XOR is the eXclusive OR; it is true if either of the inputs is true but false if both inputs are true or false.
The truth table explains the possible in- and output options mentioned above. All combinations other than all 0s return a 1 – and all 0 inputs return one as an output.
Different combinations of NOR logic (or gates) can be used to create other Boolean mathematical functions. NOR gates and NAND gates have a property called functional completeness. This means building any other logic gate using only NOR gates is possible. This is done by repeating using multiple NOR gates in specific configurations. For example, you can create an AND gate by combining three NOR gates in a particular way. This is what that would look like:
While the symbol for the AND gate in and of itself is far simpler, both of these options will deliver the same output – Q – from the same inputs – A and B. Different ways of assembling NOR gates are needed to create other types of functional gates both in physical semiconductors and in theoretical mathematic problems.
Conclusion
NOR is a logic gate; it stands for Not-OR. A NOR gate is the logical inverse of an OR gate. An OR gate only returns false if all inputs are valid. Conversely, a NOR gate is always true unless all inputs are wrong. NOR gates have a property called functional completeness that allows them to be combined to create all other logic gates. NOR gates are core components of NOR flash memory primarily used in microcontrollers or for storing firmware.
