This article describes a circuit that will compare two values and tell you if one is LESS than, EQUAL to or GREATER than the other. It can be used in a computer design or on its own. For instance it can measure a weight and tell if the weight is more than a specified minimum.

The schematic is very simple as it has only one Memory Bank at its heart (P).

Compare 2-Ops

The actual compare bank is on the left (P). It outputs a different number depending on whether Input-A is LESS, EQUAL or GREATER than Input-B. What number is output depends on the application. In most cases, it will be easiest to assign a different output BIT according to each case. For example, bit-0 could be set when LESS, bit-1 set when EQUAL or bit-2 set when GREATER.

This type of output is effectively a 'result flag' register as used in computer architectures. It could be OR'd with other result flags and stored in a holding register to be read out when needed.

The bank on the right (F) may be used to 'separate' the flag bits according to what comparisons are wanted. It may be programmed to read the bits and only respond to one single condition. That condition would be decided by the SELECT input. Essentially this would ignore all bits except the one wanted. That bit is then turned into a digital output signal, a '0' bit would become a '0'hex and a '1' bit would become a 'F'hex.

Adding the function bank also gives the opportunity to use multiple bits in the result. This way a GREATER OR EQUAL function may be achieved, for example.

In a computer design, you would typically want to have a buffer between the function bank's output and the data bus. The FLAGS output would typically be OR'd with other result flags and accessed via another register, latch or bus-enabling device.

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