# What is combinational circuit?

I'm reading the Digital Design and Computer Architecture by David Harris, Sarah Harris. The authors give the following definition of combinational logic:

A combinational circuit’s outputs depend only on the current values of the inputs; in other words, it combines the current input values to compute the output... A combinational circuit is memoryless, but a sequential circuit has memory. The functional specification of a combinational circuit expresses the output values in terms of the current input values.

However, they claim this circuit is not combinational: because "node n6 connects to the output terminals of both I3 and I4". Indeed, it's one of the designated signs when a scheme can not be combinational but, according to the authors:

Certain circuits that disobey these rules are still combinational, so long as the outputs depend only on the current values of the inputs.

As I'm able to catch on, the aforementioned circuit is the case: its output is 1 if and only if its inputs are both 1, otherwise the output is 0. So the output is defined as a function of the inputs (the AND function).

Circuit (d) cannot be written in this form [of formula], since the outputs of I3 and I4 are wired together. What is the relation between the input to the rightmost gate and the outputs of I3 and I4? Not something that can be described combinatorially.

Unfortunately, I'm still confused due to

• The circuit, regarded as a black box, is still in scope of the combinational logic definition: its output values depend only on the current values of the inputs;
• The relation between the input to the rightmost gate and the outputs of I3 and I4 can be described through the function NAND of the circuit inputs and this function is quite "memoryless". It's not obvious for me why we can't afford to depict a gate input using multiple outputs of other gates.

I need some elaboration. Maybe things would fall into place if someone provide a circuit example when two gates outputs is connected to one input and it actually causes "memory" (in contrast to the considered sample).