how does Arithmetic Logic Unit knows the logic of comparisons?

Question

I am taking a course of system tools and architecture in my University and the first class is about how CPU and RAM talks and how CPU process the data. As explained by the Prof, CPU has a ALU (Arithmetic Logic unit) which performs the arithmetics such as additions and comparisons. But he did not explain how it does it.

so I did some search found this link which directs to a youtube video that explains how addition is performed - well explained for beginners. Even this link explains that CPU directs arithmetic processing to ALU but does not deal with how ALU performs it.

My question is how about ALU does comparison. As humans, we know 5 is less than 7. But how do ALU know this, is it hard-coded in someway. I am sure it must use some logic for comparisons, what is that logic.

It would be great to know how this works being majoring in CS

Answer 1

Are you familiar with how typical ALUs set flags to indicate that overflow occurred, or that the result of an operation is zero or negative? Typically, a comparison is performed by doing a subtraction and updating the flags but discarding the numerical result.

For example, if you subtract 7 from 5, the result is negative. Therefore, 5 is less than 7.

---

I should note that this is definitely not the only way in which it is possible to implement comparison. With a little thought, one can construct an algorithm (which could be made into a circuit) to directly compare the binary representations of two integers without subtracting them. However, subtraction is such a fundamental operation that (almost) all CPUs implement it quite efficiently already, and there is no reason not to reuse that implementation for comparisons.