Can Registers inside a CPU do Arithmetics

Question

I read in many detailed articles that Data from the Registers are used as Operands for the ALU to add two 32-bit integers, and this is only one small part of what the ALU can actually do.

However I also read the Register can even do arithmetic too? The difference between the two is quite blurred to me, what is the clear cut difference between a Register and the actual ALU component?

I know ALU doesn't store values, rather it receives it, and is instructed to simply do the Logic part, but the Register can both store and do general purpose stuff?

If the latter is true, then when does one use the ALU, and when does when use the General Purpose Registers?

Answer 1

Registers don't do arithmetic. Modern cores have several “execution units” or “functional units” rather than an “ALU“. A reasonable rule of thumb is to discard any text (online or hardcopy) that speaks in terms of ALUs in the context of mainstream CPUs. “ALU” is still meaningful in the context of embedded systems, where a µC might actually have an ALU, but otherwise it's essentially an anachronism. If you see it, usually all it tells you is that the material is seriously out-of-date.

Answer 2

Registers can't do arithmetic. A "register" is just a term for a place where you can stick a value. You can do arithmetic on the values stored in registers and have the results saved back into the register. This arithmetic would be done by the "ALU," which is the generic term for the portion of the processor that does number-crunching.

If you're still confused by something specific that you read, please quote it here or post a citation and someone can try to clarify. Note that "register" and "ALU" are very generic terms and are implemented and used differently in every architecture.

Answer 3

Although it is true that registers, properly speaking, don't do arithmetic, it IS possible to construct a circuit which both stores a number and, when a particular input line is set, increments that number. One could interpret this as a register which does a (very limited amount of) arithmetic. As it happens, there is generally no use for such a circuit in a general-purpose CPU (except maybe as the Program Counter), but such circuits could be useful in very simple digital controllers.

[note: historically, the first such circuits were made from vacuum tubes and used in Geiger Counters to count how many radioactive decay events occurred over short periods of time]