# How does a computer perform computations?

I'm reading about computers and it all seems a bit complicated to me. I come across this sentence:

To perform useful computation, we need to irreversibly change distinguishable states of memory cell(s)

What does this exactly mean? I personally don't exactly know what 'distinguishable states' and 'memory cells' are, and a google search doesn't seem to illuminate me much.

If I had to guess I'd say they mean that to perform useful computation, you need to change the state of a bit from 1 to 0 and from 0 to 1. But I'm not too sure, so my questions are:

• Is my interpretation right? Does the author mean that in order to compute you need to change the 0/1's (i.e. states) of bits (i.e. memory cells)?

• If not, what does he mean? And in particular, what is a 'distinguishable state' and what is a memory cell?

This is the source: http://large.stanford.edu/courses/2012/ph250/kumar1/

-
It would help to know the source of this quote. In modern computers a human should consider anything below Y voltage as a low and anything above it as a High. In reality there is a voltage state where its niether a high or a low. A `undistinguishable state` would describe that event. – Ramhound Oct 3 '13 at 13:39
@Ramhound You are correct, I added the source in my post. – user30117 Oct 3 '13 at 13:41
Also worth remembering is that 0 and 1 are abstract concepts that people use, not the actual distinguishable state. Base 2 is a simple concept, hence its use when we talk about computer operations. – AthomSfere Oct 3 '13 at 13:47
@Ramhound I think we are more or less saying the same thing here... But yes, binary is a simple analogy of how the various components perform logic and storage. – AthomSfere Oct 3 '13 at 14:18
@user30117: about computers and computation, please have a look at the way Feynman describe computers: Richard Feynman Computer Heuristics Lecture - YouTube. In a nutshell: A computer does NOT compute. It handles data. It's just a kind of "filing clerk". And it's really, really dumb (and therefore Fast, as it is so dumb its behavior was possible to reproduce with eletricity and components) – Olivier Dulac Oct 3 '13 at 14:50

## Memory cell

A memory cell can be anything that can keep a certain state. Of course, in the end we mean something like RAM, but it could also be a coin.

A coin has two sides, heads and tails. And if you flip it, it will stay like that, until you flip it again.

## Distinguishable states

You want the states of your memory cell to be distinguishable from each other. As noted above, the coin has two sides, heads and tails, thus, the two sides are distinguishable from each other because they are different.

We need to have this property so that we can assign meaning to the states. One state can mean 0 the other can mean 1.

-
Thank you, but in the specific case of computation with a computer, am I correct when I say that the author means 'bit' with 'memory cell' and that the 'distinguishable states' are the states of a bit, namely 0 or 1? – user30117 Oct 3 '13 at 13:43
The example Oliver used is good. Because the same coin could be flipped and land on its side. This `undistinguishable state` is neither heads or tails. The memory cell needs to be designed to handle which state it would use if this were to happen. There computers are binary and not trinary and it deals with electrons and the inability to keep 100% of them contained without some leakage. A binary system allows for all the leakage ( within reason ) and it won't effect another state. – Ramhound Oct 3 '13 at 13:47
@user30117: Well, I think the author is trying to explain the concept of computation in a more abstract way. The fact that our computers use the binary system with 2 states is just one implementation of the explained concept. And the memory cell is closer to a capacitor, a "bit" is just a unit of measurement. – Oliver Salzburg Oct 3 '13 at 13:47
@user30117 - The author is indeed speaking about a High voltage and Low voltage. A computer can be design with more then 2 states. It does not have to be 0 or 1. I will be honest. I wouldn't read to much into what this user wrote. Its not even clear that PH250 is a graduate class. I hope your not using it as a source for your own classwork – Ramhound Oct 3 '13 at 13:51
@user30117: In this context, as the author is writing about the limits of Moore's Law (the idea that computing power doubles over a given time period), yes, I guess he's primarily talking about bits in our binary system (he even mentions it later on). The author wants to show the physical limits to Moore's Law and, thus, also showing the limits of computing power we can reach given our current understanding of physics. – Oliver Salzburg Oct 3 '13 at 13:53