Bathtubs are memorable

I'm going to get right to the point. All that nonsense about lined up bathtubs full of buckets? I made it up. I was trying to teach symbolically. Did it work?

Anyway, here's the breakdown. In that metaphor, the lined-up bathtubs collectively represent the memory system of a computer. The bathtubs represent individual bytes, and the buckets in the bathtubs represent individual bits. The numbers? Those were numbers. It would have taken too much imagination on my part to invent a replacement for numbers.

So am I trying to tell you that the memory system of a computer is a giant series of bytes full of bits? Pretty much. That's not how it's physically implemented in the computer (have you ever noticed that memory chips are square-ish, and not 3 meters long and impossibly thin?), but we're not quite ready to talk about the physical implementation of computers yet.

As for the loading and storing, I pretty much just explained that one directly also. There are fundamentally two things that a computer can do with its memory system. It can "load" values out of it, and you can "store" values into it. I know, it might seem like "load" should mean "put something into the memory," but remember that when a number is "loaded" out of the memory, it's "loaded" into somewhere else. Yeah, that doesn't really cut it for me either, but remember that the opposite of "load" is "store," and that sounds even more like you're storing something into the memory (and this time you really are).

All of the bytes in a memory system are numbered. There's an order, and all of the bytes in memory have an "address." The computer loads numbers out of a particular address, and stores numbers into other addresses. A computer might "store 128 into memory address 183,820,"or "load memory address 1,024 and put its contents into variable X." One of the basic things that makes this a reliable system to use is that the contents of the memory system cannot change unless the computer explicitly changes it. That allows us to confidently store numbers into the memory and get the same number back out when we load it later.

Just to recap, a computer's memory system consists of a very long string of bytes that hold their value between accesses. These bytes can be accessed using a specific numbered address for each byte. These accesses can be loads or stores. Loads read numbers out of the memory, and stores put numbers into the memory.

Next time we'll talk about what kinds of things are stored in memory, in preparation for explaining our last big ingredient to building a working computer: branching.