Remember these things

We know computer memory is a big long list of bytes that can be read and written (loaded and stored), but now we need to talk about the types of information that is contained in the memory. There are two broad categories of information that the computer keeps track of, namely, instructions and data. Both of these are just numbers when you get right down to it, because that's all the memory can hold in it, so the difference between them lies in how the computer treats these different kinds of information.

Let's talk about data first. The word "data" is the plural form of the word "datum," which just means "piece of information." So "data" means "many pieces of information." All data in a computer is represented as binary numbers, but those binary numbers can stand for a variety of different things. It just depends on what meaning the computer's programmer decides to give those binary numbers (which the computer doesn't really care about, by the way). The programmer could consider the byte 0b0001 0001 to mean the decimal number 33 or the ASCII symbol '!' (ASCII is just a standard convention for treating single-byte numbers as text symbols). It is very common for things we want to represent in a computer to take up more than one byte. For example, numbers larger than 255 must be represented by more than one byte. Also, picture and movie files are larger than one byte. Large quantities of text can be represented by very long lists of single bytes that are interpreted as ASCII symbols (like the example above). In the end, it is the value of the byte(s) and the context which the programmer gives them that really determines what the data in a computer memory means.

The other kind of information that can be stored in computer memory is computer processor instructions. I know we haven't talked about how computer processors work at all yet, but I think this one detail about their operation cannot be avoided at this time. Processors work by reading a part of the computer memory, and then depending on what was contained in that part of the computer memory (or that "instruction"), the processor will perform an action. There is a counter in the processor called the "program counter" which keeps track of the place in memory that the processor will get its next instruction. After every instruction is processed the program counter gets increased (i.e., "counts up") to move on to the next instruction. After that instruction is processed, the program counter "counts up" again, moving on to the next instruction, and so on.

Just like how data is just a series of arbitrary bytes until the programmer steps in to give those bytes meaning, instructions are just series of arbitrary bytes until the processor's designer steps in to give those bytes meaning. There are many different types of instructions, including adding, loading, storing, jumping and branching. There are also many more, but these are the basic building blocks of computer science that we're focusing on for now. We've already talked in detail about adding, loading and storing.

Jumping and branching are special instructions that deal with changing the program counter. Normally the program counter just increases (counts up) to the next instruction after each previous instruction is completed. This doesn't allow for repeating any previous instructions (or looping, as it's called in programming). Jumping and branching allow for the program counter to be set to whatever value the programmer wants. This can include increasing the program counter, or decreasing it. Next time we'll talk more about jumping and branching and what it allows computers to do that would otherwise be impossible.

Note: Yes, I know that I was inconsistent about treating the word "data" as a plural word. This is pretty much universal in all technical and academic literature. Even though the word represents a plural concept, it is almost always treated grammatically as a singular. I will be following this convention from here on out.