Today we’re going to explicitly look at programming, and how it fits into the story we’ve been building up all semester.
To review, our basic story is:
- To understand and make good use of computers, we need to think about how data is represented and transformed.
- Data at rest tends to live in files.
- We tend to transform data by running programs that read and write files.
That’s great, if the program that does the transformation we want exists and we have a copy, but frequently we want to do something that no program we have does.
Luckily, we can write a new program that does basically whatever we want, as long as we have a reasonable understanding of what computers and computer programs can do.
Key idea: Layers of Abstraction #
The basic function of a computer is pretty simple:
- The computer reads an instruction from RAM into the CPU.
- That instruction can do one simple thing:
- Arithmetic on two numbers.
- Load or store data to/from memory.
- Change what instruction runs next.
- Repeat forever.
So, for example, we might have a program with these instructions:
i = 0
sum = 0
again:
load x from address base + i
add x to sum
if i > 10, goto done
goto again
done:
That’s going to add up 10 numbers.
