P-1.1 What the Machine Actually Executes

Work through these

• What a processor does in one cycle: fetch, decode, execute

  A processor repeats a very short loop forever, reading a numbered instruction, working out what it means, and doing it. Everything a computer appears to do is that loop running billions of times a second, which is worth seeing once before any language makes sense.

• Source code, machine code, and the tools that get from one to the other

  Your source file is text and a processor cannot read text. A compiler translates the whole file into machine instructions ahead of time, an interpreter reads and acts on it line by line, and most real systems do some of both.

  CS50x: Introduction to Computer Science · Course

• Compiled and interpreted are properties of an implementation, not of a language

  There is nothing about C that requires compiling and nothing about Python that forbids it. The distinction belongs to the tool you run your code through, which is why the same language often has several with very different performance.

• Why one language runs faster than another on the same problem

  Speed differences come from how much work happens while the program is running rather than before it: checking types, looking up names, and allocating memory all cost time. Knowing where that work went is the beginning of ever making anything faster.

• Write the smallest program you can, and follow it from text to output

  Take a program of three or four lines and account for every stage it passes through before anything appears on screen. Doing this once, deliberately, replaces a great deal of vague belief about what computers do.