instructions

We’ve built a calculator that can dynamically switch between addition and subtraction. We send it two pieces of data:

• An operation telling it to add or subtract
• The numbers A and B we want it to operate on

We call this bundle of data an instruction. A list of instructions might look something like this:

	binary	meaning
0	1010001011	ADD 1 3
1	000101010	SUBTRACT 27 4
2	110000101	SUBTRACT 1 19
3	101001001	ADD 40 44
…	…	…

This is called a program.

The calculator knows how to perform two operations: ADD and SUBTRACT. This group of available operations is called the instruction set.

Everything that a computer does is just a combination of operations from its instruction set.

running a program

When we run a program:

• The computer stores the program as a list of instructions.
• It takes the first instruction and executes it.
• It advances to the next instruction, and executes that, and so on.
• Once it runs out of instructions, it stops.

Technically our calculator can run simple programs, but it doesn’t feel very computer-y yet. What’s missing?

For one thing, our instruction set is too small. Real computers typically have dozens of operations available. Think of the instruction set as the computer’s “toolbox” — to do anything interesting, we need to give it access to more tools, which we’ll do in the next tutorial.

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