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To make the Analytical Engine solve a different problem, you just needed to feed it different instructions. To make a modern computer solve a different problem, you just need to run a different program. Over a hundred years later, the first true computer would appear in 1943 when the U.S. Army funded a computer to calculate artillery trajectories. This computer, dubbed ENIAC (Electronic Numerical Integrator and Computer), consisted of vacuum tubes, switches, and cables. To give ENIAC instructions, you had to physically flip its different switches and rearrange its cables. The first ENIAC programmers were all women. Physically rearranging cables and switches to reprogram a computer would work, but it was tedious and clumsy. Rather than physically rearranging the computer’s wiring, computer scientists decided it’d be easier if you could leave the computer physically the same but just rearrange the type of instructions you give it.


By giving the computer different instructions, you could make the computer behave in different ways. In the old days, computers filled entire rooms and cost millions of dollars. Today, computers have shrunk so far in size that they’re essentially nothing more than a little silicon wafer, about the size of a potato chip. These silicon wafers are called the central processing unit (CPU), a microprocessor, or just a processor. A processor is essentially an entire computer.


To tell the processor what to do, you have to give it instructions written in machine language — a language that the processor can understand. To make faster computers, engineers smash two or more processors together and make them work as a team. So instead of having a single processor in your computer, the latest computers have two, four, six, or more processors working side by side.