The trapPeople say a CPU "does the math" — but how many different things can it actually do?
See the picture
fetch1 / 3
1. Fetch — take one instruction out of drawer 102
The CPU pulls instructions one at a time out of numbered drawers (memory). This one says "go read the value in drawer 200".
speed
what a person wrote
total = price + tax
compile ↓
what the CPU actually sees
LOAD → ADD → STORE → HALT
what to watch now
A separate slot remembers the drawer number, so as soon as one is taken the number moves on to the next.
state log
next drawer number
103
instruction in hand
LOAD R1, [200]
value in hand
—
drawer 202
—
Follow the sentences
1/38
What exactly does a CPU do?
Text mode advances to the next sentence every 3 seconds.
Picture it this way
A CPU is less a universal calculator than a remote control with very few buttons: read in, add, store, stop.
Only one button can be pressed at a time. And every press goes through the same three steps — take an instruction out of a numbered drawer, work out which button it is, then move a value exactly as told.
Press "next change" above to walk those three steps one at a time. A single line of your code is this three-step bundle, repeated many times over.
Now the numbers
computed · just now
The program on this screen uses only 4 kinds of button — read in, add, store, stop. And every single press goes through the same 3 steps.
If one press costs one beat, a 3GHz CPU can press 3,000,000,000 buttons per second. The next chapter shows why that number does not become performance on its own.
These numbers are not written into the sentence. They are worked out while this page renders.
In one sentence
A CPU is fast not because it is clever, but because it repeats something extremely simple an enormous number of times.
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Quick check
0 / 3 correct
1.How does the CPU handle one line like `total = price + tax`?
2.Memory holds instructions and values side by side. How does the CPU tell them apart?
3.If a CPU only ever presses one button at a time, why does a computer look like it does many things at once?
New words here
instruction
One button the CPU knows — "fetch this", "add these" — small enough that it cannot be split further.
register
A very small slot holding the one or two values the CPU is handling right now. It is the "value in hand" on the screen above.
memory address
The number on the drawer a value sits in. A CPU finds values by that number, never by a name.