Wirebook › Glossary › Checksum
What is a checksum?
In one sentence: A small number worked out from a piece of data. Work it out again later: if the numbers differ, the data got damaged.
A little more
A checksum is a small number calculated from all the bytes of a piece of data, such as a save file or a download. It's stored or sent along with the data. Later, the computer calculates it again; if the new number doesn't match, the data must have been damaged, so it can be thrown away or fetched again.
Games use checksums to spot a save that was only half written when the power died, and internet packets carry them so damaged pieces can be noticed. Simple checksums catch accidents; for spotting deliberate tampering, stronger fingerprints called cryptographic hashes are used.
It's a bit like… Counting the sweets in a bag before you post it and writing the number on the outside, so the person who opens it can tell if any fell out.
See it in action
- 12How does a game remember where you got to?Play a tiny game, switch the pretend tablet off and see what survives: working memory (RAM) versus the storage drawer, autosave checkpoints, a battery dying mid-save, checksums and cloud saves.6 min · interactive
- BonusHow does a TV remote work when nothing comes out of it?Press a pretend remote and see the invisible infrared flashes it sends: a wake-up flash, 32 bursts with long and short gaps, and a 38,000-a-second flicker that stops sunshine fooling the TV.8 min · interactive
- BonusHow does a photo fly to the phone next to you?Send a pretend photo to the phone next to you and watch how AirDrop and Quick Share really work: a tiny Bluetooth shout to find each other, a private Wi-Fi lane with no router, and numbered pieces that fly over like paper aeroplanes and click back together like a jigsaw.8 min · interactive