How Does a Computer Remember Things?電腦怎麼記住東西？

🔊You take a photo of the sunset over the fields in Changhua, turn off your phone, and go to sleep. The next morning, the photo is still there. But a phone is only metal, glass, and silicon. It has no brain. How can it remember a photo, a message, or your favorite song? The answer is surprisingly simple: it stores everything as 0s and 1s.

🔊Each 0 or 1 is called a bit, which is short for binary digit. The American scientist John Tukey first used the word in 1947, and Claude Shannon made it famous in 1948. In 1970, a memory chip from Intel that held 1,024 bits began to replace older kinds of computer memory. Today a phone with 128 gigabytes of storage holds about one trillion bits. Taiwan helps make memory, too: companies such as Nanya Technology, Winbond, and Macronix produce memory chips.

🔊How can 0s and 1s make words? Computers group bits in eights, and eight bits are called a byte. Eight bits can make 256 different patterns, enough to give every English letter its own number. The letter A is number 65, which in bits is 01000001. Most Chinese characters need three bytes, or 24 bits. Photos work the same way: each tiny dot, or pixel, usually uses 8 bits each for red, green, and blue. A 12-megapixel photo is about 288 million bits before the phone squeezes it into a smaller file.

🔊Where are the bits kept? In a computer's working memory, called RAM, each bit sits in a microscopic cell made of a tiny switch, called a transistor, and a tiny store of electric charge, called a capacitor. A charged cell means 1, and an empty cell means 0. Billions of cells are lined up in rows, and every byte has its own number, called an address, so the computer can find any byte in an instant.

🔊There is a problem. The charge in each cell slowly leaks away, like water from a cup with a tiny hole. So the computer refreshes its memory: it reads every cell and tops up the 1s, usually every 64 milliseconds. When you turn the power off, nothing refreshes the cells, and the 1s fade into 0s. That is why RAM forgets everything when a computer shuts down, and why work you have not saved can be lost.

🔊So how does your phone keep your photos? It also has flash memory, invented by Fujio Masuoka at Toshiba in Japan in 1980. In flash memory, electrons are pushed into a tiny box surrounded by insulating material, so they stay there even without power. A computer uses fast RAM for the work it is doing right now and flash memory to keep your files for later. So save your work often, and keep a backup copy of the photos you love.