Memory – Computer data storage, called just storage or memory, is any computer component, device or recording media retaining digital date for some amount of time. Memory provides a core computer function, retaining information. Memory and the CPU combine in the basic computer model in use since the 1940's.
Memory amounts or storage limits are:
- Byte – one characters
- KiloByte (KB) – 1,000 bytes
- MegaByte (MB) - 1,000 KB or 1,000,000 bytes, equal to 500 typed/printed pages, a short novel
- GigaByte (GB) – 1,000 MB, equal to a pick up truck filled with typed/printed pages
- TeraByte (TB) – 1 trillion bytes, equal to 150 miles of typed/printed pages on bookshelves
Memory refers to semiconductor storage known as RAM or other fast, temporary storage. Storage refers to mass storage such as hard drives, DVD and other optical discs, and other slower than RAM but more permanent storage. These distinctions reflect computer architecture. They also reflect a fundamental difference between memory and mass storage devices such as hard drives.
The CPU directly accesses primary storage, memory, but does not directly access mass storage. The CPU reads and executes instructions gathered from memory. Memory also stores data during CPU operations on that data.
Old computers used various types of mass storage.
- Delay Line Memory – serial access refreshable memory.
- Williams Tubes – stored information as charged particles draw dots on a cathode ray tube
- Rotating magnetic drums – similar to hard drives
These methods were unreliable. Magnetic core memory replaced them completely by 1954, but it wasn't much better. Transistors created a revolution in memory when they allowed super small solid state silicon chip memory.
Modern RAM has many advantages over older memory, particularly smaller size and light weight. But they have disadvantages too, including cost and volatility. They lose data when not powered.
The CPU cannot access secondary storage. Computers access data in secondary storage with I/O channels, then transfer that data to primary storage (memory) where the CPU operates on it. Secondary storage is slower, but not volatile – it retains memory even without power. Secondary storage is less expensive than memory by an order of magnitude. Modern computers use an order of magnitude or two more of secondary storage than primary storage. For example, a powerful PC has 4 GB of RAM but 400 GB, or more, hard drive space. Memory only holds data temporarily while secondary storage holds it for a long time – people want data held in secondary storage forever.
Computers access memory much faster than secondary storage. For example hard drive data access takes milliseconds (thousandths of a second) while RAM data access takes nanoseconds (billionths of a second.) The difference is memory beats hard drives by a million times, highlighting the progress in memory (primary storage) between the days of magnetic drums and modern memory such as RAM. Optical drives like DVDs have longer access times than even hard drives.
Other types of secondary storage include Flash Memory such as USB sticks, floppy disks, DAT, punch cards, and Zip Drives.
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