3.2: Memory and storage

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While a process is running, most of its data is held in main memory, which is usually some kind of random access memory (RAM). On most current computers, main memory is volatile, which means that when the computer shuts down, the contents of main memory are lost. A typical desktop computer has 2–8 GiB of memory. GiB stands for “gibibyte,” which is \( 2^{30} \) bytes.

If the process reads and writes files, those files are usually stored on a hard disk drive (HDD) or solid state drive (SSD). These storage devices are non-volatile, so they are used for long-term storage. Currently a typical desktop computer has a HDD with a capacity of 500 GB to 2 TB. GB stands for “gigabyte,” which is \( 10^{9} \) bytes. TB stands for “terabyte,” which is \( 10^{12} \) bytes.

You might have noticed that I used the binary unit GiB for the size of main memory and the decimal units GB and TB for the size of the HDD. For historical and technical reasons, memory is measured in binary units, and disk drives are measured in decimal units. In this book I will be careful to distinguish binary and decimal units, but you should be aware that the word “gigabyte” and the abbreviation GB are often used ambiguously.

In casual use, the term “memory” is sometimes used for HDDs and SSDs as well as RAM, but the properties of these devices are very different, so we will need to distinguish them. I will use storage to refer to HDDs and SSDs.