Secondary memory is a type of computer memory that is not directly accessed by the central processing unit (CPU) of a computer and is usually available as non-volatile memory. One of the most common forms of this memory is the hard drive of a computer, which is used to store the operating system (OS) and other programs. Other forms of secondary memory include disc drives for compact discs (CDs) or digital versatile discs (DVDs), as well as removable flash memory. In contrast to this, primary memory is computer memory directly accessed by the CPU, usually volatile memory, and tertiary memory refers to systems outside of a computer that can be directly accessed through various means.
There are several different types of memory utilized by computer systems, and secondary memory is one of the most common and important. This type of memory is typically recognized by how it is used by a computer system and the way in which data is stored on it. The main characteristic of this type of memory is that it is not accessed directly by the CPU, but instead through a connection that allows the CPU to utilize it. This type of memory is also usually non-volatile memory, which means it retains data even without a power source.
One of the most common types of secondary memory is the hard drive on a computer. This drive is accessed by the CPU through one of several different indirect methods and data remains on the drive even without a power supply. Other disc drives, such as those used with CDs and DVDs, are also types of secondary memory utilized for other purposes. Easily removable memory types, such as flash drives, are also secondary memory and use interfaces such as a universal serial bus (USB) port to be accessed by the CPU.
In addition to secondary memory, many computer systems also utilize primary memory, tertiary memory, and offline storage. Primary memory is the memory on a computer that is directly accessed by the CPU, namely random access memory (RAM) and the memory used to store the basic input/output system (BIOS). Tertiary memory is typically external memory a computer can access directly through some type of automated system, such as a bank of hard drives that can be connected to a computer through the use of a robotic armature. Offline storage is memory that can be used and accessed remotely, such as servers that can be utilized through an Internet connection for data backup or storage.