Chapter 5 states everything we need to know about the importance of data storage technology. A typical computer system has multiple storage devices filling a variety of needs. Primary storage devices support immediate execution of programs, and secondary storage devices provide long-term storage of programs and data. The characteristics of speed, volatility, access method, portability, cost, and capacity vary between storage devices, forming a memory-storage hierarchy. Primary storage tends to be at the top of the hierarchy, with faster access speeds and higher costs per bit of storage, and secondary storage tends to be in the lower portion of the hierarchy, with slower access speeds and lower costs. Modern computers use memory implemented with semiconductors. Basic types of memory built from semiconductor microchips include random access memory (RAM) and nonvolatile memory (NVM). The primary RAM types are static and dynamic. Static RAM (SRAM) is implemented entirely with transistors; dynamic RAM (DRAM) is implemented with transistors and capacitors. SRAM s more complex circuitry is more expensive but provides faster access times. Neither type of RAM can match current microprocessor clock rates. NVM is usually relegated to specialized roles and secondary storage because of its slower write speeds and limited number of rewrites.
Optical discs store data bits as variations in light reflection. An optical disc drive reads data bits by shining a laser beam onto a small disc location. High and low reflections of the laser are interpreted as 1s and 0s. The cost per bit of optical storage is less than magnetic storage, at the expense of slower access speed. Types of optical discs (and drives) include different forms of CDs and DVDs. CD-ROM, DVD-ROM, and BD are written during manufacture. CD-R,DVD /-R, and BD-R are manufactured blank and can be written to once. CD-RW and