A storage unit is used to store information and instructions when they are not being used in memory.
Three kinds of storage devices are a flash drive, hard disks, and CD - ROM drives.
Data access time is the time taken by a computer, or a data storage device or medium, to respond to a user's request for data.
Typical access times for a CD-ROM drive, 200 milliseconds; hard drive, 15 milliseconds; Random Access Memory (RAM), 70 nanoseconds.
There is a trade-off among the three key characteristics of memory: namely, capacity, access time, and cost. A variety of technologies are used to implement memory systems, and across this spectrum of technologies, the following relationships hold:
- Faster access time, greater cost per bit
- Greater capacity, smaller cost per bit
- Greater capacity, slower access time




