Primary storage devices consist of random access memory (RAM), which means the time it takes to access data is not affected by the data’s location in the device. That is, RAM doesn’t need to worry about things like moving parts into the correct position or rewinding tape spools. There are two widely used types of RAM, static RAM (SRAM) and dynamic RAM (DRAM), and both play an important role in modern computers. Table 1 characterizes the performance measures of common primary storage devices and the types of RAM they use.

SRAM stores data in small electrical circuits (for example, latches). SRAM is typically the fastest type of memory, and designers integrate it directly into a CPU to build registers and caches. SRAM is relatively expensive in its cost to build, cost to operate (power consumption), and in the amount of space it occupies. Collectively, those costs limit the amount of SRAM storage that a CPU can include.

DRAM stores data using electrical components called capacitors that hold an electrical charge. It’s called "dynamic" because a DRAM system must frequently refresh the charge of its capacitors to maintain a stored value. Modern systems use DRAM to implement main memory on modules that connect to the CPU via a high-speed interconnect called the memory bus.

Figure 1 illustrates the positions of primary storage devices relative to the memory bus. To retrieve a value from memory, the CPU puts the address of the data it would like to retrieve on the memory bus and signals that the memory modules should perform a read. After a short delay, the memory module sends the value stored at the requested address across the bus to the CPU.

Even though the CPU and main memory are physically just a few inches away from each other, data must travel through the memory bus when it moves between the CPU and main memory. The extra distance and circuitry between them increases the latency and reduces the transfer rate of main memory relative to on-CPU storage. As a result, the memory bus is sometimes referred to as the von Neumann bottleneck. Of course, despite its lower performance, main memory remains an essential component because it stores several orders of magnitude more data than can fit on the CPU. Consistent with other forms of storage, there’s a clear trade-off between capacity and speed.

CPU cache (pronounced "cash") occupies the middle ground between registers and main memory, both physically and in terms of its performance and capacity characteristics. A CPU cache typically stores a few kilobytes to megabytes of data directly on the CPU, but physically, caches are not quite as close to the ALU as registers. Thus, caches are faster to access than main memory, but they require a few more cycles than registers to make data available for computation.

Rather than the programmer explicitly loading values into the cache, control circuitry within the CPU automatically stores a subset of the main memory’s contents in the cache. CPUs strategically control which subset of main memory they store in caches so that as many memory requests as possible can be serviced by the (much higher performance) cache. Later sections of this chapter describe the design decisions that go into cache construction and the algorithms that should govern which data they store.

Real systems incorporate multiple levels of caches that behave like their own miniature version of the memory hierarchy. That is, a CPU might have a very small and fast L1 cache that stores a subset of a slightly larger and slower L2 cache, which in turns stores a subset of a larger and slower L3 cache. The remainder of this section describes a system with just a single cache, but the interaction between caches on a real system behaves much like the interaction between a single cache and main memory detailed later.

Physically, secondary storage devices connect to a system even farther away from the CPU than main memory. Compared to most other computer equipment, secondary storage devices have evolved dramatically over the years, and they continue to exhibit more diverse designs than other components. The iconic punch card allowed a human operator to store data by making small holes in a thick piece of paper, similar to an index card. Punch cards, whose design dates back to the U.S. census of 1890, faithfully stored user data (often programs) through the 1960s and into the 1970s.

A tape drive stores data on a spool of magnetic tape. Although they generally offer good storage density (lots of information in a small size) for a low cost, tape drives are slow to access because they must wind the spool to the correct location. Although most computer users don’t encounter them often anymore, tape drives are still frequently used for bulk storage operations (for example, large data backups) in which reading the data back is expected to be rare. Modern tape drives arrange the magnetic tape spool into small cartridges for ease of use.

Removable media like floppy disks and optical discs are another popular form of secondary storage. Floppy disks contain a spindle of magnetic recording media that rotates over a disk head that reads and writes its contents. Figure 2 shows photos of a punch card, a tape drive, and a floppy disk. Optical discs such as CD, DVD, and Blu-ray store information via small indentations on the disc. The drive reads a disc by shining a laser at it, and the presence or absence of indentations causes the beam to reflect (or not), encoding zeros and ones.