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5.1 / SEMICONDUCTOR MAIN MEMORY 159

KEY POINTS

◆ The two basic forms of semiconductor random access memory are dynamic
RAM (DRAM) and static RAM (SRAM). SRAM is faster, more expen-
sive, and less dense than DRAM, and is used for cache memory. DRAM is
used for main memory.
◆ Error correction techniques are commonly used in memory systems.These
involve adding redundant bits that are a function of the data bits to form an
error-correcting code. If a bit error occurs, the code will detect and, usually,
correct the error.
◆ To compensate for the relatively slow speed of DRAM, a number of ad-
vanced DRAM organizations have been introduced. The two most com-
mon are synchronous DRAM and RamBus DRAM. Both of these involve
using the system clock to provide for the transfer of blocks of data.

We begin this chapter with a survey of semiconductor main memory subsystems, in-
cluding ROM, DRAM, and SRAM memories. Then we look at error control tech-
niques used to enhance memory reliability. Following this, we look at more advanced
DRAM architectures.

5.1 SEMICONDUCTOR MAIN MEMORY

In earlier computers, the most common form of random-access storage for com-
puter main memory employed an array of doughnut-shaped ferromagnetic loops
referred to as cores. Hence, main memory was often referred to as core, a term that
persists to this day. The advent of, and advantages of, microelectronics has long
since vanquished the magnetic core memory.Today, the use of semiconductor chips
for main memory is almost universal. Key aspects of this technology are explored
in this section.

Organization
The basic element of a semiconductor memory is the memory cell.Although a vari-
ety of electronic technologies are used, all semiconductor memory cells share cer-
tain properties:
• They exhibit two stable (or semistable) states, which can be used to represent
binary 1 and 0.
• They are capable of being written into (at least once), to set the state.
• They are capable of being read to sense the state.

Figure 5.1 depicts the operation of a memory cell. Most commonly, the cell
has three functional terminals capable of carrying an electrical signal. The select