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162 CHAPTER 5 / INTERNAL MEMORY

Although the DRAM cell is used to store a single bit (0 or 1), it is essentially
an analog device.The capacitor can store any charge value within a range; a thresh-
old value determines whether the charge is interpreted as 1 or 0.
STATIC RAM In contrast, a static RAM (SRAM) is a digital device that uses the
same logic elements used in the processor. In a SRAM, binary values are stored
using traditional flip-flop logic-gate configurations (see Chapter 20 for a description
of flip-flops).A static RAM will hold its data as long as power is supplied to it.
Figure 5.2b is a typical SRAM structure for an individual cell. Four transistors
(T ,T ,T ,T ) are cross connected in an arrangement that produces a stable logic state.
4
3
1
2
In logic state 1,point C is high and point C is low;in this state,T and T are off and T 2
2
1
4
1
1
and T are on. In logic state 0, point C is low and point C is high; in this state,T and
2
1
1
3
T are on and T and T are off. Both states are stable as long as the direct current (dc)
3
4
2
voltage is applied. Unlike the DRAM, no refresh is needed to retain data.
As in the DRAM, the SRAM address line is used to open or close a switch.
The address line controls two transistors (T and T ).When a signal is applied to this
6
5
line, the two transistors are switched on, allowing a read or write operation. For a
write operation, the desired bit value is applied to line B, while its complement is ap-
plied to line . This forces the four transistors (T ,T ,T ,T ) into the proper state.B 1 2 3 4
For a read operation, the bit value is read from line B.
SRAM VERSUS DRAM Both static and dynamic RAMs are volatile; that is, power
must be continuously supplied to the memory to preserve the bit values. A dynamic
memory cell is simpler and smaller than a static memory cell.Thus, a DRAM is more
dense (smaller cells = more cells per unit area) and less expensive than a corre-
sponding SRAM. On the other hand, a DRAM requires the supporting refresh cir-
cuitry. For larger memories, the fixed cost of the refresh circuitry is more than
compensated for by the smaller variable cost of DRAM cells. Thus, DRAMs tend to
be favored for large memory requirements.A final point is that SRAMs are generally
somewhat faster than DRAMs. Because of these relative characteristics, SRAM is
used for cache memory (both on and off chip), and DRAM is used for main memory.
Types of ROM
As the name suggests, a read-only memory (ROM) contains a permanent pattern of
data that cannot be changed. A ROM is nonvolatile; that is, no power source is re-
quired to maintain the bit values in memory.While it is possible to read a ROM, it is
not possible to write new data into it. An important application of ROMs is micro-
programming, discussed in Part Four. Other potential applications include
• Library subroutines for frequently wanted functions
• System programs
• Function tables
For a modest-sized requirement, the advantage of ROM is that the data or program
is permanently in main memory and need never be loaded from a secondary stor-
age device.

1 The circles at the head of T 3 and T 4 indicate signal negation.