Static Random-Access Memory Dynamic Random-Access Memory

COMPUTER ORGANIZATION AND ARCHITECTURE DESIGNING FOR PERFORMANCE NINTH EDITION

RAM technology is divided into two technologies: dynamic and
static. A dynamic RAM (DRAM) is made with cells that store data as charge on
capacitors. The presence or absence of charge in a capacitor is interpreted as a
binary 1 or 0. Because capacitors have a natural tendency to discharge, dynamic

RAMs require periodic charge refreshing to maintain data storage. The term
dynamic refers to this tendency of the stored charge to leak away, even with power
continuously applied.

Computer Systems A Programmer's Perspective Second Edition

SRAM stores each bit in a bistable memory cell. Each cell is implemented with

a six-transistor circuit. This circuit has the property that it can stay indefinitely

in either of two different voltage configurations, or states.

Any other state will

be unstable—starting from there, the circuit will quickly move toward one of the

stable states. Such a memory cell is analogous to the inverted pendulum illustrated

in Figure 6.1.

The pendulum is stable when it is tilted either all the way to the left or all the

way to the right. From any other position, the pendulum will fall to one side or the

other. In principle, the pendulum could also remain balanced in a vertical position

indefinitely, but this state is metastable—the smallest disturbance would make it

start to fall, and once it fell it would never return to the vertical position.

Due to its bistable nature, an SRAM memory cell will retain its value indef-

initely, as long as it is kept powered. Even when a disturbance, such as electrical

noise, perturbs the voltages, the circuit will return to the stable value when the

disturbance is removed.

 

metastable 亚稳的，相对稳定的

 

DRAM stores each bit as charge on a capacitor. This capacitor is very small—

typically around 30 femtofarads, that is, 30×10−15

farads. Recall, however, that

a farad is a very large unit of measure. DRAM storage can be made very dense—

each cell consists of a capacitor and a single access transistor. Unlike SRAM,

however, a DRAM memory cell is very sensitive to any disturbance. When the

capacitor voltage is disturbed, it will never recover. Exposure to light rays will

cause the capacitor voltages to change. In fact, the sensors in digital cameras and

camcorders are essentially arrays of DRAM cells.

Various sources of leakage current cause a DRAM cell to lose its charge

within a time period of around 10 to 100 milliseconds. Fortunately, for computers

operating with clock cycle times measured in nanoseconds, this retention time is

quite long. The memory system must periodically refresh every bit of memory by

reading it out and then rewriting it. Some systems also use error-correcting codes,

where the computer words are encoded a few more bits (e.g., a 32-bit word might

be encoded using 38 bits), such that circuitry can detect and correct any single

erroneous bit within a word.

Figure 6.2 summarizes the characteristics of SRAM and DRAM memory.

SRAM is persistent as long as power is applied. Unlike DRAM, no refresh is

necessary. SRAM can be accessed faster than DRAM. SRAM is not sensitive to

disturbances such as light and electrical noise. The trade-off is that SRAM cells

use more transistors than DRAM cells, and thus have lower densities, are more

expensive, and consume more power.