• Following table lists the major types of semiconductor memory.
• The most common is referred to as random-access memory (RAM).
• This is, of course, a misuse of the term, because all of the types listed in the table are random access.
• One distinguishing characteristic of RAM is that it is possible both to read data from the memory and to write new data into the memory easily and rapidly.

• Both the reading and writing are accomplished through the use of electrical signals.

  

• The other distinguishing characteristic of RAM is that it is volatile. A RAM must be provided with a constant power supply.

• If the power is interrupted, then the data are lost. Thus, RAM can be used only as temporary storage. The two traditional forms of RAM used in computers are DRAM and SRAM.

DYNAMIC RAM

• 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.
• Figure(1) is a typical DRAM structure for an individual cell that stores 1 bit.
• The address line is activated when the bit value from this cell is to be read or written.
• The transistor acts as a switch that is closed (allowing current to flow) if a voltage is applied to the address line and open (no current flows) if no voltage is present on the address line.
• For the write operation, a voltage signal is applied to the bit line; a high voltage represents 1, and a low voltage represents 0.
• A signal is then applied to the addressline, allowing a charge to be transferred to the capacitor.
• For the read operation, when the address line is selected, the transistor turns on and the charge stored on the capacitor is fed out onto a bit line and to a sense amplifier.
• The sense amplifier compares the capacitor voltage to a reference value and determines if the cell contains a logic 1 or a logic 0. The readout from the cell discharges the capacitor, which must be restored to complete the operation.

• 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 threshold 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 A static RAM will hold its data as long as power is supplied to it.
• Figure (2) is a typical SRAM structure for an individual cell. Four transistors (T1, T2, T3, T4) are cross connected in an arrangement that produces a stable logic state. In logic state 1, point C1 is high and point C2 is low; in this state,T1and T4 are off and T2and T3 are on.1 In logic state 0, point C1 is low and point C2 is high; in this state,T1 and T4 are on and T2 and T3 are off.
• Both states are stable as long as the direct current (dc) 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 (T5 and T6).When a signal is applied to this 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 applied to line. This forces the four transistors (T1, T2, T3, T4) into the proper state. 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 corresponding SRAM. On the other hand, a DRAM requires the supporting refresh circuitry.
• 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.