Random access memory (RAM) is a generic term describing primary storage devices with
the following characteristics:
Microchip implementation with semiconductors
Capability to read and write with equal speed
Random access to stored bytes, words, or larger data units
RAM is fabricated in the same manner as microprocessors. You might assume that
microprocessor clock rates are well matched to RAM access speeds. Unfortunately, this
isn t the case for many reasons, including the following:
Reading and writing many bits in parallel requires additional circuitry.
When RAM and microprocessors are on separate chips, there are delays
when moving data from one chip to another.
There are two basic RAM types and several variations of each type. Static RAM (SRAM)
is implemented entirely with transistors. The basic storage unit is a flip-flop circuit
(see Figure 5.4). Each flip-flop circuit uses two transistors to store 1 bit. Additional transistors
(typically two or four) perform read and write operations. A flip-flop circuit is an
electrical switch that remembers its last position; one position represents 0 and the other
represents 1. These circuits require a continuous supply of electrical power to maintain
their positions. Therefore, SRAM is volatile unless