Datasheet

Identifying Purposes and Characteristics of Memory 39
FIGURE 1.29 Location of memory within a system
Memory
Important Memory Terms
There are a few technical terms and phrases that you need to understand, with regard to
memory and its function. These include:
Parity checking
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Error checking and correcting (ECC)
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Single- and double-sided memory
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Single- and dual-channel memory
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These terms are discussed in detail in the following sections.
Parity Checking and Memory Banks
Parity checking is a rudimentary error-checking scheme that offers no error correction. Parity
checking works most often on a byte, or 8 bits, of data. A ninth bit is added at the transmit-
ting end and removed at the receiving end so that it does not affect the actual data transmit-
ted. The four most common parity schemes affecting this extra bit are known as even, odd,
mark, and space. Even and odd parity are used in systems that actually compute parity.
Mark (a term for a 1 bit) and space (a term for a 0 bit) parity are used in systems that do not
compute parity, but expect to see a fixed bit value stored in the parity location. Systems that
do not support or reserve the location required for the parity bit are said to implement non-
parity memory.
The most basic model for implementing memory in a computer system uses eight
memory chips to form a set. Each memory chip holds millions or billions of bits of informa-
tion. For every byte in memory, one bit is stored in each of the eight chips. A ninth chip is
added to the set to support the parity bit in systems that require it. One or more of these
sets, implemented as individual chips or as chips mounted on a memory module, forms
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