Chapter 7: Computer Memory^149
Extendedmemoryisoftenconfusedwithexpandedmemory.Expandedmemory(the
upper memory area) expands conventional memory to fill up the first 1MB of RAM.
Extended memory extends RAM all the way to its limit.
The first 64KB of extended memory is reserved for use during the startu p processes of
the PC. This area is called thehigh memory area.Dealing with Memory Errors
Memory errors are a common occurrence on most PCs, although they shouldn’t be so
common that they are an everyday occurrence. There are two general types of memory
errors—hard errorsandsoft errors. There isn’t really a lot of difference between these two
types of errors. The biggest difference is that hard errors can be repeated because some-
thing is definitely broken, and soft errors are transient or intermittent and may or may
not be a one-time fluke.
A hard memory error happens when a memory module or chip, its mounting, or the
motherboardisdefective.Becausethistypeoferrorisusuallytheresultofaphysicaldefect,
the same error can be repeated consistently. For example, if a bit in the conventional
memory area is damaged by ESD (electrostatic discharge), it could cause a consistently
reported parity error. Another example is a SIMM module that is improperly seated,
causing the memory not to be detected during the boot cycles. Hard memory errors are
commonly the result of loose memory modules, system board defects, or defective memory
modules. Typically, hard errors are fairly easy to find and repair because they are easily
diagnosed and located. Because they can be repeated, you have a very good chance of
isolating the problem.
Soft errors are transient in nature. A single bit can give the wrong data value one-time
ever, or it can operate normally most of the time but malfunction intermittently. Soft
errors can be difficult to diagnose because they are moving targets. A PC that develops a
history of soft memory errors most likely has poor quality memory. However, the prob-
lem could also be with the motherboard or another component seemingly unrelated to
the memory. Diagnosing a soft error can be an exercise in patience. Soft errors are usually
not consistent, but they will eventually repeat if there is anything to worry about.Common Memory Errors
Fortunately, most hard memory errors will show u pduring the boot process and are the
result of a physical defect, system configuration, or component installation problem.
Your built-in hardware diagnostic package, the POST (Power-On Self-Test), should find
and report any hard errors it detects with either a beep code or a text message. See
Chapter 6 for more information on the POST and its error modes.