5.1 Understanding Memory
This chapter focuses on general-purpose memory, where PCs store
programs and data that are currently in use, the pipeline that
supplies data to and receives results from the processor.
General-purpose memory, called read-write
memory or Random Access
Memory
(RAM), must be
readable-from and writable-to. Two types of RAM are used on modern
PCs:
- Dynamic RAM (DRAM)
-
Dynamic RAM
stores data for only a tiny
fraction of a second before losing it. To maintain stored data, the
system must constantly refresh DRAM, which
exacts a performance penalty and limits its speed. Typical DRAM
provides 60 ns access, but is inexpensive and consumes relatively
little power.
- Static RAM (SRAM)
-
Static RAM
automatically maintains its
contents as long as power is applied to it, without requiring
refresh. SRAM provides access times an order of magnitude faster than
DRAM, but is expensive and power-hungry.
PCs use a tiered memory architecture that takes advantage of these
characteristics:
- Main memory
-
The bulk of a
PC's memory uses DRAM and is called main
memory. It is large—typically 64 MB to 512 MB or
more—but too slow to keep up with a modern CPU. Main memory is
where the CPU stores programs and data that it will soon need. Main
memory functions as a buffer between the CPU and disk and stores tens
to hundreds of megabytes.
- Cache memory
-
Cache memory
is a small amount of fast SRAM that
buffers access between the CPU and main memory.
Modern PCs have two layers of cache memory:
- Primary cache memory
-
Primary
cache, also called Level 1 cache or L1
cache, is typically 16 to 128 KB of very fast memory on
the same chip as the CPU itself. L1 cache size and efficiency are
major factors in CPU performance. The amount and type of L1 cache is
determined by the CPU you use, and cannot be upgraded.
- Secondary cache memory
-
L1
cache is not large enough to eliminate the speed disparity between
processors and main memory. Secondary cache,
also called Level 2 cache,
L2 cache, or (on Pentium-class motherboards)
external cache, bridges that gap with a
reasonable compromise between cost and performance. L2 cache is a
part of the CPU package (or of the CPU substrate itself) on all
modern processors, including the Intel Celeron/Pentium II/III/4 and
the AMD Athlon/Duron/Opteron. Modern processors have L2 cache memory
sizes ranging from 128 KB to 2 MB.
 |
Pentium and earlier processors use discrete L2 cache chips on the
motherboard. Although it's possible to upgrade the
L2 cache on many such systems, it seldom—we're
tempted to say "never"—makes
sense to spend money on a system that old. If for some reason you
must upgrade a Pentium-class motherboard, see the first or second
edition of this book for details.
Some PCs and motherboards arrive
with cache disabled. Always check the Chipset Setup section of CMOS
Setup to verify that L1 and L2 cache are both enabled.
|
|
|
Main Menu
|
