Hardware Reference
In-Depth Information
fetch unit needs access to data. A split cache allows parallel accesses; a unified
one does not. Also, since instructions are not modified during execution, the con-
tents of the instruction cache never has to be written back into memory.
Finally, a fifth issue is the number of caches. It is common these days to have
chips with a primary cache on chip, a secondary cache off chip but in the same
package as the CPU chip, and a third cache still further away.
From the early days of semiconductor memory until the early 1990s, memory
was manufactured, bought, and installed as single chips. Chip densities went from
1K bits to 1M bits and beyond, but each chip was sold as a separate unit. Early
PCs often had empty sockets into which additional memory chips could be
plugged, if and when the purchaser needed them.
Since the early 1990s, a different arrangement has been used. A group of
chips, typically 8 or 16, is mounted on a printed circuit board and sold as a unit.
This unit is called a
SIMM
(
Single Inline Memory Module
)ora
DIMM
(
Dual
Inline Memory Module
), depending on whether it has a row of connectors on one
side or both sides of the board. SIMMs have one edge connector with 72 contacts
and transfer 32 bits per clock cycle. They are rarely used these days. DIMMs
usually have edge connectors with 120 contacts on each side of the board, for a
total of 240 contacts, and transfer 64 bits per clock cycle. The most common ones
at present are DDR3 DIMMS, which is the third version of the double data-rate
memories. A typical DIMM is illustrated in Fig. 2-17.
133 mm
256-MB
memory
chip
Connector
Figure 2-17.
Top view of a DIMM holding 4 GB with eight chips of 256 MB on
each side. The other side looks the same.
A typical DIMM configuration might have eight data chips with 256 MB each.
The entire module would then hold 2 GB. Many computers have room for four
modules, giving a total capacity of 8 GB when using 2-GB modules and more
when using larger ones.
A physically smaller DIMM, called an
SO-DIMM
(
Small Outline DIMM
), is
used in notebook computers. DIMMS can have a parity bit or error correction
added, but since the average error rate of a module is one error every 10 years, for
most garden-variety computers, error detection and correction are omitted.
