Datasheet

26
Chapter 1
Hardware
Double Data Rate (DDR) SDRAM/DDR2 Essentially, clock-doubled SDRAM. The mem-
ory chip can perform reads and writes on both sides of any clock cycle (the up, or start, and
the down, or ending), thus doubling the effective memory executions per second. So, if you’re
using DDR SDRAM with a 100MHz memory bus, the memory will execute reads and writes
at 200MHz and transfer the data to the processor at 100MHz. The advantage of DDR over
regular SDRAM is increased throughput and thus increased overall system speed.
The next generation of DDR SDRAM is DDR2 (Double Data Rate 2). This allows for two
accesses per clock cycle and effectively doubles the speed of the memory. DDR2-667 chips
work with speeds of 667MHz and PC2-5300 modules, while DDR3-1600 chips support a
12800Mbps throughput.
RAMBUS A relatively new and extremely fast (up to 800MHz) technology that uses, for
the most part, a new methodology in memory system design. RAMBUS (also known as direct
Rambus) is a memory bus that transfers data at 800MHz, and is named after the company
that designed it. RAMBUS memory models (often called Rambus inline memory modules
[RIMMs]), like DDR SDRAM, can transfer data on both the rising and falling edges of a
clock cycle. That feature, combined with the 16-bit bus for efficient transfer of data, results in
the ultra-high memory transfer rate (800MHz) and the high bandwidth of up to 1.6GBps.
Memory Chip Package Types
Memory chips come in many different types of packages. Lets look at the ones most
frequently encountered.
Dual Inline Package (DIP)
Dual inline package (DIP) memory is so named because the individual RAM chips use the
DIP-style package for the memory module. Older computers, such as the IBM AT, arranged
these small chips like rows of caskets in a small memory “graveyard.” This type of memory
has long been obsolete.
SIMMs
Single inline memory modules (SIMMs) were developed because DIPs took up too much
real estate on the logic board. Someone got the idea to put several DIP chips on a small
circuit board and then make that board easily removable.
Each of these RAM circuit boards is a stick of RAM. There are two sizes of SIMMs:
30-pin and 72-pin. The 30-pin are older, 8-bit sticks. The 72-pin are 32-bit sticks. Figure 1.13
shows one of each. SIMMs are called single because theyre single-sided. When you count
the number of pins (the metal tabs) along the bottom, there are 30 or 72 of them. In contrast,
DIMMs (dual inline memory modules) are double-sided; for example, a 168-pin DIMM has
84 pins on each side.
DIMMs and RIMMs
DIMMs (dual inline memory modules) are double-sided memory chips used in modern sys-
tems (Pentium and higher). They typically have 168 pins and are 64 bits in width. Figure 1.14
shows a DIMM.
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