Processing and Memory - Strata Study Guide - page 18

Memory Bus Speeds
The pathway that delivers data to and from the memory is called a memory bus . Memory
has a bus width that determines how many columns are in each row of storage. All the
bits in a single row are read together as a single value, so the wider the memory bus width,
the more data that can be read at once. For example, in memory with an 8-bit width, you
might have a number like 01001100. In memory with a 32-bit width, you could have a
number with up to 32 binary digits.
The memory bus also has a speed, which determines how quickly data will travel on its
pathway. Memory on modern PCs is synchronized (that is, synchronous ) with the system
bus, which in turn is controlled by the system timer on the motherboard. The system timer
determines the speed at which data enters the processor. Memory that operates at the same
speed as the front-side bus is called single data rate (SDR) synchronous dynamic read-only
memory (SDRAM) .
The original successor to SDR SDRAM was double data rate (DDR) SDRAM , also
sometimes called DDR1. It makes higher transfer rates achievable by strictly controlling the
timing of the electrical data and clock signals so that data can be double-pumped into
the RAM: that is, pumped in on both the rising and the falling of each tick of the internal
system clock (see Figure 1.6). The name double data rate is a reference to DDR's capability
of achieving nearly twice the bandwidth of SDR.
FIGURE 1.6 DDRdatatransfer
Data Exchange
Data Cycle
Data Exchange
After DDR1 came DDR2 SDRAM, which enables greater throughput and requires
lower power by running the internal clock at half the speed of the data bus, in addition to
double-pumping the bus. This effectively multiplies the DDR1-level performance by two, so
that there are a total of four data transfers per internal clock cycle.
DDR3 goes even further, once again doubling the data rate, to a total of eight times
the original SDR throughput. It also uses about 30 percent less power than DDR2
modules because it uses a lower voltage. The main benefi t of DDR3, and the reason it
doubles the data rate, isn't due to a raw increase in the pumping, but due to the use of a
deeper prefetch buffer. A prefetch buffer is an extra buffer on the RAM that allows quick
access to data located on a common physical row in the memory. (A buffer is a simpler
version of a cache.)
Generally speaking, most motherboards accept only one type of RAM: SDR, DDR,
DDR2, or DDR3. Even if the motherboard is physically compatible with other types, it's
programmed to work with RAM at a certain speed.