The front-side busis another name for the system bus; on modern machines
there’s also a back-side bus,which connects the microprocessor to a second-
level cache— specialized memory that holds recently used or anticipated
commands and data. The back-side bus is very fast, but extremely limited in
its capacity and its purpose.
But now that we’ve got that down, you should also be aware that you may
not be able to find new PC1600 modules; this design has been supplanted by
PC2100 modules — which work in a system looking for PC1600. In techie talk,
an advanced technology that also works with older designs is declared to be
backward compatible.
Since I’ve brought up the subject of PC2100, here’s the skinny on these little
chips: They are meant to work with systems with a 133-MHz front-side bus,
and with DDR that yields a 266 MT/s data transfer rate and a 2.1GB total band-
width, which gives the module its PC2100 name. (PC2100 arrived with the first
Pentium III and AMD Athlon microprocessors. If you install a PC2100 module
in a machine that is looking for PC1600, it operates at the lower bandwidth.)
Next up were PC2700 and PC3200 DDR RAM. They follow the same math.
PC2700 was designed for systems with a 166-MHz front-side bus yielding
333 MT/s data transfer rate and a 2.7GB bandwidth; it is sometimes called
DDR333. PC3200 was designed for systems with a 200-MHz front-side bus
yielding 400 MT/s data transfer rate and a 3.2GB bandwidth; it is sometimes
called DDR400.
The high-end for DDR memory, not yet that common in laptops, uses a 266-
MHz front-side bus. Do the math backwards: That means a 533 MT/s data
transfer rate and a 4.2GB bandwidth. So, its name is PC4200 or DDR533.
SDRAM
Synchronous Dynamic Random Access Memory (SDRAM)is the industry’s
basic form of memory. The most modern version of SDRAM, which you
already explored, uses a technology called DDRor Double Data Rate.The
naming convention for SDRAM is straightforward; names begin with PCand
are followed by the speed of the front-side bus.
For example, PC66 works with systems based on a 66-MHz front-side bus; this
older scheme was introduced with early Pentium and Macintosh G3 systems.
PC100, for computers using a 100-MHz front-side bus, is more common and
employed on motherboards based on Pentium II, Pentium III, AMD Athlon,
AMD Duron, and Macintosh G4 microprocessors. The next step up is PC133
for 133-MHz bus systems.
Chapter 6: Brain Matters: Memory, Microprocessors, and BIOS 89