Hardware Reference
In-Depth Information
DRAMsuseonlyonetransistorandcapacitorpairperbit,whichmakesthemdense,offer-
ing more memory capacity per chip than other types of memory. Currently, DRAM chips
are being prepared for production with densities up to 4Gb (512MB) per chip, which at
one transistor per bit requires at least 4 billion transistors. The transistor count in memory
chips is much higher than in processors because in a memory chip the transistors and ca-
pacitorsareallconsistentlyarrangedina(normallysquare)gridofsimplerepetitivestruc-
tures,unlikeprocessors,whicharemuchmorecomplexcircuitsofdifferentstructuresand
elements interconnected in a highly irregular fashion.
The transistor for each DRAM bit cell reads the charge state of the adjacent capacitor. If
thecapacitor ischarged,thecell isreadtocontain a1;nochargeindicates a0.Thecharge
in the tiny capacitors is constantly draining, which is why the memory must be refreshed
constantly. Even a momentary power interruption, or anything that interferes with the re-
fresh cycles, can cause a DRAM memory cell to lose the charge and thus the data. If this
happens in a running system, it can lead to blue screens, global protection faults, corrup-
ted files, and any number of system crashes.
DRAM is used in PC systems because it is inexpensive and the chips can be densely
packed,soalotofmemorycapacitycanfitinasmallspace.Unfortunately,DRAMisalso
relatively slow—typically much slower than the processor. For this reason, many types of
DRAM architectures have been developed to improve performance. These architectures
are covered later in the chapter.
Cache Memory: SRAM
Another distinctly different type of memory exists that is significantly faster than most
types of DRAM. SRAM stands for
static RAM
, which is so named because it does not
need the periodic refresh rates like DRAM. Because of the way SRAMs are designed, not
only are refresh rates unnecessary, but SRAM is much faster than DRAM and much more
capable of keeping pace with modern processors.
SRAM memory is available in access times of 0.45ns or less, so it can keep pace with
processorsrunning2.2GHzorfaster.ThisisbecauseoftheSRAMdesign,whichcallsfor
a cluster of six transistors for each bit of storage. The use of transistors but no capacit-
ors means that refresh rates are not necessary because there are no capacitors to lose their
chargesovertime.Aslongasthereispower,SRAMrememberswhatisstored.Withthese
attributes, why don't we use SRAM for all system memory? The answers are simple.
Compared to DRAM, SRAM is much faster but also much lower in density and much
moreexpensive (see
Table6.1
).ThelowerdensitymeansthatSRAMchipsarephysically
largerandstorefewerbitsoverall.Thehighnumberoftransistorsandtheclustereddesign
mean that SRAM chips are both physically larger and much more expensive to pro-
