Hardware Reference
In-Depth Information
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 capacitors are all consistently arranged in a (normally
square) grid of simple repetitive structures, unlike processors, which are much more complex circuits
of different structures and elements interconnected in a highly irregular fashion.
The transistor for each DRAM bit cell reads the charge state of the adjacent capacitor. If the
capacitor is charged, the cell is read to contain a 1; no charge indicates a 0. The charge 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 refresh 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, corrupted files, and any number of system crashes.
DRAM is used in PC systems because it is inexpensive and the chips can be densely packed, so a lot
of memory capacity can fit in a small space. Unfortunately, DRAM is also 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 0.25ns or less, so it can keep pace with processors
running 4GHz or faster. This is because of the SRAM design, which calls for a cluster of six
transistors for each bit of storage. The use of transistors but no capacitors means that refresh rates are
not necessary because there are no capacitors to lose their charges over time. As long as there is
power, SRAM remembers what is stored. With these 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 more expensive
(see
Table 6.1
). The lower density means that SRAM chips are physically larger and store fewer bits
overall. The high number of transistors and the clustered design mean that SRAM chips are both
physically larger and much more expensive to produce than DRAM chips. For example, a high-
density DRAM chip might store up to 4Gb (512MB) of RAM, whereas similar-sized SRAM chips
can only store up to 72Mb (9MB). The high cost and physical constraints have prevented SRAM from
being used as the main memory for PC systems.
Table 6.1. Comparing DRAM and SRAM
Even though SRAM is impractical for PC use as main memory, PC designers have found a way to use
SRAM to dramatically improve PC performance. Rather than spend the money for all RAM to be
SRAM memory, they design in a small amount of high-speed SRAM memory, used as cache memory,
