The system memory is the place where the
computer holds current programs and data that are in use, and, because of the
demands made by increasingly powerful software, system memory requirements have
been accelerating at an alarming pace over the last few years. The result is
that modern computers have significantly more memory than the first PCs of the
early 1980s, and this has had an effect on development of the PC’s
architecture. Storing and retrieving data from a large block of memory is more
time-consuming than from a small block. With a large amount of memory, the
difference in time between a register access and a memory access is very great,
and this has resulted in extra layers of ‘cache’ in the storage hierarchy.
When it comes to access speed, processors
are currently outstripping memory chips by an ever-increasing margin. This
means that processors are increasingly having to wait for data going in and out
of main memory. One solution is to use ‘cache memory’ between the main memory
and the processor, and use clever electronics to ensure that the data the
processor needs next is already in cache.
Primary cache
The primary cache, or ‘Level 1’ cache, is on the CPU and is used for temporary
storage of instructions and data organised in blocks of 32 bytes. Primary cache
is the fastest form of storage. Because it’s built in to the chip with a zero
wait-state (delay) interface to the processor’s execution unit, it is limited
in size.
SRAM is manufactured in a way rather similar
to how processors are: highly integrated transistor patterns photo-etched into
silicon. Each SRAM bit is comprised of between four and six transistors, which
is why SRAM takes up much more space compared to DRAM, which uses only one
(plus a capacitor). This, plus the fact that SRAM is also several times the
cost of DRAM, explains why it is not used more extensively in PC systems.
Secondary cache
Most PCs are offered with a secondary cache to bridge the processor/memory
performance gap. Secondary cache (also know as ‘external’ or ‘Level 2’ cache)
uses the same control logic as primary cache and is also implemented in SRAM.
Secondary cache typically comes in two
sizes, 256KB or 512KB, and can be found, or soldered onto the motherboard. The
aim of the secondary cache is to supply stored information to the processor
without any delay (wait-state).
Main memory
A PC’s third and principal level of system memory is referred to as main
memory, or Random Access Memory (RAM). It is an impermanent source of data, but
is the main memory area accessed by the hard disk. It acts, so to speak, as a
staging post between the hard disk and the processor. The more data it is
possible to have available in the RAM the faster the PC will run.
Main memory is attached to the processor via
its address and data buses. Each bus consists of a number of electrical
circuits or bits. The width of the address bus dictates how many different
memory locations can be accessed, and the width of the data bus how much
information is stored at each location. Every time a bit is added to the width
of the address bus, the address range doubles.
Main memory is built up using DRAM chips,
short for Dynamic RAM.