|
Random access memory (RAM) is the best known form
of computer memory. RAM is considered "random access"
because you can access any memory cell directly if you know the row and
column that intersect at that cell.
Dynamic RAM (DRAM)
Similar to a microprocessor, a memory chip is an integrated
circuit (IC) made of millions of transistors and Capacitors.
In the most common form of computer memory, dynamic random
access memory (DRAM), a transistor and a capacitor are paired
to create a memory cell, which represents a single bit
of data. The capacitor holds the bit of information either 0 or a 1.
The transistor acts as a switch that lets the control circuitry on the
memory chip read the capacitor or change its state.
Dynamic RAM is a type of RAM that only holds its data if it is
continuously accessed by special logic called a refresh circuit. Many
hundreds of times each second, this circuitry reads the contents of
each memory cell, whether the memory cell is being used at that time by
the computer or not. Due to the way in which the cells are constructed,
the reading action itself refreshes the contents of the memory. If this
is not done regularly, then the DRAM will lose its contents, even if it
continues to have power supplied to it. This refreshing action is why
the memory is called dynamic.
Static RAM (SRAM)
Static RAM is a type of RAM that holds its data without external
refresh, for as long as power is supplied to the circuit. This is
contrasted to dynamic RAM (DRAM), which must be refreshed many times
per second in order to hold its data contents. SRAMs are used for
specific applications within the PC, where their strengths outweigh
their weaknesses compared to DRAM:
- Simplicity: SRAMs don't require external refresh
circuitry or other work in order for them to keep their data
intact.
- Speed: SRAM is faster than DRAM.
In contrast, SRAMs have the following weaknesses, compared to
DRAMs:
- Cost: SRAM is, byte for byte, several times more
expensive than DRAM.
- Size: SRAMs take up much more space than DRAMs
(which is part of why the cost is higher).
All PCs use DRAM for their main system memory, instead of SRAM, even
though DRAMs are slower than SRAMs and require the overhead of the
refresh circuitry. It may seem weird to want to make the RAM out of
something that can only hold a value for a fraction of a second. In
fact, DRAMs are both more complicated and slower than SRAMs.
The reason that DRAMs are used is simple: they are much cheaper and
take up much less space, typically 1/4 the silicon area of SRAMs or
less. To build a 64 MB core memory from SRAMs would be very expensive.
The overhead of the refresh circuit is tolerated in order to allow the
use of large amounts of inexpensive, compact memory. The refresh
circuitry itself is almost never a problem; many years of using DRAM
has caused the design of these circuits to be all but perfected
SDRAM (synchronous DRAM) is a generic name for
various kinds of dynamic random access memory (DRAM) that are
synchronized with the clock speed that the microprocessor is optimized
for. This tends to increase the number of instructions that the
processor can perform in a given time. The speed of SDRAM is rated in
MHz rather than in nanoseconds (ns). This makes it easier to compare
the bus speed and the RAM chip speed. You can convert the RAM clock
speed to nanoseconds by dividing the chip speed into 1 billion ns
(which is one second). For example, an 83 MHz RAM would be equivalent
to 12 ns
DDR SDRAM (double data rate SDRAM) is synchronous
dynamic RAM (SDRAM) that can theoretically improves memory clock speed
or doubles the data rate.
The speed of the system memory is also very important because it is
a contributing factor to overall performance. There are many different
aspects that influence the real-world speed of memory.
RAM Controller
It generates the necessary signals to control the reading and
writing of information from and to the memory, and interfaces the
memory with the other major parts of the system the memory controller
is normally integrated into the system chipset.
Memory Access
When memory is read or written, this is called a memory
access. A specific procedure is used to control each access to
memory, which consists of having the memory controller generate the
correct signals to specify which memory location needs to be accessed,
and then having the data show up on the data bus to be read by the
processor whatever other device requested it
Memory Bus
The memory bus is the set of wires that is used to carry memory
addresses and data to and from the system RAM. The memory bus in most
PCs is also shared with the processor bus, connecting the system memory
to the processor and the system chipset
Processor-memory bottleneck
The memory bus is actually a major limiting factor to overall system
performance. Older computers had the processor running at the same
speed as the memory bus, but newer ones have the processor running at
2, 3 or even more times the speed of the memory. The more that the
processor is running faster than the memory, the more often it will
have to wait for information from the memory. This is why the system
cache is so important, because it is much faster than the main memory,
which means the processor can do more useful work and less waiting.
The actual installation of the memory module does not normally
require any tools. RAM is installed in a series of slots on the
motherboard known as the memory bank. The memory
module is notched at one end so you won't be able to insert it in the
wrong direction.
The memory bank is nothing but the slots in the motherboard and the
slot is nothing but a DIMM (dual in-line memory module) is a double
SSIMM (single in-line memory module). Like a SIMM, it's a module
containing one or several random access RAM chips on a small circuit
board with pins that connect it to the computer motherboard. A SIMM
typically has a 32 data bit (36 bits counting parity bits) path to the
computer that requires a 72-pin connector. For synchronous dynamic RAM
(SDRAM) chips, which have a 64 data bit connection to the computer,
SIMMs must be installed in in-line pairs (since each supports a 32 bit
path). A single DIMM can be used instead. A DIMM has a 168-pin
connector and supports 64-bit data transfer. It is considered likely
that future computers will standardize on the DIMM.
|
