RAM (random access memory) is a type of volatile memory that can be accessed randomly and stores recently used data and instructions to allow for fast access by the CPU. It consists of small electronic chips mounted on modules that are installed in sockets on the motherboard. Different types of RAM have been developed over time with improved speeds, including DRAM, FPM, EDO, SDRAM, DDR RAM, and Rambus RAM, with later varieties operating at higher clock frequencies in a synchronous manner with the system bus. RAM modules come in various sizes and speeds depending on the memory technology.

1. RAM
Description:
Ram (random access memory) is the place in a computer where the operating
system, application programs, and data in current use are kept so that can be quickly reached by
the computer’s processor.
 Ram is much Faster to read and write than floppy disk,hard disk and CD-ROM.
 Data stays there only as long as computer is running.
 When computer is turned off it loses its data.
 It is a volatile memory.
Physical Appearance Of Ram
RAM consists of small electronic chips which are mounted in modules (small printed circuit
boards). The modules are installed in the PC’s motherboard using sockets, there are typically 2, 3
or 4 of these. On this motherboard there are only two, and that’s a bit on the low side of what is
reasonable.
 RAM modules are installed in sockets on the motherboard. In the background you see the
huge fan on a Pentium 4 processor.

2.  Each RAM module is a rectangular printed circuit board which fits into the sockets on the
motherboard:
Fig. 512 MB DDR RAM module.
 On a module there are typically 8 RAM chips which are soldered in place. There can also
be 16 if it is a double-sided module. Below is a single RAM chip:
Fig.
Figur130.
A single RAM
chip, a 256
megabit
circuit.
 On the bottom edge of the module you can see the copper coated tracks which make
electrical contact (the edge connector). Note also the profile of the module; this makes it
only possible to install it one way round and in the right socket.
Module or chip size
All RAM modules have a particular data width, which has to match the motherboard, chipset,
and ultimately the CPU. Modules using the two most common RAM types, SD and DDR RAM,
are 64 bits wide.

3. Different Types Of Ram
In the old days
 Back in the 1980’s, DRAM was used. This was dynamic RAM, which was relatively
slow. It was replaced by FPM (Fast Page Mode) RAM which was also dynamic, only a
bit faster.
 Originally, loose RAM chips were installed directly in large banks on the motherboard.
Later people started combining the chips in modules.
 FPM RAM could not run any faster than 66 MHz, but that was fine for the system bus
clock frequency in the original Pentium processors.
 After FPM came EDO RAM (Extended Data Out). EDO is a bit faster than FPM because
the data paths to and from the RAM cells have been optimised. The gain was a 3-5 %
improvement in bandwidth. The clock frequency could be increased to 75 MHz, but
basically, EDO is not very different to FPM RAM.
SDRAM
 The big qualitative shift came in around 1997, when SDRAM (Synchronous DRAM)
began to break in. This is a completely new technology, which of course required new
chipsets. SDRAM, in contrast to the earlier types of RAM, operatessynchronously with
the system bus.
 Data can (in burst mode) be fetched on every clock pulse. Thus the module can operate
fully synchronised with the bus – without so-called wait states (inactive clock pulses).
Because they are linked synchronously to the system bus, SDRAM modules can run at
much higher clock frequencies.
 The 100 MHz SDRAM (PC100) quickly became popular, and with new processors and
chipsets, the speed was brought up to 133 MHz (PC133).

4. DDR RAM
 It is expensive to produce fast RAM chips. So someone hit on a smart trick in 1999-2000,
which in one blow made normal RAM twice as fast. That was the beginning of DDR
RAM (Double Data Rate). See the module in Fig. 131.
 In DDR RAM, the clock signal is used twice. Data is transferred both when the signal
rises, and when it falls. This makes it possible to perform twice as many operations per
clock pulse, compared to earlier RAM types:
Timings
 DDR RAM exist in many versions, with different the clock frequencies and timings. The
timing indicates how many clock cycles there are wasted, when the motherboard waits
for the memory to deliver the requested data.
 With smaller numbers, we have better timings and the CPU having fewer idle clock
cycles. The better timing, the more expensive the RAM module is.
Rambus RAM
Rambus Inc., as already mentioned, has developed a completely new type of RAM technology.
Rambus uses a completely different type of chip, which are mounted in intelligent modules that
can operate at very high clock frequencies. Here is a brief summary of the system:
 The memory controller delivers data to a narrow high-speed bus which connects all the
RDRAM modules in a long series.
 The modules contain logic, which stores the data in the format the chips use.
 Data is written to one chip at a time, in contrast to SDRAM where it is spread across
several chips.

5.  The modules work at 2.5 volts, which is reduced to 0.5 volts whenever possible. In this
way, both the build up of heat, and electromagnetic radiation can be kept down. They
are encapsulated in a heat conducting, aluminium casing.
 Rambus RAM thus has a completely new and different design. The modules are only 16
bits wide. Less data is transferred per clock pulse, but the clock frequencies are much
higher.
 As the advanced Rambus modules are quite costly to produce, the technology is on its
way out of the market.
(Reference Book: PC Architecture By B.Karbo)