1.0 Personal Computer Components
1.1 Identify the fundamental principles of using personal
Types of memory (e.g. DRAM, SRAM,
SDRAM, DDR / DDR2, RAMBUS)
PC memory stores data and programs currently being executed by the computer.
It is important that the information is fetched by the CPU quickly to further
processing. There are several memory types available. Important among there
include the following:
Dynamic RAM (DRAM)
Synchronous RAM (SRAM)
Synchronous DRAM (SDRAM)
Rambus DRAM (RDRAM)
Video RAM (VRAM)
RAM stands for Random Access Memory.
Dynamic RAM (DRAM):
In dynamic RAM, the RAM gets refreshed continually by the controller. DRAM
has been introduced in the earlier stages, and RAM versions available today are
much bigger and faster than the earlier simple DRAMs.
DRAMs store data in the form of capacitive charges. Since any capacitor tends
to be leaky, a DRAM needs to be refreshed on a continual basis.
Synchronous RAM (SRAM):
SRAM contains a clock built onto the memory module, enabling the SRAM to be
in synchronization with the motherboard cloak. SDRAM doesn't require frequent
recharge like DRAM. L-2 memory caches are usually made of SRAM and exhibit very
fast read and write operations.
Synchronous DRAM (SDRAM): SDRAM works in sync with the motherboard, and hence
works quite fast. SDRAMs have speeds of the order of 133MHz, 800MHz, etc.
Double Data Rate SDRAM (DDR
DDR SDRAM is similar to SDRAM, but for the difference that DDR reads data on
both the rising and falling edges of the clock. SDRAM reads only on the rising
edge of a signal. This technique allows the DDR module to achieve speeds twice
that of SDRAM. For example, instead of a data rate of 133MHz, DDR memory
transfers data at 266MHz.
Rambus DRAM (RDRAM):
RDRAM technology was developed originally by
Rambus, Inc. Rambus memory is
integrated onto Rambus Inline Memory Modules (RIMMs). RDRAM chips are
synchronized to the processor's memory bus.
CPU synchronized to the motherboard. DRAM is neither synchronized to the
motherboard nor CPU.
CPU and memory module are synchronized to the motherboard.
Synchronizes to the memory bus clock. Memory bus clock is much faster
than the motherboard clock. Hence faster data transfer between the CPU and
the memory module occurs.
Video RAM (VRAM): VRAM is primarily used on video cards. It is dual ported,
in the sense that while one device write to VRAM, another device can
simultaneously do read operation. This is quite useful in animation and other
speed sensitive video applications. VRAMs are more expensive than DRAMs,
but provide better graphic display.
Memory chips (8, 16, 32)
Parity versus non-parity
Parity enables some basic error detection, and
correction capabilities. This comes at the cost of increased
overhead, and additional cost. Normally, parity is not used for
memory chips in personal computers. If there is any specific
requirement for added error detection and correction capabilities,
one may go for parity enabled memory modules.
ECC vs. non-ECC
ECC memory, short for Error-Correcting Code memory,
a type of memory that enables some basic error detection and
correction capabilities, similar to parity. Here again, you need to
pay additional cost for ECC memory modules, and regular memory
modules do not support ECC or parity.
Single-sided vs. double-sided
Memory modules are available with single sided contacts (SIMM)
and double sided contacts (DIMM). SIMM stands for Single In-line Memory Module,
and DIMM stands for Dual In-line Memory Module. Even the memory chips may be
soldered on either side of the PCB or on a single side. Double sided soldering
of memory chips enables higher memory density.30-pin SIMMs are single-sided.
72-pin SIMMs are either single-sided or double-sided.
SIMMs have contacts on either side of the circuit board but they
are electrically shorted. So a 72-pin (or a 30-pin) SIMM has pads on each side
of the circuit board, but each pair of pads is connected together. DIMMs
however have different connections on each side of the circuit board. So a
168-pin DIMM has 84 pads on each side and they are NOT shorted electrically.
This results in smaller packaging volume for DIMM modules. DIMMs support 64-bit
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