Memory
CSIT 301 (Blum)
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Types of DRAM
• Asynchronous
– The processor timing and the memory timing
(refreshing schedule) were independent. Thus the
processor might have to wait until the memory
“window” was open for access.
• Synchronous (SDRAM)
– The processor and memory timing are linked. This
allows for more efficient processor-memory interaction.
– Note SDRAM is NOT static dynamic RAM – that
doesn’t make sense
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The Role of Cache
• It’s important to remember the role of cache (SRAM)
when trying to understand the distinction between various
types of DRAM.
• Over 90% of the time the processor finds what it needs
in cache.
• But when one needs to access memory, one caches not
only that particular value held in that particular location
but also many of the values in the nearby locations
(because they are likely to be needed as well – locality of
reference).
• So the differences in memory types are often not in an
individual access but in the accessing of a larger amount of
data for the purposes of caching it.
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Asynchronous DRAM
• Asynchronous DRAM was common until the mid
to late 1990’s but now is out-dated.
– Fast Page Mode
• What made FPM fast was that the same row but different
columns of data could be accessed without forcing one to
reselect the row strobe.
– Extended Data Out (EDO)
• What was “extended” about EDO was that it could go longer
between refreshes.
– Burst Extended Data Out (BEDO)
• Consecutive data was fetched in “bursts” saving on the
addressing part of access time.
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Synchronous DRAM
• Since the mid to late 1990’s SDRAM has
taken over as the standard for use in main
memory.
– JEDEC (Joint Electron Device Engineering
Council) or PC66 or Ordinary SDRAM
operates with bus speeds up to 66MHz, now is
outdated.
– PC100 SDRAM that works at higher bus speed
of 100 MHz
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Synchronous DRAM (Cont.)
– PC133 SDRAM operates at bus speed of 133
MHz and slower. This is a standard memory
these days.
– There are versions of PC133 SDRAM that
differ in “latency.”
– Latency is the time you spend waiting until
conditions are right to proceed with some
action.
– This is the CAS Latency mentioned previously.
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DDR-SDRAM
• Double Data Rate Synchronous DRAM
• One allows data to be accessed on both the
positive and negative edge of the clock (double
pumping). This effectively doubles the
throughput.
– The associated chips go by PC200 (double PC100) or
PC266 (double PC 133)
– But the memory modules are designated by
throughput. With a 64-bit bus (8 bytes) operating at
PC200 (double pumped 100MHz bus), the DDR
module goes by PC1600
• 1600 = 200  8
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DDR2 from Wikipedia
http://en.wikipedia.org/wiki/DDR2_SDRAM
• “The key difference between DDR and
DDR2 is that in DDR2 the bus is clocked at
twice the speed of the memory cells,
allowing transfers from two different cells
to occur in the same memory cell cycle.
Thus, without speeding up the memory cells
themselves, DDR2 can effectively operate
at twice the bus speed of DDR.”
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Rambus DRAM
• Rambus DRAM or RDRAM
• A proprietary memory technology produced
by Rambus.
• It is a competitor of the DDR-SDRAM but
it seems to be losing out.
– Which is better depends to the situation.
– For very high memory usage in P4 systems,
RDRAM might be better.
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Serial Presence Detect
• With all of the different types of memory, the
computer needed a way to determine the specs of
the memory installed so that it could be used
efficiently.
• This is the job of the Serial Presence Detect
(SPD).
• A set of data usually stored on the memory stick
itself that informs the BIOS of the module's size,
data width, speed, and voltage.
• Previously done by the parallel presence detect
(PPD).
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Memory Packaging
• How memory is packaged
– DIP: Dual inline pin Package (OLD)
• Rectangular, has pins on both sides
• 386
– SIPP: single inline chip packages (OLD)
• Little circuit board with memory chip
• Has pins, hard to install
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Memory modules (Cont.)
• SIMM single inline memory module
– Like SIPP but no pins, easier to install
– a 32-bit path to the memory
30-pin SIMM
72-pin SIMM
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Memory modules (Cont.)
• DIMM dual in-line memory module,
– a 64-bit path
• RIMM the Rambus version of DIMM (is not
really an acronym)
• SO-DIMMs (small outline) are memory
modules for laptops
– Initially laptop memory was proprietary and not easily
changed or updated. Furthermore there was no easy
access to its location.
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Row versus Bank
• Row: Physical unit of memory
• Bank: logical unit
• The bank is based on bus width. Pentiums
use a 64-bit width.
– Two SIMMs to make a bank
– One DIMM to make a bank
– One RIMM to make a bank
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Installing Memory
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References
•
•
•
•
http://www.pcguide.com/ref/ram/types.htm
http://www.webopedia.com
http://www.whatis.com
PC Hardware in a Nutshell, Thompson and
Thompson
• All-in-One A+ Certification, Meyers and
Jernigan
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