Memory Technologies

1
Memory Technologies

• Memory is an essential part of modern day digital
  systems
• There are many different types of memory
  architecture each of which has advantages and
  disadvantages
• In this lecture we will discuss the some of these
  memory types

2
Random Access Memory - RAM

• This memory can be written to and read from as
  often as you like
• Random access refers to the fact that we can
  access any position in the memory equally fast
• This type of memory is used for short term
  storage, such as program variable storage
• It is a volatile memory type
• This means that it will only store its contents
  as long as there is power supplied to the device

3
RAM Signal Lines

• A typical RAM device will have at least the
  following signal lines
• DATA BUS This is the set of lines which carry
  data into and out of memory. The memory width is
  how many bits can be written or read
  simultaneously
• ADDRESS BUS Set of lines which is used to
  specify location of data to be written to or read
  from
• WR line Tells RAM it is going to be written to
  or read from
• OE line Enables the data bus as an output for
  when we read the device
• CS line Used to select device

4
Writing to RAM

• The process of writing to RAM is illustrated with
  timing diagrams below. To summarize the essential
  steps
• Ensure that the line is high
• Assert the address on the address lines
• Pull the Chip select ( ) line low
• Place the data onto the data bus
• Pull the Write Enable ( ) line low
• Pull , and high.

5
RAM Timing Diagrams
6
RAM types

• There various types of RAM devices and they
  differ in what types of components they use to
  store information
• We will discuss a few of the RAM types in the
  next few slides

7
Basic RAM types DRAM

• DRAM stands for Dynamic RAM
• It uses tiny capacitors to store each bit of
  information
• It is cheap and a high density technology
• The capacitors are not perfect and leak charge
• They therefore need to be recharged periodically,
  DRAM needs to be refreshed frequently
• It forms the basis for most computer memory
• It is typically used with a DRAM controller which
  handles all the refresh cycles

8
DRAM Illustration
Taken from Wikipedia
9
Basic RAM types SRAM

• SRAM stands for static RAM
• It uses a flip-flop to store each bit in memory.
• Because it uses flip-flops there is no need to
  refresh SRAM
• It is more expensive and lower density than DRAM
• But it has better access times which means that
  it is faster to store and retrieve data then DRAM
• It is simpler to use than DRAM as it does not
  require memory refreshes

10
SRAM Illustration
Taken from Wikipedia
11
Special RAM types SDRAM

• SDRAM stands for synchronous dynamic RAM
• It is a DRAM based technology which has been
  enhanced and improve the performance of the
  computing system it is in
• When a computer uses memory it often exhibits a
  property called spatial locality
• When one memory location is accessed we will
  often access other memory locations close to it
  soon afterwards
• SDRAM is modified so that it operates
  synchronously with the computers clock line

12
Special RAM types DDR

• This has now become one of the most popular RAM
  types for PCs
• It is based around SDRAM except the difference is
  that the access bus has been enhanced to allow
  memory access on both clock edges
• DDR stands for Double Data Rate

13
Popular RAM Form Factors

• A form factor is the physical size and shape of a
  device
• RAM comes in a number of standard sizes
• These generally originate in the computer
  industry
• Early desktop computers used individual RAM chips
  which took up a large amount of space on the
  mother board

14
History of RAM Packages
DIP
SIPP
30 pin SIMM
72 pin SIMM
DIMM
DDR DIMM
Adapted from Wikipedia
15
SIMM

• Single Inline Memory Modules are small circuit
  boards which has RAM chips on them
• They occupied less space on the mother board
• They first came out in 30 pin packages and then
  in 72 pin packages
• The 72 pin devices FPM (slower) or EDO (faster)
  RAM chips on them

16
DIMM Modules

• When PCs started using SDRAM the memory form
  factor changed
• The new devices were called Dual Inline Memory
  Modules
• These had even more pins than the SIMMs
• DIMMs had a 10ns (PC100) access time device and a
  7.5ns device (PC133) when connected to a 100MHz
  bus

17
DDR DIMM Modules

• When PCs started using DDR the memory form
  factor changed again
• The new devices were called DDR Dual Inline
  Memory Modules
• They come in a variety of speeds