Download Physical Memory and Physical Addressing

Physical Memory and
Physical Addressing
By Alex Ames
Types of RAM
Static Ram
SRAM
Dynamic Ram
DRAM
SRAM





SRAM uses a flip flop to store a bit
Very fast
Consume a lot of power
Larger than DRAM, so less density
Typically used for memory applications that are
small but fast
DRAM





DRAM use a capacitor to store a charge
representing if it is set or cleared.
Capacitors are smaller than flip flops, so it can be
more densely packed
Slower than Flip Flops
Low cost, low power, high density
Generally used for main memory
Problems with DRAM



There is an odd problem with DRAM where they
can be prematurely discharged by naturally
occurring gamma rays
Early PC's did not have error correction, but it was
not a large problem because it was a rare event and
PCs were usually turned off at the end of the day
Now most computers have error correction
algorithms because many computers run for very
long periods of time, and errors would accumulate.
Structure

The processor sends data to the memory
controller, which modifies the RAM
2-1/2D One-Bit Ram
Example of RAM Organization



During a read operation, an entire row is fed to the
column MUX which selects the bit for reading
During a write operation, the bit to be written is
distributed by the DEMUX to the target column,
while the row decoder selects which row to write
to
This example shows a square configuration, but
usually a 2x1 rectangle is used to reduce the pin
count
RAM Properties


Size – Determines the density.
Speed – Determines how fast read and write
operations are

Read operations and write operations often have
different speeds and should be measured separately
RAM Size and Density



The size of RAM determines how much can be
packed on to one chip
Higher density chips have more memory in the
same space
However, they use power power and generate
more heat
Flash Memory


Each flash cell stores a
single bit in a floating
gate as a negative
charge
Before updating, the
charge must be cleared
by applying high
voltage to it (flashing)
Flash Memory

Made of dielectric
material




Poor conductors of
electricity
Good at holding a
charge
That's why flash
memory doesn't need
power
However, flash
memory can only
survive about 100000
write operations
SDRAM


SDRAM is synchronized to the clock, meaning
that it waits for a clock signal before performing
operations.
This allows the chip to have a more complex
pattern of operation than DRAM which does not
have synchronizing control circuits.
Virtual Memory



Virtual Memory allows a program to treat
segmented memory as contiguous memory
This makes programming much easier, as the
operating system will ensure that the program
believes it has all the memory it needs
It can even extend to more memory than is
available, by moving some of the data in RAM to
the hard drive


Allows for much more main memory
However, it is much slower than regular RAM
Memory Organization