DRAM
DRAM memory is different from static ram in a sense that the cells are not
bi-stable elements but just addressed capacitors holding a charge which
represents the state of the cell. As this charge leaks away over time the cell
needs to be read out and reprogrammed regularly. Typically this must be done
each 4-100ms for the whole array. Since the state of the cell is a mere
analogue voltage the technology is called dynamic. The technology has several
disadvantages : Since the data is a charge which can be disturbed by internal
or external events the soft error rate is higher than with other types of
memory. Also the complexity of the circuits controlling the memory is much
higher since the address bus is generally multiplexed and the cells must be
refreshed regularly. During refresh the memory may not be available for data,
dependent on the refresh scheme used. Finally dynamic ram is slower than static
ram. Its only advantage over static ram is PRICE. Dynamic ram is a fourfold
cheaper per bit over static ram, explaining its wide use in computer
applications where large memory sizes are required as data or program memory.
Today Dram's come in different technologies which are all designed to overcome
one of the biggest drawback of drams : speed.
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Fast page mode (FPM)
: This type was the most widespread until 1996. It used to represent over 90%
of all dram sales. Fast page dram allows the system to select a row address and
then a column address as usual. The page is then determined by the row address,
and the system can switch within a very short time to another column address
while keeping the rom address identical (within the same page).
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Nibble mode
: This type allows to read or write data on 4 successive addresses in a burst
mode. A row and column address is selected, and the next three locations become
the logical next addresses without full addressing needing to reoccur.
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Static column mode
: This type allows static addressing once a row address has been selected. As
soon as this has been done (a page has been chosen) the column address can
freely change without column strobing, making the device faster.
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EDO mode (Extended Data out)
: This type is basically fast page memory with an added data latch which hold
the data at the output while the address changes, allowing a higher memory
bandwith. This type of memory has become mainstream since 1996 or personal
computers.
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SDRAM (Synchronous Dram) : This is basically a dram with a fixed clock
making data transfers more precise and thus faster. SDrams come in many
different technologies and can have important internal architectural
differences. All SDrams are designed to boost speed. Some will be better for
cpu applications, other will be more suitable for video applications. Basically
SDRAM memory is FPM memory made synchronous, has several banks that can be
accessed simultaneously and can accept memory handling commands.