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C3
1. Basic concepts of RAM operation
A DRAM memory array can be thought of as a table of cells. These cells are
comprised of capacitors, and contain one or more ‘bits’ of data, depending upon the
chip configuration. This table is addressed via row and column decoders, which in
turn receive their signals from the RAS and CAS clock generators. In order to
minimize the package size, the row and column addresses are multiplexed into row
and column address buffers. For example, if there are 11 address lines, there will be
11 row and 11 column address buffers. Access transistors called ‘sense amps’ are
connected to the each column and provide the read and restore operations of the chip.
Since the cells are capacitors that discharge for each read operation, the sense amp
must restore the data before the end of the access cycle.
The capacitors used for data cells tend to bleed off their charge, and therefore require
a periodic refresh cycle or data will be lost. A refresh controller determines the time
between refresh cycles, and a refresh counter ensures that the entire array (all rows)
are refreshed. Of course, this means that some cycles are used for refresh operations,
and has some impact on performance.
A typical memory access would occur as follows. First, the row address bits are
placed onto the address pins. After a period of time the RAS\ signal falls, which
activates the sense amps and causes the row address to be latched into the row address
buffer. When the RAS\ signal stabilizes, the selected row is transferred onto the sense
amps. Next, the column address bits are set up, and then latched into the column
address buffer when CAS\ falls, at which time the output buffer is also turned on.
When CAS\ stabilizes, the selected sense amp feeds its data onto the output buffer.
Virtual Memory:
A technique your operating system uses to make up for a lack of actual physical
memory. When your computer needs more memory than it actually has installed, it
uses the hard drive as virtual memory. Just as data is saved and read from memory,
but at a much higher speed, the system does this on the hard drive. Due to the
mechanical nature of the hard drive, it is slower. This explains why your hard drive
may be quite active while it appears you are doing nothing. It also explains a lot of
drawbacks on performance.
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