Re: What is a memory bank?

Cliff Lewis wrote:
The instruction book for my A7V8X-X MB says that it is limited to
"PC3200 maximum to 2 banks only and PC2700 maximum to 4 banks only."
It seems obvious that a bank is a memory module, but the board only
has 3 slots, so there is no way I could exceed the 4-bank limit. Does
bank have another meaning? If you had to choose between 2 banks of
PC3200 and 4 banks of PC2700, which would you pick?

Thanks,
Cliff Lewis

It's not that obvious.

Try Figure 2 on page 6 here.
http://download.micron.com/pdf/datasheets/modules/ddr/DD16C32_64_128_256x64A.pdf

There are two groups of eight chips.

A group of eight chips is making a 64 bit wide memory array.
Think of it as the eight chips being an independent entity.
This is termed a "rank", but in some documentation, it is
also called a "bank".

x8 x8 x8 x8 x8 x8 x8 x8

Now, when two of those are put on the same module (a double
sided module), the chips get wired to similar lanes on the
memory bus. The DIMM connector has the 64 wires for the
64 bit memory array. The wiring looks like this (compare to
Figure 2 if you like).

x8 x8 x8 x8 x8 x8 x8 x8 Bank 0 Double
| | | | | | | | Sided
x8 x8 x8 x8 x8 x8 x8 x8 Bank 1 Module
| | | | | | | | Sample Configuration
Memory_slot_64_bits_wide

Each bank has its own chip select (CS#), with all the chips in the
bank wired together. The chip select tells a group of eight chips
to "go active". The other chips in the opposite bank "snooze", because
their chip select is not active at the same time. Thus the two
banks are independent of one another.

The above example is for a double sided module. There are a number
of other issues that I won't get into here, but the above is a
start at addressing the question of "what is a bank". If you
install three conventional double sided modules, that is a total
of six banks.

*******

Higher speed memory, is backward compatible with slower operating
speeds. You can buy two PC3200 sticks, then adjust the BIOS to run
them at PC2700 (DDR333). A PC3200 stick can operate all the way down
to PC1600 if you like.

So you don't have to buy the "exact" speed.

There are cases, where the "exact" speed purchase is necessary, and
there are some motherboards (like in prebuilt computers), where
memory is not adjustable in the BIOS. For example, there is one
motherboard, where the computer will crash if it runs at PC3200.
If you install a PC3200 module, you cannot get into the BIOS, because
it will crash. The BIOS in that case, reads the SPD chip on the module,
and when it sees the PC3200 speed rating, that is what it uses.
Apparently the author of the BIOS, was clueless about the fact that
the hardware was not rated to do that speed. Since the end user has
no control over their fate with that hardware, they would be advised
in that restricted case, to buy PC2700 modules. The BIOS will read
the PC2700 speed info in the SPD, and as a result, won't have any
"speeding" accidents :-)

For DIY home-build motherboards, generally there are some BIOS
adjustments, and in your case, since the system is guaranteed to
run a PC3200 single stick of RAM, you can plug in one stick of
your new RAM, and then turn the speed down to PC2700 (DDR333)
in the BIOS screens. Save and Exit. Shut off the power to the
computer, and add the second stick of RAM. It should then run
the two sticks at PC2700.

Always shut off all power, before adding or removing hardware
from the computer. I like to unplug my computer, as a
convenient way to assure myself that all power is off.
Certainly the switch on the back can do it, but at least
one person had a busted power switch, and the power actually
stayed on. Asus motherboards have a green LED, and if the green
LED is lit, it is unsafe to add or remove RAM. When the green
LED goes out, then you can add RAM.

Have fun,
Paul
.

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