SDRAM Bank Interleaving – What is It?

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Where The Confusion Stems From

The confusion is mainly due to a poor choice of terminology, and the term in question is the word “bank“. Unknown to a lot of people, the term “bank” has two different meanings when talking about memory, and the meaning depends on the context.

The memory slots on your motherboard are arranged in “banks”. These slots are what you plug a memory module (a DIMM, SIMM or RIMM) into. For the sake of simplicity, I’ll just refer to DIMM’s./

A memory module (a DIMM) is either single sided or double sided. If it is single sided* (memory chips on only one side), it occupies one “bank”. If it is double sided, it occupies two “banks” of memory on your motherboard.

The memory controller has a limit to the number of these types of banks it can access (be it 2, 4, 6 or 8). This is mainly due to timing issues and is part of the design of the chipset.

FUD Alert: These banks have nothing to do with memory interleave. Memory interleave does not depend on how many sticks of RAM you have.

A memory module (a DIMM) generally has, in the case of unbuffered, non-parity memory, 8 chips (single sided module) or 16 chips (double sided module) on it, and 9 or 18 for parity or ECC memory. More chips than that is considered a ‘high density’ module (and will likely be ‘Registered), and won’t be supported by most PC motherboards. Memory chips are rated in ‘megabits’ (or Mb), with common SDRAM chip sizes currently being 16Mb, 64Mb, 128Mb and 256Mb. Therefore, a 64MB module will usually be comprised of 64Mb chips, will be single sided, and there will be eight chips. A 128MB module might have sixteen 64Mb chips (double sided module), or might have eight 128Mb chips (single sided module). Of course, you could have a 64MB module with four 128Mb chips , or even a 128MB module with four 256Mb chips, but these are less common and could be either single sided or double sided.

The determining factor for ‘sidedness’ is the chip configuration (discussed below), which determines how many chips are necessary for reading/writing 64 bits of data. By counting all of the bits that can be read/written by each chip, you will either end up with 64 (single sided) or 128 (double sided). If you get more than this, you have either an ECC module (eight to sixteen extra bits, depending on sidedness) or a Registered module (usually 256 bits). Registered modules are usually not supported in desktop PCs.

Inside each chip, the memory is arranged much like a spreadsheet (rows and columns), called an ‘array’. Access to data depends on the row and column you are looking for. To make the design of memory chips easier (read: less complex), memory manufacturers adopted a scheme whereby the arrays on each chip are arranged in “banks” – there’s that term again. Since about the days of 16Mb chips, memory chips (not modules) have been arranged in either two banks or four banks. If your memory chips are 64Mb or larger, then it is most likely that there are four banks internal to the chip. This is the “bank” that is referred to when people speak about memory interleave.

The only way to be sure that your memory supports bank interleave is to examine your DIMM, write down the chip number (it should be the same on each chip), and go to the manufacturer’s web site and look it up. The tech docs will tell you if the memory is arranged as two “bank” (2 bank interleave is possible) or four “bank” (4 bank interleave is possible).

Remember, the number of banks on the motherboard (accessed by the memory controller) is not the same as the number of banks on each chip.

Footnote:

* This is not strictly true. it is probably more accurate to say that a single sided DIMM has only eight chips on an unbuffered (common) DIMM. Usually this is arranged as eight chips on one side of the circuit board. There is nothing stopping the memory manufacturer putting four chips on each side however (and I have seen modules arranged just so).

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