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CrossBrand - SIMMs and DIMMs Explained
Applicable to: World-Wide
SIMM vs DIMM
Memory on IBM ValuePoint and PS/2 products has been implemented via SIMMs. However, the term DIMM has recently been introduced. The following is a comparison of SIMM versus DIMM.
First, some definitions:
- A 'tab' on a memory module is the metal contact at the bottom of the memory module. Usually, IBM uses gold as the metal for this tab, but it could be tin. Sometimes the tab is called a 'pin' or 'contact'.
- A 'bank' of memory is a set of memory chips in which all chips/DRAMs are accessed simultaneously. A 'bank' of memory is not necessarily on one side of a SIMM/DIMM. For example, all 16 MB SIMMs are considered a single bank, but have chips/DRAMs on both sides.
- SIMM (Single In-line Memory Module)
Tabs on both sides of the module can read out the same signal because they are shorted together. The same signal is duplicated on both sides of the tabs on the SIMM.
A 72-pin SIMM has 72 gold/tin tabs on both sides of the SIMM. Tab 1 on the 'A' side and tab 1 on the 'B' side are shorted together, that is, it is the same tab/pin/contact. Therefore, there are a total of 144 (72 x 2) gold/tin tabs to make contact with the SIMM connector, but only 72 independent signals.
The reason that tabs are shorted together on both sides of the SIMM is because the connector for 72-pin SIMMs is unreliable if it tries to provide signals to the card on only one side of the SIMM. The only way to make the connector design reliable is to have a duplicate contact on the other side of the SIMM.
SIMMs are used on all PS/2s and ValuePoints (as of March 1994).
- DIMM (Dual In-line Memory Module)
Tabs on both sides of the module are independent and therefore have different signals.
A 168-pin DIMM has 168 total tabs, 84 on each side. Because the tabs are independent, there are a total of 168 independent signals, as opposed to the 72 independent signals on the 72-pin SIMM.
168-pin DIMMs are a convenient way to support 64-bit processors and 64-bit memory transfers (for Pentium and PowerPC). Sometimes they are called '8-byte DIMMs' because 8 bytes equals 64 bits. However, there is talk in the industry of 8-byte, 200-pin DIMMs.
DIMMs are a method to get more signals per inch of connector.
Most connectors in PCs are DIMMs. For example, Micro Channel and ISA cards are DIMM connectors.
A 168-pin DIMM has buffers for critical signals in the module, which provides faster memory access and better signal quality. Buffers on the module reduce the need for buffers on the planar. So the cost of buffers is incremental as memory is added, compared to adding buffers on the planar for a maximum memory environment. However, some DIMMs (that are not 168 pin) do not have buffers
DIMMs are designed to support x4, x8/x9, x16/x18 memory chips. These are defined below:
- 8-byte DIMMs have 3 to 4 times the number of Vcc GND signals to improve performance and signal quality.
- 8-byte DIMMs can be parity, non-parity, or ECC.
- 8-byte DIMMs can support 3.3 volt technology and 64Mbit and 256 Mbit DRAMs.
- The Options by IBM brand announced DIMMs on PCC 94-065as 'IBM 8MB Memory Upgrade; 70ns; 168 pin' as well as 16MB and 32MB versions. They are all 5 volts technology.
The 72-pin SIMM and 168-pin DIMM are incompatible; they will not fit in each other's socket/connector.
How Do DIMMs Relate to 'Single RAS' and 'Dual RAS' SIMMs?
Single RAS is defined as memory modules with one bank; so the chips/DRAMs are all accessed simultaneously. Single RAS modules may or may not have all the chips/DRAMs on one side of the module (single RAS = single bank). Dual RAS means that there are two banks on the module; so only one bank can be accessed at a time (dual RAS = two banks). Dual RAS means two banks, but this does not imply that memory is always located on both sides.
Also, there is no relationship between sides of the SIMM/DIMM containing memory chips and the pins/tabs on that side.
Single RAS SIMMs still have the signal duplicated on the tabs on each side of the SIMM, even though only one bank exists and the chips may be on only one side of the module.
The reason some products (like ValuePoint Si) require single RAS memory SIMMs is because they have fewer RAS signals. Twice as many RAS signals are needed to support dual RAS SIMMs as single RAS SIMMs. The ValuePoint Si memory controller has four RAS signals to support four SIMM sockets. If Dual RAS SIMMs were supported, eight RAS signals are required to support four SIMM sockets.
The 168-pin DIMM is architecturally compatible with the 72-pin SIMM, meaning that the DIMM can be used in some applications to replace 72-pin SIMMs. The same signals used to drive two 72-pin SIMMs can drive a single 168-pin DIMM. This is true for non-parity, parity, and ECC memory. The DIMM specification allows for many types of uses:
- 2 x 32-bit non-parity
- 1 x 64-bit non-parity
- 2 x 32-bit parity
- 1 x 72-bit parity
- 2 x 39/40-bit ECC (IBM uses 39-bit ECC, but the industry standard is 40)
- 1 x 72-bit ECC.
Therefore, a 168-pin DIMM can be used for a 32-bit or a 64-bit data bus. The notation, 2 x 36-bit parity, means that the DIMM can be treated as if it combined two 36-bit parity SIMMs (two 72-pin SIMMs) into a single DIMM. The DIMM was designed so that if a memory controller was designed to work with pairs of 32-bit (72-pin) SIMMs, then the two 72-pin SIMMs could be replaced by a single DIMM (so no changes are needed in memory controller architecture). Remember though, that the connectors are incompatible; so you cannot put DIMMs into SIMM sockets.
Memory chips are provided in organisations of:
- 1 data bit/chip (x1 organisation)
- 4 data bits/chip (x4 organisation)
- 8 data bits/chip (x8 organisation)
- 9 data bits/chip (x9 organisation)
- 16 data bits/chip (x16 organisation)
- 18 data bits/chip (x18 organisation).
This organisation determines the amount of memory chips on the module. For example, if a module is a 72-pin SIMM with no parity (that is, 32-bit), it can use x4 organisation with 8 chips/DRAMs of memory. A 72-pin SIMM supports two banks, so up to 16 chips/DRAMs would be on the SIMM. If x8 organisation were used, only half as many chips/DRAMs (8) would be needed for both banks of the SIMM.
All DIMMs are not 168-pin DIMMs. There are 72-pin DIMMs about half the size of a standard SIMM. These would be used in notebooks and are sometimes called 'small outline DIMMs'. They have 72 (36 x 2) independent signals 36 pins each side). The small size of these DIMMs requires 8, x9, x18, x19 organisation DRAMs, so that very few memory chips are put on the DIMM, thereby allowing the size of the card to be reduced.
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Date Created |
10-08-98 | |
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11-08-98 | |
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