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Packaging Modules

We've discussed how memory chips work on the inside, but you'll need to know how these chips are installed on a motherboard. Most of the changes in form came about either to make maintenance easier or to avoid bad connections. Keep in mind that installing a combined unit or module of some kind is less expensive than having many individual units to install. (This is one reason why Intel wanted to get the Level 2 cache onto the die, rather than onto a separate processor IC board.)

Everything about the packaging of memory chips rests on the concept of modules. These modules are vaguely like tiny motherboards within a motherboard, in that they, too, are integrated circuit boards. The big difference between DRAM and SDRAM is the synchronization feature. Be sure you understand how SDRAM uses timing cycles to more efficiently interrupt the CPU. Remember, SRAM is extremely fast and is used in secondary caches; SDRAM is a type of main memory.

It's all well and good to know how SDRAM differs from Rambus RAM, but you're also going to have to be able to differentiate between SIMMs and DIMMs. Inline memory modules are the small IC cards you install in your machine when you upgrade your memory. You won't have to remember clock speeds and the exact number of pins, but you'll definitely be tested on the different types of modules. (That being said, keep in mind that SIMMs are usually 30-pin or 72-pin modules, and DIMMs often use a 168-pin configuration.)

Dual Inline Package (DIP)

Originally, DRAM came in individual chips called dual inline packages (DIPs). XT and AT systems had 36 sockets on the motherboard, each with one DIP per socket. Later, a number of DIPs were mounted on a memory board that plugged into an expansion slot. It was very time consuming to change memory, and there were problems with chip creep (thermal cycling), where the chips would work their way out of the sockets as the PC turned on and off. Heat expanded and contracted the sockets, and you'd have to push the chips back in with your fingers.

To solve this problem, manufacturers finally soldered the first 640 kilobytes of memory right onto the board. Then the problem was trying to replace a bad chip. Finally, chips went onto their own card, called a single inline memory module or SIMM. On a SIMM, each individual chip is soldered onto a small circuit board with an edge connector. Prices had fallen, so it was cost effective to simply replace the whole module if a memory chip failed.

Connectors: Gold Versus Tin

SIMMs and DIMMs come with either tin (silver-colored) or gold edge connectors. Although you may assume that gold is always better, that's not true. You'll want to match the metal of the edge connectors to the metal in the board's socket. If the motherboard uses gold sockets, use a gold SIMM. Tin sockets (or slots) should use tin edge connectors. The cost difference is minimal, but matching the metal type is critical.

Although it's true that gold won't corrode, a gold SIMM in a tin connector will produce much faster corrosion in the tin connectors. This quickly leads to random glitches and problems, so look at the board and match the color of the metal.

It's important to note, too, that each module is rated for the number of installations, or insertions. Each insertion causes scratches, and the more metal that is scratched off, the worse the connection becomes. In flea market exchanges and corporate environments, modules are subjected to constant wear and tear, and nobody is looking at the rated number of insertions.

Single Inline Memory Modules (SIMMs)

When DRAM chips were placed in a line on their own circuit board, it gave rise to the term inline memory. After the chips were formed into a module, the entire module would fit into a socket on the board. These modules and sockets are referred to as memory banks. Depending on how the chips are connected on their own little circuit board, the module is called either a single or dual inline memory module (SIMM or DIMM).

CAUTION

Remember that SIMM, with an S, is a Single inline memory module. D is for double, and DIMM is a dual (two) inline memory module, with connectors on two sides, making it a double-edged connector. DIP is a dual inline package, but refers to single chips.

SIMMs come in both 30-pin and 72-pin versions. The 30-pin module is an 8-bit chip, with 1 optional parity bit. The 72-pin SIMM is a 32-bit chip, with 4 optional parity bits.

The memory bus grew from 8 bits to 16 bits, and then from 32 bits to 64 bits wide. The 32-bit bus coincided with the development of the SIMM, which meant that the 32-bit-wide data bus could connect directly to one SIMM (4 sets of 8 bits). However, when the bus widened to 64 bits, rather than making a gigantic SIMM, boards started using two SIMMs in paired memory banks. The 64-bit-wide DIMM was developed after the SIMMs, and went back to using only one module per socket again.

CAUTION

SIMMs and DIMMs are sometimes referred to as chips, but they are really series of chips (modules). DRAM itself is a chip, and many chips are grouped together to form SIMMs and DIMMs. SIMMs can come with a varying number of pins, including 30-pin and 72-pin. (Even though the 72-pin module could have chips on both sides, it was still a SIMM.)

Be careful when you read a question on the exam that you don't accidentally agree that a SIMM is a memory chip. A good way to keep alert is that chips have RAM in their name: DRAM, SRAM, SDRAM, RDRAM, and so forth.

Dual Inline Memory Modules (DIMMs)

Dual inline memory modules are very similar to SIMMs in that they install vertically into sockets on the system board. DIMMs are also a line of DRAM chips, combined on a circuit board. The main difference is that a dual module has two different signal pins, one on each side of the module. This is why they are dual inline modules.

The differences between SIMMs and DIMMs are as follows:

  • A DIMM has opposing pins on either side of its board. The pins remain electrically isolated to form two separate contacts—a dual set of electrical contacts (sort of like a parallel circuit).

  • A SIMM also has opposing pins on either side of the board. However, the pins are connected, tying them together. The connection forms a single electrical contact (sort of like a series circuit).

DIMMs began to be used in computers that supported a 64-bit or wider memory bus. Pentium MMX, Pentium Pro, and Pentium II boards use 168-pin modules. They are 1 inch longer than 72-pin SIMMs, with a secondary keying notch so they'll fit into their slots only one way.

Don't Mix Different Types of Memory

Mixing different types of SIMMs or DIMMs within the same memory bank prevents the CPU from accurately detecting how much memory it has. In this case, the system will either fail to boot, or will boot and fail to recognize or use some of the memory.

  • You can, however, substitute a SIMM with a different speed within the same memory bank, but only if the replacement is equal to or faster than the replaced module.

  • All memory taken together (from all memory banks) will be set to the speed of the slowest SIMM.

Rambus Inline Memory Modules (RIMMs)

Rambus inline memory modules (RIMMs) use Rambus memory chips. On a standard bi-directional bus, prior to Rambus memory, data traveled down the bus in one direction, with returning data moving back up the same bus in the opposite direction. This same process took place for each bank of memory, with each module being addressed separately. As a result, the system entered a wait state until the bus was ready for either type of transfer.

RIMMs use a looped system, where everything is going in one direction (uni-directional) all the time. In a looped system, data moves forward from chip to chip and module to module. Data goes down the line, and then the results data continues forward on the wire in the same direction. The results data doesn't have to wait for downstream data to finish being sent.

Continuity Modules

Because Rambus memory works with serial transfers, there must be a memory module in every motherboard memory slot. Even if all the memory is contained in a single module, the unused sockets must have an installed printed circuit board, known as a continuity module, to complete the circuit. This is similar to a string of lights, wired in series, where every socket requires a bulb.

RDRAM chips are set on their modules contiguously (next to each other in a chain) and are connected to each other in a series. This means that if an empty memory bank socket is in between two RIMM chips, you must install a continuity module. These are low-cost circuit boards that look like a RIMM, but with no memory chips. All the continuity module does is allow the current to move through the chain of RDRAM chips.

NOTE

RDRAM is fast for two reasons: It doesn't have to wait for the bus to turn around, and the cycle time is running at a fast 800MHz, so it doesn't have to wait very long for the next cycle. Using RDRAM chips, signals go from one module to the next to the next, and the throughput is triple that of 100MHz SDRAM. There can also be four RDRAM channels (narrow channel memory) at the same time. This can increase throughput to either 3.2GB (dual channel) or 6.4GB (all four channels).

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