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Dual Inline Packages (DIPs) and Memory Modules
Most memory chips are packaged into small plastic or ceramic packages called dual inline packages or DIPs. A DIP is a rectangular package with rows of pins running along its two longer edges. These are the small black boxes you see on SIMMs, DIMMs or other larger packaging styles. The DIP has been the standard for packaging integrated circuits since the invention of the PC, and in fact the earliest processors were also packaged as (large) DIPs.
Older computer systems used DIP memory directly, either soldering it to the motherboard or placing it in sockets that had been soldered to the motherboard. At that time most systems had a small amount of memory (less than one megabyte) and this was the simplest way to do things. However, this arrangement caused many problems. Chips directly soldered onto the motherboard would mean the entire motherboard had to be trashed if any of the memory chips ever went bad.
Chips inserted into sockets suffered reliability problems as the chips would (over time) tend to work their way out of the sockets. Due to thermal contraction and expansion as the machine was turned on and off, the chips would actually slowly come loose, a process called chip creep. Anyone who has worked at keeping an old XT running for many years probably remembers opening up the box and pushing all the memory chips back into their sockets with their thumbs to fix a memory problem. Dealing with individual chips also made upgrading or troubleshooting difficult.
Newer systems do not use DIP memory packaging directly. The DIPs are soldered onto small circuit boards called memory modules; the two most common being the single inline memory module or SIMM and the dual inline memory module or DIMM. The circuit boards are inserted into special sockets on the motherboard that are designed to eliminate the chip creep problem. This arrangement makes for better reliability and easier installation. Also, since SIMMs and DIMMs are (for most PCs) industry standard, it makes upgrades much simpler as well.