Memory Chips and SIMMs
Figure1 shows a typical memory chip, in actual size. The memory chip itself is a small sliver of silicon no bigger than a fingernail. However, since it is so sensitive to air that exposure would destroy it, the chip is hermetically sealed in a plastic case, as shown in the figure. The larger size of the case also provides enough room for the pins.
Memory chips are generally placed in computers in memory banks consisting of 8 or 9 chips per bank. In early PC systems, adding additional memory required carefully inserting these chips, one by one, into sockets on the motherboard or on a memory expansion board. If a fragile pin was bent, it could easily break, rendering the chip useless.
These days, most PCs are designed to accept packages of chips that are permanently soldered onto their own small circuit boards. The most popular version of this is the single inline memory module, or SIMM, shown in Figure2. SIMMs are much easier to install and remove than individual chips.
One edge of the SIMM board has the metal contacts that take the place of pins. This edge snaps easily into a SIMM socket, eliminating the problem of breaking a pin. Thus, SIMMs are much easier to install and remove than individual memory chips, bringing the process of upgrading memory into the realm of the average user. Figure3 shows how the letter E (as an ASCII codeword) might be stored in a SIMM module inside a PC.
