memory, refers to computer components, devices, and recording media that retain digital data used for computing for some interval of time. Computer data storage provides one of the core functions of the modern computer, that of information retention. It is one of the fundamental components of all modern computers, and coupled with a central processing unit (CPU, a processor), implements the basic computer model used since the 1940s.
In contemporary usage, memory usually refers to a form of semiconductor storage known as random access memory (RAM) and sometimes other forms of fast but temporary storage. Similarly, storage today more commonly refers to mass storage - optical discs, forms of magnetic storage like hard disks, and other types slower than RAM, but of a more permanent nature. Historically, memory and storage were respectively called primary storage and secondary storage.
The contemporary distinctions are helpful, because they are also fundamental to the architecture of computers in general. As well, they reflect an important and significant technical difference between memory and mass storage devices, which has been blurred by the historical usage of the term storage. Nevertheless, this article uses the traditional nomenclature.
Single In-line memory module (SIMM) - A slender circuit board dedicated to storing memory chips. Each chip is capable of holding 8 to 9 chips per board, the ninth chip usually an error checking chip (parity / non parity). The typical bus from the chip to the motherboard is 32-bits wide. When upgrading a Pentium motherboard you will be required to upgrade 2 of the same type of chips at the same time to accommodate the Pentium processor.
SPEED: You can determine the size amount of the chip by looking at the part number of each chip on the SIMM board. For 2-, 8- and 9- chip SIMMs, all the chips should have the same part numbers. Look at the number that ends with a dash and a digit such as "-7". This is the rate speed or nanoseconds of the chip. With "-7" this would indicate that the memory is 70ns.
SIZE: Look at the four digits to the left of this number; these often carry information about the number of bits in the chip. A 4256 indicates 256K bits arranged in sets of four, for a total of 1Mb. "1000" indicates 1MB of bits arranged in one set.
With some types of memory, the last one or two digits may be changed to indicate different kinds of memory; there are 1MB chips that end with 4256, 4257, and 4258. In this case, round the last digits to an even 256 or thousand. Three-chip SIMMs will typically have two larger chips that are four times the capacity of the third chip (because 4 plus 4 plus 1 makes 9, which is the number of bits needed per byte including parity).