The System Memory

The system memory is the part of a system that is used to store data and programs temporarily. Memory capacity is measured in bytes or, more conveniently, in kilobytes (units of 1,024 bytes), megabytes (units of 1,024 kilobytes) or gigabytes (1,024 megabytes).

Immediate access memory, or internal memory, describes the memory locations that can be addressed directly and individually by the central processing unit. It is either read-only - stored in ROM, PROM (Programmable ROM), and EPROM (Erasable Programmable ROM) chips - or read/write (stored in RAM chips). Read-only memory stores information that must be constantly available and is unlikely to be changed. It is non-volatile that is, it is not lost when the computer is switched off. Read/write memory is volatile, it stores programs and data only while the computer is switched on. Read/write memory is referred to as RAM (Random Access Memory) and is semiconductor based (electronic) memory. The storage locations within RAM can be accessed in any order. Note that the various types of ROM memory are capable of random access, but cannot be written to. The term RAM, however, is generally understood to refer to volatile memory that can be written to as well as read.

There are a variety of RAM types on the market.

DIMM

DIMMs (Dual Inline Memory Modules) are the current standard in system memory, and carry SDRAM (Synchronous RAM). SDRAM has several advantages over EDO RAM, speed being one of them. SDRAM access times range from 7 ns to 9ns and can make use of 100MHz bus speeds. With software packages demanding more and more memory resources, DIMM chips are usually distributed in 32MB, 64MB or 128MB on a single chip. As DIMMs have a count of ‘2’ on the motherboard, they do not need to be fitted in pairs like SIMMs need to be in Pentium Systems.

DIMMs have 168 contact lines (sometimes referred to as 'fingers') and are fitted right from the top into the DIMM socket. Two cut-outs in between the contact lines ensure that they can only be fitted one way around. Once seated properly, two clips (one on each side of the socket) will snap closed, holding the module in position. Always make sure that your memory module is seated correctly, otherwise you will have no Power On Self Test (see Fault Finding).

SIMM

SIMMs (Single Inline Memory Modules) are still in use in older Pentium PCs and even older 486es. A standard 72-pin SIMM 72 ‘fingers’ that connect to the board. Most old Pentium boards have four sockets made from two banks of two. Each bank has to be filled by an equally balanced pair of SIMMs at a time. Single chips are not allowed on a Pentium motherboard and bank 0 (being socket one and two) must be filled before bank 1. That means for a 16 MB system you need to add either two pairs of 4 MB SIMMs or one pair of 8 MB SIMMs in bank 0.

Pentium systems that still take SIMMs need to be fitted with EDO RAM (Enhanced Data Output RAM) as supposed to Standard RAM as used with 486 systems. Very few systems of either generation will take memory modules designed for the other because both systems run at different FSB (Front Side Bus) speeds and have different ways of addressing memory. Also, EDO RAM is about 10 percent faster than standard RAM, which makes it too fast for 498 motherboards. But you need not worry about that, as you are unlikely to come across either generation any more unless you get hold of an old second hand PC.

SIMMs have to be fitted in to their sockets at a slight angle and then clipped into place. One side next to the fingers has a small cut-out (bottom right in the picture above), which fits the little 'step' in the SIMM socket. Don't force the SIMM in the wrong way around! Once it it clipped into the socket, the two clips will hold the module in place.

When you purchase your RAM DIMMs or SIMMs always make sure you get a money back guarantee, regardless of the manufacturer. Usually SIMM or DIMM chips either work the first time or fail almost immediately. If you are thinking of upgrading your system and don’t know where to start first: Memory - That way you get the loudest bang out of your money!

Level 2 Cache Memory

Level 2 cache memory, is used by the system to store and retrieve data from the RAM. Frequently accessed segments of data may be stored in the cache for quicker access compared with secondary storage devices such as disks. Depending on the system you buy, the cache memory comes either with the motherboard (e.g. Socket 7 and earlier) or with the CPU (Pentium 2 and 3, AMD Athlon).

Level 1 Cache Memory

The Intel 486 CPU used to have an 8k build-in cache among its 1.2 million transistors. This build in cache is called level 1 cache or L1. Since it does not have to travel outside the CPU it operates at the same speed as the CPU itself, making it the fastest memory of a computer system. The Pentium CPU uses two separate 8k level 1 caches, doubling the number of cache ports and with it the overall performance of the processor. The Pentium II has 5.5 million transistors and a build in L2 cache of 512k within its design alongside two 16k L1 caches.