RAM

What Does RAM Do?


The CPU uses RAm as its active workplace. Programs stored on the hard drive need to be loaded into RAM to enable the CPU to swap data in and out quicly. Without RAM, the modern PC simply does not function.


You can conceptualize RAM as the CPU's short-term memory. RAM has no direct connection to either the external data bus or the address bus; the Northbridge chip acts as the intermediary for all data transfers between RAM abd the CPU. The CPU sends requests for data stored in RAM the Northbridge chip via the address bus, and the Northbridge chip retrieves the data from RAM and drops in onto the external data bus for pickup by the CPU.


The CPU uses RAM to store active programs and data temporarily. As is changes data, the CPU tells the Northbridge to update that date in RAM. So, as you work on a document, such as a letter to your friend Sally, you can type merrily away and watch the data on the screen change to match what you're doing.


RAM does the job of storing programs just fine - but RAM also has a tiny problem! If your computer suddenly lost power, everything that you typed would be irretrievably lost. RAM used in the PC is a volatile medium, meaning that it requires continuous electricity to hold data. Losing power for even a split second clears RAM completely.


RAM Sticks

The RAM in a typical PC consists of one or more small circuit boards, called sticks, usually about 4 inches (13 cm) long. The average PC has at least ons stick of RAM, although two, three, or four are not at all uncommon.

If you look closely at those sticks of RAM, you see that they're covered with small chips. These chips are the actual physical location of the RAM. These chips work together to form the many millions of rows of 64-bit wide RAM used in today's PCs.

TYPES of RAM Technologies

RAM manufacturers have developed many ways to hold data over the lifespan of the PC. Five technologies stand out because of their amazing longevity and market share; all are commonly known by their abbreviations: FPM, EDO, SDRAM, DDR SDRAM, and RDRAM.

FPM RAM

RAM chips in early PCs were all 1 bit wide, but even tha RAM commands had to be at least 8 bits wide. These early provided 8 bits at a time with eight little RAM chips all nicely mounted on the mothereboard.

Southland Micro 8 MB FPM RAM (SMC8MB-A1100)


EDO RAM

The Northbridge chip accessed FPM RAM by engaging a pair of wires called the Row Array Strobe (RAS) and Coulmn Array Strobe (CAS) in rapid sequence. To get a single row of RAM, the Northbridge needed to light up the RAS and CAS for each bit of data stored.

EDO RAM extend the amount of data you got without relighting the RAS wire for each bit.





SDRAM

In the mid-1990s, RAM technology finally incorporated a clock, in Synchronous Dynamic Random Access Memory (SDRAM). SDRAM shared the same organization as the earlier types of RAM, but for the first time the North bridge could accurately predeict the number of clocl cycles needed for RAM to produce requested data, speeding RAM access dramatically.

As you might imagine, given that it has a clock speed, SDRAM speed is measured in hertz (Hz), just like CPU speed. SDRAM speeds generally matched the speed of the motherboard-that is, if the motherboard ran at 66 MHz, the you need to use 66-MHz SDRAM. If the motherboard ran at 100MHz, you would need 100-MHz SDRAM.

* Motherboard documentation showing the required SDRAM speed

Memory * 2 168-pin DIMM sockets

* Supports PC-100/PC-133 SDRAM

* Supports up to 1.0GB DRAM

* Supports only 3.3V SDRAM DIMM

DDR SDRAM

As its name implies, Double Data Rate SDRAM (DDR SDRAM) is SDRAM, but it can make two RAM accesses in each clock cycle, hence the "double." DDR SDRAM comes in four common speeds: 200, 266, 333 and 400 MHz.

DDR SDRAM is extremely popular today. Odds are good that unless you have a higher-end Intel Pentium 4 or better, the system you have in front of you uses DDR SDRAM. Again, the motherboard documentation will clearly state both the type of RAM and the speed it needs.

RDRAM

Way back in 1992, a company called Rambus invented a new type of memory technology called Rambus Dynamic Random Access Memory (RDRAM). RDRAM is a very hig-speed type of clocked RAM that runs at 400 or 533MHz, achieving these high speeds by accessing RAM four times per cliock cycle.

RAM PACKAGES

Understanding RAM technologies enables you to make informed recommendations to clients, but techs work with hardware: selecting, installing and configuring.

RAM Technology Typical CPU Today

FPM Obsolote; Pentium and earlier

EDO Obsolote; Pentium II, Pentium, early Celeron and Athlon

SDRAM ALL (Pentium and later)

DDR SDRAM Later Celeron and Duron, Athlon XP, Pentium 4

RDRAM Pentium 4