Copyright Michael Karbo and ELI Aps., Denmark, Europe.
Chapter 19. Different types of RAM
RAM works in synch with the system bus, as I described in the previous chapter. But what is RAM actually? RAM is a very central component in a PC, for without RAM there can be no data processing. RAM is simply the storage area where all software is loaded and works from.
Silicon storage area
RAM is made in electronic chips made of so-called semiconductor material, just like processors and many other types of chips.
In RAM, transistors make up the individual storage cells which can each “remember” an amount of data, for example, 1 or 4 bits – as long as the PC is switched on.Fig. 126. Manufacturing RAM.
Normal RAM is dynamic (called DRAM), and requires constant electronic recharging to preserve its data contents. Without power, all RAM cells are cleared. RAM is very closely linked to the CPU, and it is very important to both have enough RAM, and to have fast RAM. If both conditions are not met, the RAM will be a bottleneck which will slow down the PC. What follows is an introduction to RAM, as it is used in modern PC’s. After this I will discuss the various types I more detail.
At the time of writing, there are several types of RAM, which are quite different. This means that they normally cannot be used on the same motherboard – they are not compatible.
However, some motherboards can have sockets for two types of RAM. You typically see this during periods where there is a change taking place from one type of RAM to another. Such motherboards are really designed for the new type, but are made “backwards compatible”, by making room for RAM modules of the old type.
The RAM types on the market at the moment are:
Figur 127. Very different types of RAM.
In any case, there is a lot of development taking place in DDR. A number of new RAM products will be released within the next few years. The modules are packaged differently, so they cannot be mixed. The notches in the sides are different as the he bottom edges of the modules are.
SDRAM is an old and proven type, which is used in the majority of existing PC’s. DDR RAM is a refinement of SDRAM, which is in reality double clocked. Rambus RAM is an advanced technology which in principle is superior to DDR RAM in many ways. However, Rambus has had a difficult birth. The technology has been patented by Rambus Inc., which has been involved in many legal suits. A number of important manufacturers (such as VIA) have opted out of Rambus, and only develop products which use DDR RAM. With the new DDR2 standard, there is no obvious need for Rambus RAM.
Notes on the physical RAM
RAM stands for Random Access Memory. Physically, RAM consists of small electronic chips which are mounted in modules (small printed circuit boards). The modules are installed in the PC’s motherboard using sockets — there are typically 2, 3 or 4 of these. On this motherboard there are only two, and that’s a bit on the low side of what is reasonable.Figur 128. RAM modules are installed in sockets on the motherboard. In the background you see the huge fan on a Pentium 4 processor.
Each RAM module is a rectangular printed circuit board which fits into the sockets on the motherboard:
Fig. 129. A 512 MB DDR RAM module.
On a module there are typically 8 RAM chips which are soldered in place. There can also be 16 if it is a double-sided module. Below is a single RAM chip:
On the bottom edge of the module you can see the copper coated tracks which make electrical contact (the edge connector). Note also the profile of the module; this makes it only possible to install it one way round and in the right socket.Fig. 131. Anatomy of the SD RAM module.
Module or chip size
All RAM modules have a particular data width, which has to match the motherboard, chipset, and ultimately the CPU. Modules using the two most common RAM types, SD and DDR RAM, are 64 bits wide.
The modules are built using chips which each contain a number of megabits. And since each byte needs 8 bits, more than one chip is needed to make a module. Look at the RAM chip in Fig. 130. You can see the text ”64MX4”:Fig. 132. Text on a RAM chip.
The text in Fig. 132 indicates that the chip contains 64 x 4 mega bits of data, which is the same as 256 megabits. If we want to do some calculations, each chip contains 1024 x 1024 x 64 = 67,108,864 cells, which can each hold 4 bits of data. That gives 268,435,456 bits in total, which (when divided by 8) equals 33,554,432 bytes = 32,768 KB = 32 MB.
So a 64MX4 circuit contains 32 MB, and is a standard product. This type of chip is used by many different manufacturers to make RAM modules. Modules are sold primarily in four common sizes, from 64 to 512 MB:
Fig. 133. The module size is determined by what RAM chips are used.
For each type of RAM, there are modules of various sizes. But there are also modules with various speeds. The faster the RAM chips are, the more expensive the modules.
Fig. 134. The speed is measured in megahertz, but it is not possible to compare directly between the three types of RAM shown in the table. The names shown are those which are being used on the market at the time of writing, but they may change.