PCs have quite a bit of memory of different kinds. Let’s take a look at some general concepts that will help us understand what we are talking about when we say “memory.”
One of the things we expect our PCs to do is to store information. This information can be our data files, it can be our application software, or our operating system. If this information were not stored in the PC itself, we would have to re-install it every time we turned on the PC, and then it would have to be stored or kept somewhere while it was being used, as well.
So right here we can see that there are two kinds of storage – short-term or temporary, and long-term or permanent. Permanent storage is the kind where the information will be accessible the next time we turn on the PC – that is, the information does not disappear when the power is off. Permanent storage is more properly known as secondary storage. The kind of devices that are secondary storage devices are those that maintain data when the power is off: hard drives, floppy disks, CD and DVDs, ZIP disks and so on.
Temporary storage, on the other hand, is where data, applications or programs reside while the system is using them. This kind of storage is called primary storage. RAM is primary storage. Information in primary storage is volatile – it is lost when the power is off.
It’s important to remember that the CPU can only access something that is in primary storage. The CPU has no direct connection with secondary storage devices. If it needs to work with a particular piece of data, that data must be moved from secondary storage (your hard drive) into primary storage (RAM). Therefore, the more RAM a PC has, the more information can be readily available to the CPU and the faster the data can be processed.
PCs have a great deal of secondary storage and much less primary storage. Secondary storage is where all information is kept on a permanent basis, until deleted, and primary storage only holds information being used by the processor.
Chapter 4 - RAM
Main memory (DRAM) is where most of the data and instructions that are being used by the processor are kept until needed. DRAM holds its contents in capacitors. This technology is inexpensive, making it feasible to have quite a bit of main memory in your system very cost effectively. The disadvantage to DRAM is that the capacitors hold information for only a very short time. In order for the contents to be maintained, the data must be refreshed or rewritten many times per second.
DRAM has not changed much over time. In early PCs, DRAM was found on the motherboard in groups of individual, socketed chips. The potential for upgrading was not very easy, so this was quickly superceded by putting these memory chips on small cards or strips which plugged directly into slots on the motherboard.
The first strips were called SIMMs or single in-line memory modules. These strips had 30 pins or connecting places on the bottom edge and were known as 30-pin SIMMs. Then came 72 pin SIMMs, which may still be found in some older working systems, such as earlier Pentiums. SIMMs have a 32-bit data path.
Next came DIMM technologies, or Dual in-line memory modules. Like SIMMs, DIMMs have pins along the bottom edge, but the pins on one side are counted separately from the other side. DIMMs can have 168, 184 or 240 pins.
Early DIMMs were SDRAM or synchronous dynamic RAM strips with 168 pins. This memory is rated by the system bus speed and it operates in sync with the system clock (PC100 or PC133). These modules can hold anywhere from 8 MB to 2 GB of RAM. The data path on SDRAM is 64 bits wide.
DDR SDRAM (double data rate SDRAM) runs twice as fast as regular SDRAM by processing data twice for every clock beat. These strips have 184 pins, and can hold up to 2 GB of memory. The data path is 64 bits wide.
RIMM technology consists of Rambus DRAM. 184 pin RDRAM modules are physically incompatible with DDR...