CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [91]
Figure 6-6 That’s a lot of real estate used by RAM chips!
Figure 6-7 A 72-pin SIMM
Modern CPUs are a lot smarter than the old Intel 8088. Their machine languages have some commands that are up to 64 bits (8 bytes) wide. They also have at least a 64-bit frontside bus that can handle more than just 8 bits. They don’t want RAM to give them a puny 8 bits at a time! To optimize the flow of data into and out of the CPU, the modern MCC provides at least 64 bits of data every time the CPU requests information from RAM.
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NOTE Some MCCs are 128 bits wide.
Modern DRAM sticks come in 32-bit- and 64-bit-wide form factors with a varying number of chips. Many techs describe these memory modules by their width, so ×32 and ×64. Note that this number does not describe the width of the individual DRAM chips on the module. When you read or hear about by whatever memory, simply note that you need to know whether that person is talking about the DRAM width or the module width. When the CPU needs certain bytes of data, it requests those bytes via the address bus. The CPU does not know the physical location of the RAM that stores that data, nor the physical makeup of the RAM—such as how many DRAM chips work together to provide the 64-bit-wide memory rows. The MCC keeps track of this and just gives the CPU whichever bytes it requests (Figure 6-8).
Figure 6-8 The MCC knows the real location of the DRAM.
Consumer RAM
If modern DRAM modules come in sizes much wider than a byte, why do people still use the word “byte” to describe how much DRAM they have? Convention. Habit. Rather than using a label that describes the electronic structure of RAM, common usage describes the total capacity of RAM on a stick in bytes. John has a single 512-MB stick of RAM on his motherboard, for example, and Sally has two 256-MB sticks. Both systems have a total of 512 MB of system RAM. That’s what your clients care about, after all, because having enough RAM makes their systems snappy and stable; not enough and the systems run poorly. As a tech, you need to know more, of course, to pick the right RAM for many different types of computers.
Essentials
Types of RAM
Development of newer, wider, and faster CPUs and MCCs motivate DRAM manufacturers to invent new DRAM technologies that deliver enough data at a single pop to optimize the flow of data into and out of the CPU.
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EXAM TIP Old RAM—really old RAM—was called fast page mode (FPM) RAM. This ancient RAM used a totally different technology that was not tied to the system clock. If you ever hear of FPM RAM, it’s going to be in a system that’s over a decade old. Be careful! CompTIA likes to use older terms like this to throw you off!
SDRAM
Most modern systems use some form of synchronous DRAM (SDRAM). SDRAM is still DRAM, but it is synchronous—tied to the system clock, just like the CPU and MCC, so the MCC knows when data is ready to be grabbed from SDRAM. This results in little wasted time.
SDRAM made its debut in 1996 on a stick called a dual inline memory module (DIMM). The early SDRAM DIMMs came in a wide variety of pin sizes. The most common pin sizes found on desktops were the 168-pin variety. Laptop DIMMs came in 68-pin, 144-pin (Figure 6-9), or 172-pin micro-DIMM packages; and the 72-pin, 144-pin, or 200-pin small-outline DIMM (SO-DIMM) form factors (Figure 6-10). With the exception of the 32-bit 72-pin SO-DIMM, all these DIMM varieties delivered 64-bit-wide data to match the 64-bit data bus of every CPU since the Pentium.
Figure 6-9 144-pin micro-DIMM (photo courtesy of Micron Technology, Inc.)
To take advantage of SDRAM, you needed a PC designed to use SDRAM. If you had a system with slots for 168-pin DIMMs, for example, your system used SDRAM. A DIMM in any one of the DIMM