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CompTIA A+ Training Kit: Understanding RAM and CPUs

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In this chapter, you’ll learn about two important concepts for any A+ technician to understand: random access memory (RAM) and central processing units (CPUs). A CPU is the brain of the computer, performing most of the processing, and RAM is used to store applications and data being used by the CPU. Both continue to be steadily improved and include a significant amount of technical detail that can easily confuse a regular user. This chapter will help you understand many of the terms used when describing them.

Exam 220-801 objectives in this chapter:

  • 1.2 Differentiate between motherboard components, their purposes, and properties.

    • CPU sockets

  • 1.3 Compare and contrast RAM types and features.

    • Types

      • DDR

      • DDR2

      • DDR3

      • SDRAM

      • SODIMM

      • RAMBUS

      • DIMM

      • Parity vs. non-parity

      • ECC vs. non-ECC

      • RAM configurations

        • Single channel vs. dual channel vs. triple channel

      • Single sided vs. double sided

    • RAM compatibility and speed

  • 1.6 Differentiate among various CPU types and features and select the appropriate cooling method.

    • Socket types

      • Intel: LGA, 775, 1155, 1156, 1366

      • AMD: 940, AM2, AM2+, AM3, AM3+, FM1, F

    • Characteristics

      • Speeds

      • Cores

      • Cache size/type

      • Hyperthreading

      • Virtualization support

      • Architecture (32-bit vs. 64-bit)

      • Integrated GPU

    • Cooling

      • Heat sink

      • Fans

      • Thermal paste

      • Liquid-based

Exam 220-802 objectives in this chapter:

  • 4.2 Given a scenario, troubleshoot common problems related to motherboards, RAM, CPU and power with appropriate tools.

    • Common symptoms

      • Unexpected shutdowns

      • System lockups

      • Overheating


httpatomoreillycomsourcemspimages1445701.jpg When technicians are talking about a computer’s memory, they are primarily talking about random access memory (RAM). RAM is used for short-term storage of applications or data so that the processor can access and use this information. In contrast, computers use hard drives for long-term storage of data.

Most RAM is volatile. This doesn’t mean that it’s explosive; it means that data in RAM is lost when power is removed.

As an introduction, the following list identifies commonly used types of RAM. All of these types of RAM are volatile.

  • Dynamic RAM (DRAM). Dynamic refers to how bits are stored in an electrical component called a capacitor. The capacitor holds the bit as a charge, but the capacitor needs to be regularly refreshed to hold the charge. This configuration uses very few components per bit, keeping the cost low, but the constant refresh reduces the speed.

  • httpatomoreillycomsourcemspimages1445701.jpg Synchronous DRAM (SDRAM). SDRAM is synchronized with a clock for faster speeds. Almost all primary DRAM used in computers today is SDRAM, but it’s often listed as DRAM to avoid confusion with SRAM.

  • Static RAM (SRAM). Static RAM uses switching circuitry instead of capacitors and can hold a charge without a constant refresh. It requires more components per bit so it is more expensive, but due to how the switching works, it is quicker than DRAM. Due to the speed, SRAM is commonly used for CPU cache (described later in this chapter) but is rarely used as the primary RAM because of its cost.

Flash memory is very popular, but not as the primary RAM used in a system. USB flash drives, solid-state drives (SSDs), and memory cards used in cameras and other mobile devices all use flash memory. Flash memory is used for BIOS in many motherboards. Unlike DRAM and SRAM, flash memory is not volatile and retains data without power.

Double Data Rate SDRAM

While the original SDRAM versions were quick and efficient for their time, manufacturers have steadily improved them. Double data rate (DDR) is one of the improvements and is used in almost all SDRAM. As a reminder, SDRAM is tied to a clock, and when the clock ticks, data is transferred.

httpatomoreillycomsourcemspimages1445701.jpg SDRAM uses only the leading edge for the clock. However, each of the DDR SDRAM versions uses both the leading and trailing edge of the clock. This is often called double pumping. Figure 3-1 compares the two over two cycles of a clock. You can see that SDRAM has two clocks from these cycles and that DDR has four clocks from the same two cycles.

Figure 3-1

Figure 3-1. SDRAM compared with double-pumping DDR.

The following list provides an overview of the different DDR versions:

  • Double Data Rate (DDR) SDRAM. DDR uses double pumping to double the data rate of SDRAM.

  • DDR2. DDR2 doubles the data rate of DDR. In addition to double pumping, it modifies the way that data is processed and can transfer twice as much data as DDR SDRAM.

  • DDR3. DDR3 doubles the data rate of DDR2. It uses double pumping and further modifies the way that data is processed. It can transfer four times as much data as DDR and eight times as much data as SDRAM.

DDR4 isn’t included in the objectives, but it is on the horizon as a replacement for DDR3. It’s expected to double the speed of DDR3.


RAM comes on cards plugged into the slots in the motherboard. They are smaller than expansion cards, and technicians commonly call memory cards sticks. The two most common types of memory sticks are:

  • httpatomoreillycomsourcemspimages1445701.jpg Dual in-line memory module (DIMM). A DIMM is the circuit board that holds the memory chips.

  • Small outline dual in-line memory module (SODIMM). SODIMM chips are smaller and are used in smaller devices such as laptop computers and some printers.

Figure 3-2 shows a DIMM (top) and a SODIMM (bottom).

Figure 3-2

Figure 3-2. Comparing a DIMM and a SODIMM.

DIMMs and SODIMMs have a different number of pins depending on the type used.

  • DDR SDRAM DIMM: 184 pins

  • DDR2 SDRAM DIMM: 240 pins

  • DDR3 SDRAM DIMM: 240 pins

  • DDR SDRAM SODIMM: 200 pins

  • DDR2 SDRAM SODIMM: 144 or 200 pins

  • DDR3 SDRAM SODIMM: 204 pins

Single Channel, Dual Channel, and Triple Channel

httpatomoreillycomsourcemspimages1445701.jpg Many motherboards and CPUs support single-channel, dual-channel, and triple-channel memory architectures. Each single channel represents a separate 64-bit line of communication that can be accessed independently. With dual channel, the system can access 128 bits at a time; triple channel gives it access to 192 bits at a time.

Using dual and triple channels provides an additional performance enhancement to DDR, DDR2, and DDR3, in addition to double pumping and other enhancements provided by the DDR versions. If you use a dual-channel motherboard with DDR3, it doubles the throughput of DDR3, providing 16 times more data throughput than SDRAM.

If you are upgrading a computer’s memory, it’s important to understand these channels. You can purchase DIMMs in matched pairs. Where you install each DIMM determines how many channels your system will use and can affect the performance of RAM.

Single Channel vs. Dual Channel

httpatomoreillycomsourcemspimages1445701.jpg Dual-channel motherboards are very common. If you look at a dual-channel motherboard, you see that it has four memory slots, two slots of one color and two slots of another color. Figure 3-3 shows a diagram of four memory slots labeled for a motherboard using an Intel-based CPU. Slots 1 and 3 are one color, and slots 2 and 4 are another color.

Figure 3-3

Figure 3-3. Intel-based DDR slots (S), banks (B), and channels (C).

  • Slots: Each slot can accept one DIMM.

  • Banks: A bank is composed of two slots. In Figure 3-3, Bank 0 includes slots 1 and 3 and these two slots are normally blue. Bank 1 includes slots 2 and 4 and these slots are normally black. This is standard for Intel CPU-based motherboards.

  • Channels: Each channel represents a separate 64-bit communication path. Slots 1 and 2 make up one channel, and slots 3 and 4 make up the second channel.

You can install a single DIMM in slot 1, and the system will have a single-channel RAM. You can purchase DIMMs in matched pairs, and it’s important to know in which slots to install them. For the best performance, you should install matched DIMMs in the same bank. Looking at Figure 3-3, you should install the matched pair of DIMMs in slots 1 and 3 (Bank 0), leaving slots 2 and 4 empty. The system will take advantage of the dual-channel architecture by using two separate 64-bit channels.

What happens if you install the DIMMs in slots 1 and 2 instead? The system will still work; however, both DIMMs are installed in channel 1, so the system will work with only a single channel. RAM will be about half as fast as it could be if it were installed correctly to take advantage of the dual channels.

Figure 3-3 and the previous explanation describe the color coding, banks, and channels for Intel-based CPU motherboards. However, most motherboards designed for AMD CPUs are organized differently, as shown in Figure 3-4. On these motherboards, slots 1 and 2 make up Bank 0, and slots 3 and 4 make up Bank 1. Channel 1 includes slots 1 and 3, and channel 2 includes slots 2 and 4.

Figure 3-4

Figure 3-4. AMD-based DDR slots (S), banks (B), and channels (C).

While this can be confusing between different motherboards, the good news is that most motherboard manufacturers use the same color for each bank. For Intel-based motherboards, Bank 0 includes slots 1 and 3, and these will be the same color (often blue). Bank 1 includes slots 2 and 4, and they will be a different color (often black). AMD motherboards also use one color for Bank 0 (slots 1 and 2) and another color for Bank 1 (slots 3 and 4).

Triple Channel

httpatomoreillycomsourcemspimages1445701.jpg On some motherboards, you see six DIMM slots instead of four. This indicates the system supports triple-channel memory usage. Table 3-1 shows the configuration of the slots, banks, and channels for a motherboard using triple-channel RAM.

Table 3-1. Triple-Channel DIMMs




Slot 1

Bank 0

Channel 1

Slot 2

Bank 1

Channel 1

Slot 3

Bank 0

Channel 2

Slot 4

Bank 1

Channel 2

Slot 5

Bank 0

Channel 3

Slot 6

Bank 1

Channel 3

Slots in each bank are commonly the same color, so you might see a motherboard with Bank 0 slots (slots 1, 3, and 5) all blue and with Bank 1 slots all black.

Triple-channel DIMMs are sold in matched sets of three, similar to how dual-channel DIMMs are sold in matched pairs. When you install triple-channel DIMMs, you should install the matched set in the same bank. For example, if you bought one set, you’d install it in slots 1, 3, and 5.

Single Sided vs. Double Sided

httpatomoreillycomsourcemspimages1445701.jpg You’d think that single-sided and double-sided RAM refers to how many sides of a DIMM have chips. That makes sense, but it’s not entirely accurate. Instead, single sided or double sided refers to how a system can access the RAM.

In double-sided RAM, the RAM is separated into two groups known as ranks, and the system can access only one rank at a time. If it needs to access the other rank, it needs to switch to the other rank. In contrast, single-sided (or single-rank) RAM is in a single group; the system can access all RAM on the DIMM without switching.

If you have a DIMM with chips on only one side, it is most likely a single-sided (single-rank) DIMM. However, if it has chips on both sides, it can be single rank, dual rank, or even quad rank. You often have to dig into the specs to determine how many ranks it is using.

Usually, you’d think that double is better than single, but in this case, more rank is not better. Switching back and forth between ranks takes time and slows down the RAM. Single-sided RAM doesn’t switch, and if all other factors are the same, single-sided RAM is faster than double-sided RAM.

RAM Compatibility and Speed

An important point about DDR, DDR2, and DDR3 is that they aren’t compatible with each other. You can’t use any version in a slot designed for another type. For example, you can use DDR3 DIMMs only in DDR3 slots. From a usability perspective, that’s not so great, but if you’re trying to remember which types are compatible, it’s a lot easier. You can’t mix and match them.

Figure 3-5 shows a comparison of the keyings of DDR, DDR2, and DDR3, with a dotted line as a reference through the middle of each one. You can see that the notched key at the bottom of the circuit card is different for each. The standards aren’t compatible, and this keying prevents technicians from inserting a DIMM into the wrong slot.

Figure 3-5

Figure 3-5. Comparing DDR versions.


Some RAM is faster than other RAM, and with faster RAM you often see faster overall performance. As you’d expect, faster RAM is more expensive. If you’re shopping for RAM, you want to ensure that you buy exactly what you need. This includes the correct DDR version, the correct number of channels if your motherboard supports multiple channels, and the correct speed.

The speed of RAM is expressed as the number of bytes it can transfer in a second (B/s) or, more commonly, as megabytes per second (MB/s). However, the speed of most RAM isn’t listed plainly. Instead, it’s listed using standard names and module names such as DDR3-800 or PC3-12800, respectively. These names indicate their speed, but not directly. If you need to shop for RAM, you need to understand these names and how they relate to the speed.

You can calculate the overall speed of any SDRAM DDR type by using a specific mathematical formula for that type. The formula includes the speed of the clock (Clk), a clock multiplier (Clk Mult) for DDR2 and DDR3, and doubling from double pumping (DP). The speed is calculated for a single channel, which is 64 bits wide, and then converted to bytes by dividing it by 8. The following formulas show how to calculate the speed of each of the DDR versions by using a 100-MHz clock:

  • DDR speed calculation:

    • Clk x 2 (DP) x 64 (bits) / 8 (bytes)

    • 100 MHz x 2 x 64 / 8 = 1,600 MB/s

  • DDR2 speed calculation:

    • Clk x 2 (Clk Mult) x 2 (DP) x 64 (bits) / 8 (bytes)

    • 100 MHz x 2 x 2 x 64 / 8 = 3,200 MB/s

  • DDR3 speed calculation:

    • Clk x 4 (Clk Mult) x 2 (DP) x 64 (bits) / 8 (bytes)

    • 100 MHz x 4 x 2 x 64 / 8 = 6,400 MB/s

Table 3-2 shows how these speeds relate to the different naming conventions used with DDR types. You can see that the standard name is derived from the clock, the clock multiplier, and double pumping. For example, DDR3 uses a 4-times multiplier and double pumping. Therefore, it’s eight times faster than SDRAM. The standard name is derived by multiplying the clock by 8. The module name is a little more cryptic, but if you calculate the speed by using the clock, you can see that the PC name indicates the calculated speed in MB/s. Also, you can see that the names include the version (DDR, DDR2, or DDR3).

Table 3-2. DDR Standard Names and Module Names

100 MHz

166 2/3 MHz

200 MHz

DDR Standard Name

DDR Module Name







DDR2 Standard Name

DDR2 Module Name








DDR3 Standard Name

DDR3 Module Name







Each DDR version supports multiple clock speeds, and each newer version supports faster clocks. Some of the clock speeds supported by different DDR versions are as follows:

  • DDR: 100, 133 1/3, 166 2/3, and 200 MHz

  • DDR2: 100, 133 1/3, 166 2/3, 200, and 266 2/3 MHz

  • DDR3: 100, 133 1/3, 166 2/3, 200, 266 2/3, and 400 MHz

A key consideration when purchasing RAM is to ensure that the RAM speeds are supported by the motherboard. If the speeds don’t match, the motherboard defaults to the slower speed. For example, if your motherboard has a 100-MHz clock and you install PC3-12800 RAM, the RAM will run at 100 MHz instead of 200 MHz. It still works, but you won’t get the benefit of the higher-speed RAM.

Compatibility within Banks

In addition to matching the RAM speed with the motherboard speed, you should also match the RAM speed within banks when using dual-channel and triple-channel configurations. If one DIMM in a bank fails, you should replace both with a matched set. However, if you have to replace the failed DIMM with a spare, look for a spare that uses the same speed.

For example, if Bank 0 currently has two PC3-12800 sticks and one fails, you should replace the failed stick with a PC3-12800 stick. PC3-12800 uses a 200-MHz clock. If you replaced it with a PC3-6400 (designed for a 100-MHz clock), both sticks would run at the slower speed or revert to single channel.

Shopping for RAM

When shopping for RAM, you need to determine the clock speed of your computer and then determine the DDR name. You can boot into BIOS, as shown in Chapter 2, “Understanding Motherboards and BIOS,” to identify the clock speed used by RAM and then plug it into the formula to determine the standard name and module name.

If you have access to the Internet, there’s an easier way. You can go to one of the memory sites, such as Crucial.com or Kingston.com, and use one of their tools. You can enter the make and model of your computer, and the tool will tell you what memory is supported. Crucial.com also has an application that you can download and run to identify your motherboard, the type and speeds of supported RAM, how much RAM is installed, and recommendations for upgrading the RAM. Another tool that can help is CPU-Z (described at the end of this chapter).

Parity and ECC

httpatomoreillycomsourcemspimages1445701.jpg Desktop systems rarely need extra hardware to detect or correct memory errors, but some advanced servers need this ability. The two primary error-detection technologies are parity and error correction code (ECC). When shopping for RAM on desktop systems, you’ll almost always buy non-parity and non-ECC RAM.

Parity works by using 9 bits for every byte instead of 8 bits. It sets the ninth bit to a 0 or a 1 for each byte when writing data to RAM. Parity can be odd parity or even parity, referring to odd and even numbers.

Odd parity is common, and when used, it ensures that the 9 bits always have an odd number of 1s. For example, if the 8 data bits were 1010 1010, it has four 1s. Four is an even number, so the parity bit needs to be a 1. Whenever data is written to RAM, the parity bit is calculated and written with each byte.

When the data is read, the system calculates the parity from the 9 bits. If it ever detects an even number of 1s, it knows there is an error, meaning that the data isn’t valid and should not be used. Parity can’t fix the problem; it just reports the error.

ECC RAM uses additional circuitry and can detect and correct errors. This extra circuitry adds significantly to the cost of the RAM and should be purchased only when necessary. For example, spacecraft that might be exposed to solar flares commonly use ECC RAM. Additionally, some high-end scientific and financial servers need it to ensure that the data in RAM remains error-free.

Rambus and RDRAM

httpatomoreillycomsourcemspimages1445701.jpg Another type of DRAM is Rambus DRAM (RDDRAM). More commonly, you see it referred to as Rambus, Rambus DRAM, or RDRAM. RDRAM is not compatible with any of the DDR versions and is rarely used.

The circuit boards are called Rambus in-line memory modules (RIMMs) instead of DIMMs. When installing RDRAM, you must install it in pairs. In some cases, only one circuit card has memory and the second circuit card in the pair is needed to complete the circuit. The second card is called a continuity RIMM (CRIMM).

RDRAM generates quite a bit of heat. To dissipate the heat, the chips are covered with a piece of metal acting as a heat sink or heat spreader. This makes them easy to identify because DDR SDRAM is not covered with metal.