Memory Banks

All memory must be added in "banks." In systems that use single-channel memory access, a bank of memory is the amount of memory (in bits) equal to the data bus of the CPU. Therefore, a bank isn't a fixed amount of memory but varies with the data bus of the CPU. In other words, for a CPU with a 64-bit data bus (Pentium, PII, PIII, Celeron, K6, Athlon, and so on), a bank of memory is the total of one or more identical modules (same type, size , speed, and so on) that add up to 64 bits in width. Refer back to Figure 7.3 to see how different processors and module combinations work together to provide a bank of memory.

tip

For the A+ Certification Exam, you need to know the meaning of the major chip type names and their features.

A growing number of systems use dual-channel memory access. In these systems, two matched 64-bit memory modules (identical in size, speed, and timing) must be used per bank instead of the single 64-bit module that would be used in single-channel mode.

If your system needs multiple modules and you don't add the full number, the system ignores partial banks. See the "Using a RAM Calculator" section later in this chapter to see how to use this information to determine how many modules to add to a system.

Using a RAM Calculator

If you know three variables about a system, you can determine how many memory modules that system needs. You need to know the following:

• The system's data bus width (in bits)

• The data bits in each memory module your system uses (ignore parity or ECC bits, if any)

• Whether the system requires or supports dual-channel memory

To calculate the amount or number of memory modules that your system needs, divide the data bus width (D) by the number of bits per memory module (M) that your system uses. The result will be the number of modules needed (N). Multiply N by 2 for dual-channel systems.

Table 7.3 shows sample calculations for systems you might encounter.

Table 7.3. Calculating Memory Modules Needed

CPU Data Bus Width (D)	Bits per Module (M)	Calculation (D/M)	Number of Modules Needed (N=D/M)	Number of Identical Modules Needed for Dual-Channel System (Nx2)
64-bit [1]	64-bit [3]	64/64	1	2
64-bit	32-bit [4]	64/32	2	N/A
32-bit [2]	32-bit	32/32	1	N/A
32-bit	8-bit [5]	32/8	4	N/A

^[1] Processors such as Pentium class and newer (including Pentium 4 and Athlon XP )

^[3] DIMMs and Rambus RDRAM modules

^[4] 72-pin SIMM

^[2] Processors such as 386 and 486 class (for comparison )

^[5] 30-pin SIMM or SIPP

On systems that use RDRAM modules, you will need to consult the system manual for the details of adding memory. Although each module can function as a bank, some systems use a memory access technique called interleaved memory, which requires that a pair of identical modules be installed in the system.

tip

Buy the largest-capacity modules you can that will work with the system. For example, it's better to use a single 256MB DIMM module than to use two 128MB modules if you need to add 256MB of RAM to a system. This allows you to use fewer modules, which reduces heat and can leave room for additional memory upgrades in the future. However, many older systems can't use the full capacity of newer, larger modules of the same connector type; check the system manual to verify which sizes will work in a given system.

Specifying Memory Modules

Wouldn't it be nice if you could go to your friendly neighborhood computer store and ask for a "16MB 72-pin SIMM" or a "256MB 184-pin DIMM?" Well, you could, but if the clerk is on the ball, you'll be asked questions in return such as "How fast?", "Do you want parity checking?", "Gold or tin?", "What CL setting?", and more. Provide the wrong information, and you'll get memory you can't use or memory that will slow down the system when you install it instead of speeding it up.

Sometimes you can skip the technicalities, particularly if your system uses proprietary memory, by looking up your computer or motherboard at the interactive Web sites provided by major memory vendors such as Kingston (www. kingston .com) or Crucial (www.crucial.com) to determine the proper memory to use. However, sooner or later, you'll need to understand how to decode the standard designation for any given memory module. As you'll learn in the following sections, the standard designation for any memory module is shorthand for its size, speed, whether it supports parity/ECC, and other information you need. The following sections break down each part of the standard designation for typical memory modules so you know what you're getting.

tip

Check your system or motherboard manual to determine what memory types are used by your system. In most cases, vendors will use standard designations such as those shown in the following sections to specify the memory you should use. Systems that use proprietary modules instead might mention particular memory modules made by major vendors instead. In such cases, look up the data sheet for the module to learn more about it.

The methods used for specifying memory modules vary with the type of memory module used by the system. 72-pin SIMMS, 168-pin DIMMs, 184-pin DIMMs and SODIMMs, and Rambus RDRAM modules are covered in the following sections.

tip

If you're wondering what the actual size of a 72-pin SIMM memory module is and all you know is the standard designation (see Table 7.4 for examples), multiply the first number listed in the standard designation by 4 to get the actual size of the module. For example, the 16MB module shown in Table 7.4 is listed as a 4Mx36 module. Multiply 4M by 4 and you get 16(MB).

Specifying 72-Pin SIMMs

To specify a 72-pin SIMM, you need to specify the following:

• Size (MB)

• Memory type (FPM or EDO)

• Speed (ns)

• Parity/ECC or non-parity

• Socket metal (gold or tin)

Table 7.4 shows some examples of 72-pin SIMM specifications.

In the standard designation, 32 is the number of data bits; 36 is the number of data plus parity bits. Note that if EDO is not mentioned, the memory module uses slower FPM memory.

Before you order memory, you should determine which metal is used for the SIMM sockets on your motherboard, gold or tin. Match the metal in the socket to the metal on the memory module contacts to avoid corrosion caused by mixing metals.

Table 7.4. Specifying 72-Pin SIMMs (Examples)

Size	Parity/Non-Parity	Memory Type	Speed	Connector Standard Type	Designation
4MB	Non-parity	FPM	70ns	Tin	1Mx32-70 tin
16MB	Parity	EDO	60ns	Gold	4Mx36EDO-60 gold

To learn more about the problems that can result from mixing tin and gold connectors, see "Avoid Mixing Metals in RAM and Sockets," p. PDF:846 .

Specifying 168-Pin DIMMs

Some of the factors used to specify a 168-pin DIMM are similar to those used to specify a 72-pin SIMM:

• Size (MB); multiply the first number in the designation by 8 to determine the actual size

• Non-parity (64-bit); ECC (72-bit)

• Memory type (EDO or SDRAM); most memory sold in 168-pin form today is SDRAM, but some older systems use EDO DIMMs

Use 36-bit or 72-bit memory modules when your motherboard or system supports parity checking or ECC error correction. If you add 36-bit or 72-bit memory modules to a system that doesn't perform parity checking or ECC error correction, the additional bits are ignored.

However, there are several new factors you also need to specify:

• The speed of a 168-pin SDRAM DIMM is not listed in ns (nanoseconds), but instead by the clock speed of the memory bus: 66MHz is known as PC-66, 100MHz is known as PC-100, and 133MHz is known as PC-133. EDO memory is 50ns or slower.

• There are three types of memory: buffered (which re- drives memory signals to make higher memory amounts possible), unbuffered (which doesn't re-drive memory signals), and registered (a newer form of buffering). Buffered DIMMs are usually EDO modules and are designed for older PCs or for Macs. Almost all desktop PCs use unbuffered DIMMs, and newer servers might use registered DIMMs.

  

  The examples in Table 7.4 are just a few of the possible configurations available for 72-pin SIMM memory. Smaller and larger sizes are available, and slower speeds (larger ns ”or nanoseconds ” rating) have also been produced.

  
  

• DIMMs with SDRAM memory use 3.3v; DIMMs with EDO memory use 5v.

• CAS Latency , or CL (the number of clock cycles required between the issuance of a command to memory and the start of data flow). Lower latency (smaller CL number) is faster. SDRAM DIMMs use CAS Latency values of CL2.5 and CL3.

• Standard (168-pin) or small outline modules (used by notebook and portable computers).

Check the manual to determine what speed(s), sizes, types, and CL values to specify for memory.

Table 7.5 provides examples of how typical SDRAM DIMMS are specified.

Table 7.5. Specifying SDRAM DIMMs

Size	ECC	Speed	CAS Latency	Registered	Standard Designation
64MB	No	66MHz	CL3	No	8Mx64 66MHz
128MB	Yes	133MHz	CL2.5	No	16Mx64 133MHz ECC CL2.5
512MB	Yes	100MHz	CL2	Yes	64Mx72 100MHz Registered ECC CL2

Specifying 184-Pin DDR SDRAM DIMMs

The process of specifying DDR SDRAM memory is similar to that of specifying SDRAM memory, except that memory speeds can be specified by MHz (DDR333 is 333MHz) or by a PC factor referring to the memory's throughput in MBps. For example, PC2700 = 2,700MBps = DDR333. CL factors range from CL3 to CL2.

You can normally make the following substitutions:

• PC-133 memory can be used in place of PC-66 or PC-100 memory on most recent systems. However, some systems that use Intel chipsets cannot use PC-133 memory.

• If CL3 memory is not available, CL2.5 memory can be used in its place.

Table 7.6 provides some examples of how to specify DDR SDRAMs.

Table 7.6. Specifying DDR SDRAM DIMMs ”Typical Examples

Size	ECC	Speed	PC Type	CAS Latency	SODIMM	Standard Designation
128MB	No	266MHz	PC2100	CL2.5	No	DDR266 PC2100 128MB non-ECC CL2.5
256MB	Yes	400MHz	PC3200	CL3	No	DDR400 PC3200 256MB ECC CL3
512MB	No	333MHz	PC2700	N/A	Yes	DDR333 PC2700 512MB SODIMM [1]

^[1] You don't need to specify CAS Latency values for SODIMM memory .

As you review Tables 7.5 and 7.6, keep in mind that these are just a few examples of the many SDRAM and DDR SDRAM memory module sizes and types on the market. All types of memory in these tables are available in 128MB, 256MB, and 512MB sizes as well as smaller sizes, and DDR memory is also available in a 1GB size. ECC memory is also available in a variety of sizes and types.

Specifying Rambus RDRAM Modules

To order a Rambus module, you need to specify the following:

• Memory speed (800MHz or 1,066MHz)

• Memory size

• ECC or non-ECC

• Pin configuration

• Number of devices (chips)

Each device in the standard designation represents an RDRAM chip. Some systems require a specific number of devices in each module.

Table 7.7 provides some examples of how to specify Rambus RIMMs.

Table 7.7. Specifying Rambus RIMMs

Size	ECC	Speed	Pins	# of Devices	Standard Designation
128MB	No	800MHz	232	4	128MB PC800 non-ECC four-device RIMM
256MB	No	1,066MHz	168	2	256MB PC1066 non-ECC two-device RIMM
512MB	Yes	800MHz	168	18	512MB PC80040 ECC 18-device RIMM

Specifying Memory by System or Motherboard Type

As you can see from the preceding sections, there are many variables involved in specifying memory for a particular system. The complexity of the specifications for modern memory modules can make getting the right module difficult. As an alternative, you also can use the following methods to specify memory:

• Check the system or motherboard manual for recommended brands and part numbers .

• Use interactive memory order databases provided by major vendors such as Crucial.com or Kingston.