Glossary - Computer Memory

System Memory - The system memory is the place where the computer holds current programs and data that are in use. The term "memory" is somewhat ambiguous; it can refer to many different parts of the PC because there are so many different kinds of memory that a PC uses. However, when used by itself, "memory" usually refers to the main system memory, which holds the instructions that the processor executes and the data that those instructions work with. Your system memory is an important part of the main processing subsystem of the PC, tied in with the processor, cache, motherboard and chipset.

Memory plays a significant role in the following important aspects of your computer system:

Performance: The amount and type of system memory you have is an important contributing factor to overall performance. In many ways, it is more important than the processor, because insufficient memory can cause a processor to work at 50% or even more below its performance potential. This is an important point that is often overlooked.
Software Support: Newer programs require more memory than old ones. More memory will give you access to programs that you cannot use with a lesser amount.
Reliability and Stability: Bad memory is a leading cause of mysterious system problems. Ensuring you have high-quality memory will result in a PC that runs smoothly and exhibits fewer problems. Also, even high-quality memory will not work well if you use the wrong kind.
Upgradability: There are many different types of memory available, and some are more universal than others. Making a wise choice can allow you to migrate your memory it to a future system or continue to use it after you upgrade your motherboard.

Memory Technology Types - The system memory itself is made from DRAM, but the other types are explained here to show the other major technology types in use in the PC, and how they differ from DRAM.

ROM: Read-Only Memory. One major type of memory that is used in PCs is called read-only memory, or ROM for short. ROM is a type of memory that normally can only be read, as opposed to RAM which can be both read and written. There are two main reasons that read-only memory is used for certain functions within the PC: Permanence. The values stored in ROM are always there, whether the power is on or not. A ROM can be removed from the PC, stored for an indefinite period of time, and then replaced, and the data it contains will still be there. For this reason, it is called non-volatile storage. A hard disk is also non-volatile, for the same reason, but regular RAM is not.Security: The fact that ROM cannot easily be modified provides a measure of security against accidental (or malicious) changes to its contents. You are not going to find viruses infecting true ROMs, for example; it's just not possible. (It's technically possible with erasable EPROMs, though in practice never seen.)
Read-only memory is most commonly used to store system-level programs that we want to have available to the PC at all times. The most common example is the system BIOS program, which is stored in a ROM called (amazingly enough) the system BIOS ROM. Having this in a permanent ROM means it is available when the power is turned on so that the PC can use it to boot up the system. Remember that when you first turn on the PC the system memory is empty, so there has to be something for the PC to use when it starts up.
RAM: Random Access Memory. The kind of memory used for holding programs and data being executed is called random access memory or RAM. RAM differs from read-only memory (ROM) in that it can be both read and written. It is considered volatile storage because unlike ROM, the contents of RAM are lost when the power is turned off. RAM is also sometimes called read-write memory or RWM. It's a better name because calling RAM "random access" implies to some people that ROM isn't random access, which is not true. RAM is called "random access" because earlier read-write memories were sequential and did not allow random access. Sometimes old acronyms persist even when they don't make much sense any more. Obviously, RAM needs to be writable in order for it to do its job of holding programs and data that you are working on. The volatility of RAM also means that you risk losing what you are working on unless you save it frequently. RAM is much faster than ROM is, due to the nature of how it stores information. This is why RAM is often used to shadow the BIOS ROM to improve performance when executing BIOS code. There are many different types of RAMs, including static RAM (SRAM) and many flavors of dynamic RAM (DRAM).
SRAM: Static RAM. Static RAM is a type of RAM that holds its data without external refresh, for as long as power is supplied to the circuit. This is contrasted to dynamic RAM (DRAM), which must be refreshed many times per second in order to hold its data contents. SRAMs are used for specific applications within the PC, where their strengths outweigh their weaknesses compared to DRAM: Simplicity: SRAMs don't require external refresh circuitry or other work in order for them to keep their data intact. Speed: SRAM is faster than DRAM. In contrast, SRAMs have the following weaknesses, compared to DRAMs: Cost: SRAM is, byte for byte, several times more expensive than DRAM. Size: SRAMs take up much more space than DRAMs (which is part of why the cost is higher).
These advantages and disadvantages taken together obviously show that performance-wise, SRAM is superior to DRAM, and we would use it exclusively if only we could do so economically. Unfortunately, 32 MB of SRAM would be prohibitively large and costly, which is why DRAM is used for system memory. SRAMs are used instead for level 1 cache and level 2 cache memory, for which it is perfectly suited; cache memory needs to be very fast, and not very large.
DRAM: Dynamic RAM. Dynamic RAM is a type of RAM that only holds its data if it is continuously accessed by special logic called a refresh circuit. Many hundreds of times each second, this circuitry reads the contents of each memory cell, whether the memory cell is being used at that time by the computer or not. Due to the way in which the cells are constructed, the reading action itself refreshes the contents of the memory. If this is not done regularly, then the DRAM will lose its contents, even if it continues to have power supplied to it. This refreshing action is why the memory is called dynamic.
All PCs use DRAM for their main system memory, instead of SRAM, even though DRAMs are slower than SRAMs and require the overhead of the refresh circuitry. It may seem weird to want to make the computer's memory out of something that can only hold a value for a fraction of a second. In fact, DRAMs are both more complicated and slower than SRAMs.
The reason that DRAMs are used is simple: they are much cheaper and take up much less space, typically 1/4 the silicon area of SRAMs or less. To build a 64 MB core memory from SRAMs would be very expensive. The overhead of the refresh circuit is tolerated in order to allow the use of large amounts of inexpensive, compact memory. The refresh circuitry itself is almost never a problem; many years of using DRAM has caused the design of these circuits to be all but perfected.
DRAMs are smaller and less expensive than SRAMs because SRAMs are made from four to six transistors (or more) per bit, DRAMs use only one, plus a capacitor. The capacitor, when energized, holds an electrical charge if the bit contains a "1" or no charge if it contains a "0". The transistor is used to read the contents of the capacitor. The problem with capacitors is that they only hold a charge for a short period of time, and then it fades away. These capacitors are tiny, so their charges fade particularly quickly. This is why the refresh circuitry is needed: to read the contents of every cell and refresh them with a fresh "charge" before the contents fade away and are lost. Refreshing is done by reading every "row" in the memory chip one row at a time; the process of reading the contents of each capacitor re-establishes the charge. For an explanation of how these "rows" are read, and thus how refresh is accomplished, refer to this section describing memory access.
DRAM is manufactured using a similar process to how processors are: a silicon substrate is etched with the patterns that make the transistors and capacitors (and support structures) that comprise each bit. DRAM costs much less than a processor because it is a series of simple, repeated structures, so there isn't the complexity of making a single chip with several million individually-located transistors.
There are many different kinds of specific DRAM technologies and speeds that they are available in. These have evolved over many years of using DRAM for system memory, and are discussed in more detail in the following.

Memory Access and Access Time - When memory is read or written, this is called a memory access. A specific procedure is used to control each access to memory, which consists of having the memory controller generate the correct signals to specify which memory location needs to be accessed, and then having the data show up on the data bus to be read by the processor or whatever other device requested it.

In order to understand how memory is accessed, it is first necessary to have a basic understanding of how memory chips are addressed. Let's take as an example a common 16Mbit chip, configured as 4Mx4. This means that there are 4M (4,194,304) addresses with 4 bits each; so there are 4,194,304 different memory locations--sometimes called cells--each of which contains 4 bits of data.. 4,194,304 is equal to 2^22, which means 22 bits are required to uniquely address that number of memory locations. Thus, in theory 22 address lines are required.

However, in practice, memory chips do not have this many address lines. They are instead logically organized as a "square" of rows and columns. The low-order 11 bits are considered the "row" and the high-order 11 bits the "column". First the row address is sent to the chip, and then the column address. For example, let's suppose that we want to access memory location 2,871,405 in this chip. This corresponds to a binary address of "10101111010 00001101101". First, "00001101101" would be sent to select the "row", and then "10101111010" would be sent to select the column. This combination selects the unique location of memory address 2,871,405. This is analogous to how you might select a particular cell on a spreadsheet: go to row #34, say, and then look at column "J" to find cell "J34".

Intuitively, it would seem that designing memory chips in this manner is both more complex and slower than just putting one address pin on the chip for each address line required to uniquely address the chip--why not just put 22 address pins on the chip? It may not surprise you to learn that the answer is "cost". By using the row/column method, it is possible to greatly reduce the number of pins on the DRAM chip. Here, 11 address pins are required instead of 22 (though you lose a small part of the "savings" of 22-11=11 to additional control signals that are needed to manage the row/column timing.) You also save some of the buffers and other circuitry that are required for each address line. Certainly having to send the address in two "chunks" slows down the addressing process, but keeping the chip smaller and with fewer inputs allows it to use less power, which makes it possible to run the chip faster, partially offsetting the loss in access speed.

Of course, a PC doesn't have a single memory chip; most have dozens, depending on total memory capacity and the size of DRAMs being used. The chips are arranged into modules, and then into banks, and the memory controller manages which sets of chips are read from or written to. Since a modern PC reads or writes 64 bits at a time, each read or write involves simultaneous accesses to as many as 64 different DRAM chips.

Here is a simplified walkthrough of how a basic read memory access is performed. This is a conventional asynchronous read, because the timing signals are not tied to the main system clock; synchronous DRAM uses different timing signals:

The address for the memory location to be read is placed on the address bus.
The memory controller decodes the memory address and determines which chips are to be accessed.
The lower half of the address ("row") is sent to the chips to be read.
After allowing sufficient time for the row address signals to stabilize, the memory controller sets the row address strobe (sometimes called row address select) signal to zero. (This line is abbreviated as "RAS" with a horizontal line over it. The horizontal line is a short-hand code that tell engineers working with the circuit that the signal is "active low", meaning that the chip is looking for it to be set to zero as a signal to "do something". There's no way in HTML to reliably use this notation so instead, I will write "/RAS".)
When the /RAS signal has settled at zero, the entire row selected (all 2^11 columns in the example above, or 2048 different cells of 4 bits each) is read by the circuits in the chip. Note that this action refreshes all the cells in that row; refreshing is done one row at a time.
The higher half of the address ("column") is sent to the chips to be read.
After allowing sufficient time for the column address signals to stabilize, the memory controller sets the column address strobe (or column address select) signal to zero. This line is abbreviated as "CAS" with a horizontal line over it, or "/CAS".
When the /CAS signal has settled at zero, the selected column is fed to the output buffers of the chip.
The output buffers of all the accessed memory chips feed the data out onto the data bus, where the processor or other device that requested the data can read it.
Note that this is a very simplified example, since it doesn't mention all of the various timing signals, and it also ignores common performance enhancements such as multiple-banked modules, burst mode, etc. A write process is performed similarly, except of course that the data is read into the chips instead of being sent out by them. A special signal called "R/W" (actually written with a horizontal line over the "W") controls whether a read or write is being performed during the access.

The amount of time that it takes for the memory to produce the data required, from the start of the access until when the valid data is available for use, is called the memory's access time, sometimes abbreviated tAC. It is normally measured in nanoseconds (ns). Today's memory normally has access time ranging from 5 to 70 nanoseconds. This is the speed of the DRAM memory itself, which is not necessarily the same as the true speed of the overall memory system. Note that much of the difference in access times of various DRAM technologies has to do with how the memory chips are arranged and controlled, not anything different in the core DRAM chips themselves.

Asynchronous and Synchronous DRAM - Conventional DRAM, of the type that has been used in PCs since the original IBM PC days, is said to be asynchronous. This refers to the fact that the memory is not synchronized to the system clock. A memory access is begun, and a certain period of time later the memory value appears on the bus. The signals are not coordinated with the system clock at all, as described in the section discussing memory access. Asynchronous memory works fine in lower-speed memory bus systems but is not nearly as suitable for use in high-speed (>66 MHz) memory systems.

A newer type of DRAM, called "synchronous DRAM" or "SDRAM", is synchronized to the system clock; all signals are tied to the clock so timing is much tighter and better controlled. This type of memory is much faster than asynchronous DRAM and can be used to improve the performance of the system. It is more suitable to the higher-speed memory systems of the newest PCs.

Note that there are several different flavors of both asynchronous DRAM and synchronous DRAM; they are discussed on the page covering DRAM technologies.
 
DRAM Speed Ratings

There are two different ways that DRAM chips are rated for speed. Conventional asynchronous DRAM chips have a rated speed in nanoseconds (ns, or a billionth of a second), a speed which represents the minimum access time for doing a read or write to memory. This includes the entire access cycle. Most asynchronous memory in modern systems is 50, 60 or 70 ns in speed. Older systems (386 and earlier) use usually 70 or 80 ns RAM. Very old systems use even slower memory: 100, 120 or even 150 ns. Systems running with a clock speed of 60 MHz or higher generally require 60 ns or faster memory to function at peak efficiency. 70 ns is fine for 486 or older PCs.

Synchronous memory is much faster than conventional asynchronous RAM. It is usually rated at 12, 10 or even 7 nanoseconds; however you have to be careful here. An SDRAM module rated at 10 ns is not "5 times faster" than an EDO module rated at 50 ns. Since SDRAM is synchronized to the internal system clock, SDRAM speed ratings refer to the maximum speed at which the SDRAM module can burst data onto the bus. This does not include the addressing latency time the way asynchronous DRAM speed ratings do, which is why the numbers are much smaller. The core DRAMs inside the SDRAM module are usually not any faster than those of older technologies; the increase in usable speed is due to how the module is constructed and controlled. See the following section on SDRAM for more details.

DRAM chips are usually marked with their speed via a suffix at the end of the part number. If you look at the chips themselves, you'll see something like "-6" or "-60". This usually means 60 nanosecond DRAM. The suffix found on SDRAM chips is often "-12", "-10" or "-07". Note that older memory running at 100 or 120 ns also used "-10" and "-12" sometimes. This memory hasn't been used in years so there really shouldn't be any confusion between the two types. However, 70 ns memory uses "-7" and this can be readily confused with 7 ns SDRAM memory if you are not careful.

In addition to being referred to using a nanosecond speed rating, SDRAMs are also often rated in terms of their maximum frequency, in MHz. This is really the same thing, just expressed in a different way: for example, an SDRAM module with a 10ns rating would be called instead a "100 MHz SDRAM". 100 MHz is 100 million cycles per second, which is the reciprocal of 10ns, one-hundred-millionth of a second per cycle. This MHz number is not the same as saying that the SDRAM with that rating is designed for a system of that speed. A 100 MHz SDRAM may not function in a 100 MHz system bus PC. See here for more.

The rated speed of the memory is a maximum. In theory, the memory cannot support memory timing that requires a faster speed of RAM. However, in practice many companies rate their DRAM conservatively, so that the memory will function at a higher speed than what is indicated. This is why many Pentium systems running on a 66 MHz bus will work with 70 ns memory, even when set to 60 ns timing. However, this is not reliable and cannot be counted on (in a way, it is a form of overclocking) and is not recommended. You can usually compensate for slower memory by turning down the system timing level, which will cause a small performance decrease but give you better reliability.

SDRAM speed ratings and selection criteria are more complicated than those for conventional asynchronous RAM. Refer to the section on SDRAM.

Mixing DRAM Speeds - Mixing memory speeds refers to the use of DRAM of more than one speed in the same computer. For example, you might have bought a machine in 1994 that came with 70 ns DRAM (the fastest speed generally available then) and later upgraded with more memory in 1997 that was 60 ns. While it's generally preferable to avoid doing this, it can work without problems as long as certain caveats are followed:
Use Identical Memory Within a Bank: PCs read a certain bit width of memory at a time, typically 32 or 64, and the memory making up this width is called a bank. PCs always read data from more than one chip at a time within a single bank of memory. If different memory is used within a bank, some of the bits may arrive later than others and all sorts of system problems can result. For this reason you should only use the same type and speed of memory within a bank. This also means using the same technology--never mix EDO and FPM memory (for example) within a bank.
Put The Slowest Memory in the First Bank: Some memory systems automatically detect the speed of the memory being used, and set the system timing accordingly. They usually only look at the speed of the memory in the first bank when setting the timing. If you have 60 ns RAM in the first bank and 70 ns in the second bank, the system may set the timing at a rate that works fine for the 60 ns memory, but causes problems for the 70 ns. You should put the 70 ns memory in the first bank instead. (If your system doesn't do autodetection this won't be an issue but it is still good practice). Note that the first bank of memory is often called "Bank 0".
Some systems just generally have a hard time working with dissimilar banks of memory. I once tried to upgrade a system that had a pair of 8 MB, fast page mode 60 ns SIMMs, with another pair of 8 MB, fast page mode 60 ns SIMMs of another brand. The two pairs just would not work together in any configuration, even though they worked fine separately. See this section of the Troubleshooting Expert for more on RAM problems.
 
Memory Bus Speed and Required DRAM Speed - Most modern systems generally require the DRAMs they use to be a certain minimum speed. The speed required is normally a function of the speed of the memory bus. Faster memory buses require faster speed DRAMs and in some cases, faster technologies. While it is not always cut-and-dried what speed is necessary, the table below is a guideline. It is usually possible to use slower memory in any system if you slow down the memory timing, but of course we're trying to make our systems run as fast as we can.

Processor Generation	Memory Bus Speed	Usual DRAM Technology	Usual Required DRAM Speed (ns)
First, Second	4.77-20	Conventional	100-120
Third, Fourth	16-40	Conventional, Page Mode, FPM, EDO	70-100
Fifth, Sixth	50-100	FPM, EDO, BEDO, SDRAM	8-10 (SDRAM) 50-70 (Asynchronous)
Future	125+	SDRAM, DDR SDRAM, DRDRAM, SLDRAM, Other?	?

 
DRAM Technologies - DRAM is available in several different technology types. At their core, all of these different memory types are similar. They differ mostly in the way that they are organized and how they are accessed. As processors get faster, memory needs to run increasingly faster and more efficiently. Memory companies have invented progressively faster memory architectures to allow memory speeds to increase.

In the real world, the differences between many of the DRAM technologies is not that great. Most requests for data by the processor are satisfied from either the primary or secondary caches on a modern PC, which masks much of the improvement in DRAM efficiency. Also, memory is just one piece of the puzzle in overall performance. Often, more system memory is more important to performance than better system memory.

Also bear in mind that at its core, DRAM is DRAM. The differences between the various acronyms of DRAM technologies are primarily a result of how the DRAM inside the module is connected, configured and addressed, in addition to any special enhancement circuits added to the device. For example, some fancy modules include SRAM (cache) directly in the DRAM module to improve performance.

Generally, there are Conventional DRAM, Fast Page Mode (FPM) DRAM, Extended Data Out (EDO) DRAM, Burst Extended Data Out (BEDO) DRAM, Synchronous DRAM (SDRAM), Double Data Rate SDRAM (DDR SDRAM), Direct Rambus DRAM (DRDRAM), Synchronous-Link DRAM (SLDRAM), Video RAM (VRAM) and Other Video DRAM Technologies. Today, the most commonlly used is SDRAM.

Synchronous DRAM (SDRAM) - A relatively new and different kind of RAM, Synchronous DRAM or SDRAM differs from earlier types in that it does not run asynchronously to the system clock the way older, conventional types of memory do. SDRAM is tied to the system clock and is designed to be able to read or write from memory in burst mode (after the initial read or write latency) at 1 clock cycle per access (zero wait states) at memory bus speeds up to 100 MHz or even higher. SDRAM supports 5-1-1-1 system timing when used with a supporting chipset. SDRAM accomplishes its faster access using a number of internal performance improvements, including internal interleaving, which allows half the module to begin an access while the other half is finishing one.

SDRAM is rapidly becoming the new memory standard for modern PCs. The reason is that its synchronized design permits support for the much higher bus speeds that have started to enter the market. SDRAM doesn't offer that much "real world" additional performance over EDO in many systems, due to the system cache masking much of that differential in speed, and the fact that most systems are running on relatively slow 66 MHz or lower system bus speeds. As 100 MHz bus system PCs become mainstream, SDRAM will largely replace older technologies, since it is designed to work at these higher operating speeds and conventional asynchronous DRAM is not.

There are several important characteristics and concerns regarding SDRAMs that are relatively unique to the technology. In addition to the notes below, you will want to read this informative article that goes into more depth on choosing SDRAM modules:

Speed and Speed Matching: SDRAM modules are generally speed-rated in two different ways: First, they have a "nanosecond" rating like conventional asynchronous DRAMs, so SDRAMs are sometimes referred to as being "12 nanosecond" or "10 nanosecond". Second, they have a "MHz" rating, so they are called "83 MHz" or "100 MHz" SDRAMs for example. Because SDRAMs are, well, synchronous, they must be fast enough for the system in which they are being used. With asynchronous DRAMs such as EDO or FPM, it was common to add extra wait states to the access timing for the memory to compensate for memory that was too slow. With SDRAM however, the whole point of the technology is to be able to run with zero wait states. In order to do this, the memory must be fast enough for the bus speed of the system. One place where people run into trouble in this regard is that they take the reciprocal of the "nanosecond" rating of the module and conclude that the module can run at that speed. For example, the reciprocal of 10 ns is 100 MHz, so people assume that 10 ns modules will definitely be able to run on a 100 MHz system. The problem is that this allows absolutely no room for slack. In practice, you really want memory rated slightly higher than what is required, so 10 ns modules are really intended for 83 MHz operation. 100 MHz systems require faster memory, which is why the PC100 specification was developed (see below).
Speed Rating: Due to the confusion inherent in the speed rating system described immediately above, and the likelihood of problems running slower SDRAM modules on new 100 MHz system bus motherboards, Intel created a formal specification for SDRAM capable of being used in these new PCs. Dubbed PC100, these modules generally are rated at 8 ns as previously mentioned, but there are other internal timing characteristics that must be met in order to have a module certified as PC100-compliant. While relying on a specification is never foolproof, it is definitely a good idea to ensure that any SDRAM you intend to use on a 100 MHz system bus motherboard is in fact PC100 specification compliant.
Latency: SDRAMs are still DRAMs, and therefore still have latency. The fast 12, 10 and 8 nanosecond numbers that everyone talks about refer only to the second, third and fourth accesses in a four-access burst. The first access is still a relatively slow 5 cycles, just as it is for conventional EDO and FPM memory.
2-Clock and 4-Clock Circuitry: There are two slight variations in the composition of SDRAM modules; these are commonly called 2-clock and 4-clock SDRAMs. They are almost exactly the same, and they use the same DRAM chips, but they differ in how they are laid out and accessed. A 2-clock SDRAM is structured so that each clock signal controls 2 different DRAM chips on the module, while a 4-clock SDRAM has clock signals that can control 4 different chips each. You need to make sure that you get the right kind for your motherboard. The current trend appears to be toward 4-clock SDRAMs.
Serial Presence Detect: Some motherboards are now being created that require the use of special SDRAM modules that include something called a Serial Presence Detect (SPD) chip. This is an EEPROM that contains speed and design information about the module. The motherboard queries the chip for information about the module and makes adjustments to system operation based on what it finds. A great idea in theory, but you won't think it's great if you buy an SDRAM module without the chip on it when your board requires SPD...
CAS2 vs. CAS3: "CAS" stands for column address strobe, one of the main signals used in accessing DRAM chips; see here for an explanation of what CAS is all about. The terms "CAS2" and "CAS3" are used to distinguish between slight variants in SDRAM modules. In fact, the term is a misnomer; the "2" and "3" refer to the latency of the CAS line, so the terms should be "CL2" and "CL3". Theoretically a "CAS2" module is slightly faster than a "CAS3" module, making it more likely to function if the system bus is being overclocked beyond 100 MHz, but the whole matter of "CAS2" and "CAS3" has been overhyped to the Nth degree by many vendors.  Dean Kent's article on SDRAM terminology explains this in greater detail.
Packaging Concerns: To make matters even more confusing, SDRAM usually comes in DIMM packaging, which itself comes in several different formats (buffered and unbuffered, 3.3 volts and 5 volts). You need to make sure you get the right type of packaging as well; see DIMMs section for detail about it..
Now that you've read that, do you feel a bit confused about exactly what type of SDRAM you need? I don't blame you! This is why I strongly advise working closely with your motherboard manufacturer and/or a trusted vendor in choosing your SDRAM. It's one thing to try to figure all of this out from specifications, but it's much better to contact the company that made your motherboard and have them say definitively that you need "10 ns, 4-clock, unbuffered, 3.3 volt SDRAM modules with serial presence detect", or whatever.

Double Data Rate SDRAM (DDR SDRAM) - Only a few years ago, "regular" SDRAM was introduced as a proposed replacement for the older FPM and EDO asynchronous DRAM technologies. This was due to the limitations the older memory has when working with systems using higher bus speeds (over 75 MHz). In the next couple of years, as system bus speeds increase further, the bell will soon toll on SDRAM itself. One of the proposed new standards to replace SDRAM is Double Data Rate SDRAM or DDR SDRAM.

Memory Size - Every system has a maximum amount of memory and maximum cacheable memory that it will support. There are in fact several limiting factors that dictate how much memory can be used in any system. See following for how it influences the system performance.

Memory Size and System Performance

The amount of memory in a PC has a significant impact on its overall performance. Using too little RAM can be the biggest anchor dragging down overall system speed. This is something that many PC users fail to realize; I will often see someone posting to USEnet saying that their PC is "too slow" and so they want to upgrade to a faster processor. Then I will find out they are trying to run multiple applications or high-end games under Windows 95 but have only 8 MB of RAM. In a situation like this, upgrading the processor is a waste of money until the system memory is brought up to a more reasonable level.

Strictly speaking, the amount of memory in the computer has no impact on the speed that the memory runs, nor on the speed that the processor, chipset, motherboard and other major system components run. However, this is only if all of the programs running on the PC fit into the system RAM! All multitasking operating systems use virtual memory, which lets the PC think it has more memory than the actual physical RAM; the extra virtual memory is stored in a swap file on the hard disk. When more programs and data are in use than physically fit in memory, the virtual memory manager swaps parts of memory to disk. This is described in detail in the section discussing virtual memory.

When the amount of virtual memory in use greatly exceeds the amount of real memory, the operating system spends a lot of time swapping pages of memory around, which greatly hampers performance. The reason is simple: the hard disk is thousands of times slower than the system memory, if not more. Remember that hard disk access time is measured in thousandths of a second; memory access time is measured in billionths of a second. This doesn't tell the whole story but it gives you a general idea of the difference.

Let's suppose you are running a word processor, spreadsheet and a calendar program on Windows 95 on a system with 8 MB of real system memory. The total amount of virtual memory required by the applications you are using combine to most likely about 24 MB, depending on what versions of the software you are using. Windows 95 itself needs about 8 MB for system tasks. Since you only have 8 MB of memory, this means basically every time you do anything, the PC will have to pause and swap information to disk before proceeding. If you were to increase your system RAM to 32 MB, you could hold most (if not all) of the data in memory and the hard disk would be quiet. The improvement in performance is dramatic.

The best way to understand the importance of having enough memory is to compare PCs with more memory and with less when running similar tasks. I have seen 80486DX2-66s that feel faster than Pentium 133s, when the 486 has 32 MB of memory and the Pentium has only 8 MB. (I'm not exaggerating.) Even though the Pentium has probably three times the raw power, it is wasted because the system is spending so much time thrashing to the hard disk (while the CPU figuratively twiddles its thumbs).

So how much memory do you need? This is not an easy question to answer. It depends entirely on what you are using the system for. If you are running a single DOS application on a slower PC, 4 MB can be sufficient as long as the application doesn't need more than that. Many high-end CAD or graphics workstations use 256 MB of RAM or more. The amount of memory needed by PCs continues to increase as programs and data get larger and larger. A few years ago, 8 MB was considered a configuration for a high-end system; last year this would have been considered "entry level"; today, it is considered totally unacceptable. The trend towards much larger quantities of RAM will continue in the future, since the price of RAM continues to drop dramatically, and is now in the environs of $1/MB.

Tip: Watch your hard disk LED. If you see it come on and flicker rapidly when switching between tasks for example, this probably means that your operating system is being forced to use virtual memory. If this happens often, it is a clue that you may need more memory. (Note that when loading programs or doing other obviously disk-intensive work, having the disk light come on does not necessarily imply anything about virtual memory, of course.)
 

In general, more memory is better, however you have to watch out for the cacheability issue, since some PCs will not cache memory above a certain value. In addition, the law of diminishing returns definitely applies to memory size: each time you increase your system's memory, you get less improvement than you did the previous time you increased it. Going from 8 MB to 16 MB results in a huge performance increase for most Windows 95 users. Going from 16 to 24 MB results in some improvement but much less than that resulting from going from 8 to 16, and so on.

For most systems, there is a point beyond which adding more memory does not add appreciably to system performance. Where this point lies depends a great deal on what you use your system for--if you are using huge multimedia files for example your PC can probably make use of as much memory as you can throw at it. For most users the point of serious diminishing returns lies between 24 and 48 MB. Above about 48 MB, you will not see much noticeable improvement unless you are doing high-end graphics work, manipulating large files, or multitasking like crazy.

The table below shows some sample operating systems and application types, along with general ranges for recommended memory. This is a guideline only and should not be taken as definitive law; if you are using Windows NT and several applications, I can guarantee that 32 MB is going to be too little, pretty quickly. Only you can assess how you use your PC and therefore what makes the most sense. (That said, I will also say that very few people will want to be below the numbers in the "minimum" column below):

Operating System and Application	Minimum RAM for Tolerable Performance	Minimum RAM for Good Performance	Typical Point of Diminishing Returns
DOS, Single Tasking (not games)	4 MB	8 MB	16 MB
Office work, Windows 3.x	8 MB	16 MB	32 MB
Windows 95 Multitasking, High-end DOS games	16 MB	32 MB	64 MB
Windows NT Workstation	24-32 MB	48 MB	128+ MB
High-end use, Graphics processing, Multimedia, Servers	32 MB	64+ MB	In many cases, none

There is one other way that memory size impacts on overall system performance. Having more memory allows you to dedicate some of it for use as a disk cache. A disk cache lets you store recently-used information from the disk in a special area of memory, to save time reading to the disk when it is needed again. This improves system performance by avoiding unnecessary reads and writes to the slow hard disk.

Memory Packaging - Like processors, memory is made from tiny semiconductor chips and must be packaged into something less fragile and tiny in order to be integrated with the rest of the system. However, in many cases the chip packages are themselves further integrated into larger packages. There are many different kinds of memory packages in the PC world today, and it can be difficult to know which type needs to be used with a given system design. The following are common memory packaging types: Dual Inline Packages (DIPs) and Memory Modules, Parity, Non-Parity and ECC Memory, Standard and Proprietary Memory Modules, Single Inline Memory Modules (SIMMs), Dual Inline Memory Modules (DIMMs).

Single Inline Memory Modules (SIMMs) - The single inline memory module or SIMM is still the most common memory module format in use in the PC world, largely due to the enormous installed base of PCs that use them (in new PCs, DIMMs are now overtaking SIMMs in popularity.) SIMMs are available in two flavors: 30 pin and 72 pin. 30-pin SIMMs are the older standard, and were popular on third and fourth generation motherboards. 72-pin SIMMs are used on fourth, fifth and sixth generation PCs.

SIMMs are placed into special sockets on the motherboard created to hold them. The sockets are specifically designed to ensure that once inserted, the SIMM will be held in place tightly. SIMMs are secured into their sockets (in most cases) by inserting them at an angle (usually about 60 degrees from the motherboard) into the base of the socket and then tilting them upward until they are perpendicular to the motherboard. Special metal clips on either side of the socket snap in place when the SIMM is inserted correctly. The SIMM is also keyed with a notch on one side, to make sure it isn't put in backwards.

The 30 pin SIMMs are generally available in sizes from 1 to 16 MB. Each one has 30 pins of course, and provides one byte of data (8 bits), plus 1 additional bit for parity with parity versions. 72-pin SIMMs provide four bytes of data at a time (32 bits) plus 4 bits for parity/ECC in parity/ECC versions. Package bit width is discussed in detail here.

SIMMs are available in two styles: single-sided or double-sided. This refers to whether or not DRAM chips are found on both sides of the SIMM or only on one side. 30-pin SIMMs are all (I am pretty sure) single-sided. 72-pin SIMMs are either single-sided or double-sided. Some double-sided SIMMs are constructed as composite SIMMs. Internally, they are wired as if they were actually two single-sided SIMMs back to back. This doesn't change how many bits of data they put out or how many you need to use. However, some motherboards cannot handle composite SIMMs because they are slightly different electrically.

72-pin SIMMs that are 1 MB, 4 MB and 16 MB in size are normally single-sided, while those 2 MB, 8 MB and 32 MB in size are generally double-sided. This is why there are so many motherboards that will only work with 1 MB, 4 MB and 16 MB SIMMs. You should always check your motherboard to see what sizes of SIMMs it supports. Composite SIMMs will not work in a motherboard that doesn't support them. SIMMs with 32 chips on them are almost always composite.

Warning: Lately, some 16 MB and 64 MB SIMMs have been seen that are composite. These can cause significant problems with some motherboards, since they are specified to support 16 MB SIMMs on the expectation that 16 MB SIMMs will all be single-sided. You may not be able to use double-sided 16 MB SIMMs in some systems, especially older or cheaper ones.

Most motherboards support either 30-pin or 72-pin SIMMs, but not both. Some 486 motherboards do support both, however. In many cases these motherboards have significant restrictions on how these SIMMs can be used. For example, only one 72-pin socket may be usable if the 30-pin sockets are in use, or double-sided SIMMs may not be usable.

Dual Inline Memory Modules (DIMMs) - The dual inline memory module or DIMM is a newer memory module, intended for use in fifth- and sixth-generation computer systems. DIMMs are 168 pins in size, and provide memory 64 bits in width. They are a newer form factor and are becoming the de facto standard for new PCs; they are not used on older motherboards. They are also not generally available in smaller sizes such as 1 MB or 4 MB for the simple reason that newer machines are rarely configured with such small amounts of system RAM.

Physically, DIMMs differ from SIMMs in an important way. SIMMs have contacts on either side of the circuit board but they are tied together. So a 30-pin SIMM has 30 contacts on each side of the circuit board, but each pair is connected. This gives some redundancy and allows for more forgiving connections since each pin has two pads. This is also true of 72-pin SIMMs. DIMMs however have different connections on each side of the circuit board. So a 168-pin DIMM has 83 pads on each side and they are not redundant. This allows the packaging to be made smaller, but makes DIMMs a bit more sensitive to correct insertion and good electrical contact.

DIMMs are inserted into special sockets on the motherboard, similar to those used for SIMMs. They are generally available in 8 MB, 16 MB, 32 MB and 64 MB sizes, with larger DIMMs also available at a higher cost per megabyte. DIMMs are the memory format of choice for the newest memory technology, SDRAM. DIMMs are also used for EDO and other technologies as well.

DIMMs come in different flavors, and it is important to ensure that you get the right kind for the machine that you are using. They come in two different voltages: 3.3V and 5.0V, and they come in either buffered or unbuffered versions. This yields of course a total of four different combinations. The standard today is the 3.3 volt unbuffered DIMM, and most machines will use these. Consult your motherboard or system manual.

A smaller version of the DIMM is also sometimes seen; called the small outline DIMM or SODIMM, these packages are used primarily in laptop computers where miniaturization is key.

Related Items: | How to Choose Memory | Installation Guide | FAQ |

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