MEMORY

 

MOTHERBOARDS AND COMPONENTS

2. MEMORY

PHYSICAL MEMORY

When we say the word memory, we are most often referring to Random Access Memory or

RAM. However, there are other types of memory.

Physical Memory Types

There are two types of memories namely RAM and ROM.

RAM types:

• SRAM

One type of memory is known as static random access memory (SRAM). It is called static

because the information doesn’t need a constant update (refresh). SRAM stores information

as patterns of transistor ons and offs to represent binary digits. This type of memory is

physically bulky and somewhat limited in its capacity. It can generally store only 256Kb

(kilobits) per IC. The original PC and XT, as well as some notebook computer systems, use

SRAM chips for their memory.

Most new computers are moving away from SRAM, to the newer, more efficient type of

memory known as DRAM.

• DRAM

Dynamic random access memory (DRAM) was an improvement over SRAM. DRAM uses a

different approach to storing the 1s and 0s. Instead of transistors, DRAM stores information

as charges in very small capacitors. If a charge exists in a capacitor, it’s interpreted as a 1.

The absence of a charge will be interpreted as a 0. Because DRAM uses capacitors instead of

switches, it needs to use a constant refresh signal to keep the information in memory. DRAM

requires more power than SRAM for refresh signals and, therefore, is mostly found in

desktop computers. DRAM technology allows several memory units, called cells, to be

packed with very high density. Therefore, these chips can hold very large amounts of

information. Most PCs today use DRAM of one type or another.

Assignment: explain different types of DRAM.

ROM

Read-only memory (ROM) is used to store information permanently for easy and quick

retrieval. This type of memory chip contains transistors that are manufactured permanently in

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the on or off position, which is the main reason why this memory is called “read only.” Once

these transistors have been set, they can’t be changed. Because these switches are

permanently in these positions, accessing the information contained in ROMs is extremely

fast.

ROMs are expensive to develop and manufacture. They are mainly used for very specialized

purposes, such as storing information about how a device needs to operate. A computer’s

BIOS is typically stored on a type of ROM chip.

ROM types:

PROM

For purposes more general than those required by ROM, a type of ROM chip was

developed called the Programmable ROM (PROM). The PROM is a ROM that is first

manufactured with all of its circuits as logical 1s (that is, with all switches on); then,

when the PROM is to be programmed, the connections that need to be set to 0 are

destroyed, using a high voltage electrical pulse. This makes the settings permanent.

EPROM

The main disadvantage to ROM is that it can’t be changed once it has been

manufactured. To resolve this, IC developers came up with Erasable Programmable

Read Only Memory (EPROM). EPROMs are erasable and able to be reprogrammed,

making them more flexible than ROMs. They work by storing binary information as

electrical charges deposited on the chip. These electrical deposits are almost

permanent. They will stay until dislodged by a special-frequency ultraviolet light

shone through a small window. Exposure to this light returns the chip to its blank

state. The chip can then be completely reprogrammed. These chips are usually easily

identified by their small, circular windows. Some older computers, such as the IBM

PC or XT, used EPROMs for their BIOS information.

EEPROM

It is very inconvenient to remove an IC every time it needs to have the software it

contains upgraded. It can be a real pain and can be dangerous. A way was needed to

permit erasure of these chips “on the fly” while still maintaining their capability of

keeping information intact once power is removed. Electrically Erasable PROM

(EEPROM) chips were designed to solve this problem. They can be erased by sending

a special sequence of electric signals to the chip while it is still in the circuit. These

signals then erase all or part of the chip.

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Although it might seem a good idea to use a EEPROM chip for the main memory in a

computer, it would be very expensive. The primary use of this type of chip is for

BIOS information; you’ll see CMOS BIOS chips in most computers.

The CMOS memory keeps the computer’s BIOS settings while the computer is turned

off. These special EEPROM chips keep their information by means of a small battery.

Although the battery’s charge lasts for several years, it will eventually lose its ability

to keep the CMOS settings. It’s easy to tell when this is happening, though, because

the computer begins to lose its ability to keep BIOS settings when powered off.

Because the BIOS settings can eventually be lost when the CMOS battery

finally loses its charge, we encourage all technicians (and PC owners in

general) to record their BIOS settings (on paper or save them to a floppy) so

that they may be reset if you have to replace the CMOS battery. The BIOS

settings are available from the computer’s Setup program, which is accessible

by a special key or key combination during startup. Some computers use the

Delete key, one of the function keys, or the Escape key; others use

Ctrl+Alt+Esc.

Memory Chip Package Types

The memory chips themselves come in many different types of packages. The ones most

frequently encountered are discussed in the following sections.

• Dual Inline Package (DIP)

The first type of memory chip package is Dual Inline Package (DIP) memory, so named

because the individual RAM chips use the DIP-style package for the memory IC. Older

computers, such as the IBM AT, arranged these small chips like rows of caskets in a small

memory “graveyard.” There are typically eight chips in one of these rows, although there

may be nine. If data is written to memory 8 bits at a time, why the ninth chip? The answer is

that the ninth chip is used for parity, a kind of error-checking routine. Chips that have an

extra chip for error checking are known as parity chips. Those without error checking are

known as non-parity

• Single Inline Memory Module (SIMM)

The next type of RAM packaging that is commonly seen in computers is called the Single

Inline Memory Module (SIMM). SIMMs were developed because DIPs took up too much

“real estate” on the logic board. Someone got the idea to put several of the DIP chips on a

small circuit board and then make that board easily removable.

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The first SIMMs had nine small DIP chips on them and took up less room than before,

because four of them could be installed in the same space as one row of the older DIP

memory chips. In order to accomplish this, the SIMMs are installed very close to each other

at approximately a 45º angle. This design was also meant to prevent “chip creep”—whereby

the chips that have been placed in sockets on the board start to slowly move out of their

sockets (caused by the repeated heating and cooling of the system board).

Most memory chips are 32-bit; so are several of the processors. You have a problem,

however, when you have 32-bit memory chips and a 64-bit processor. To solve this, you must

either install the SIMMs in pairs (always installing multiples of two—this is especially true

for Pentium computers) or change to a DIMM installation.

• Dual Inline Memory Module (DIMM)

The final type of memory package is known as a DIMM (Dual Inline Memory Module).

DIMMs are dual-sided memory chips that hold twice as many chips as a SIMM. (And, except

for the fact that they have chips on both sides, they look just like a SIMM.) Generally, the

DIMMs you’ll run into will have either 72 or 168 pins. Some DIMMs are 32-bit, but more

and more are 64-bit and only have to be installed one at a time in Pentium-class computers.

• Rambus Inline Memory Module (RIMM)

It looks almost like identical to DIMMs, but slightly bigger (with several keys between the

metal contact fingers). Also called the Direct Rambus Memory Module, these advanced

memory devices transfer data in 16-bit chunks along dedicated memory channels. Early

RIMM implementations used 168 pins, but the 600 MHz (PC600), 711 MHz, and 800 MHz

(PC800) RIMMs available today use 168 pins. Rambus modules also include a long heat sink

(or heat spreader) used to manage the elevated operating system temperatures encountered

with the RDRAM chips.

NOTE:

Specialized Memory Types

There are four major specialized applications for memory besides main memory.

Video RAM

Video memory (also called video RAM or VRAM) is used to store image data for processing

by the video adapter. The more video memory an adapter has, the better the quality of image

that it can display. Also, more VRAM allows the adapter to display a higher resolution of

image.

Windows RAM (WRAM)

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Windows RAM (WRAM) is a specialized memory for Windows accelerator cards.

Developed by Samsung, it is similar to video RAM, except that it’s much faster. While

information is being read from one set of WRAM addresses to draw the screen, other

information can be written to another set of addresses. This is faster than normal VRAM,

where all addresses can only be either read from or written to. This ability of WRAM to be

read from or written to simultaneously is called dual-ported memory.

Portable Memory

The memory styles for portable computers are many and varied. Each portable computer

manufacturer comes up with their own specification for portable memory. Installing memory

in a laptop usually involves removing a specially attached panel on the bottom of the laptop

and installing the memory in the slot that is under the removed panel. Then you can replace

the panel. Because each laptop’s memory could potentially install in completely

different ways, check with the manufacturer of your laptop to determine how to

upgrade the memory.

Cache Memory

When a CPU goes to get either its program instructions or data, it always has to get them

from main memory. However, in some systems, there is a small amount of very fast SRAM

memory, called cache memory, between the processor and main memory, and it is used to

store the most frequently accessed information. Because it’s faster than main memory and

contains the most frequently used information, cache memory will increase the performance

of any system.

There are two types of cache memory: on-chip (also called internal or L1 Cache) and off-chip

(also called external or L2 Cache). Internal cache memory is found on Intel Pentium, Pentium

Pro, and Pentium II processors, as well as on other manufacturer’s chips. The original

Pentium contains two 8KB-on-chip caches, one for program instructions and the other for

data. External cache memory is typically either a SIMM of SRAM or a separate expansion

board that installs in a special processor-direct bus.

To get the most out of cache memory, if you have the option of installing an external

cache card onto your motherboard, do it. It can give you as much as a 25 percent

boost in speed.

Logical Memory

Logical memory is the way the physical memory is “put together” for the operating system. In

order to use the physical memory installed in a computer, we need to organize it in some

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logical manner. Most people don’t understand this concept. Let’s reduce that number by at

least one right now.

There is a model that helps us understand the way that memory is laid out. This model is

actually called the “MS-DOS Memory Map.” It was not created all at once but has evolved

over time. The first computers to run DOS were based on the Intel 8088 processor. That

processor could only access a maximum of 1MB (1,024KB) of memory. The memory map

allows us to describe how the memory is being used. It is important to remember that this

memory map is also called a stack, because for purposes of visualizing concepts the memory

blocks are stacked on top of one another.

MEMORY OVERVIEW

When selecting memory for a computer, there are several items to consider:

Type: Motherboards can accept certain types of memory, such as SDRAM, DDR SDRAM,

Rambus, EDO RAM, and burst-EDO RAM. Check the motherboard documentation or the

motherboard manufacturer’s Web site.

Speed: Motherboards can accept memory modules in certain ranges of speed; for example,

100 or 133 MHz. The faster the memory, the faster the performance of the computer. As in

memory type, all memory installed in a single computer should be the same speed. If you

install two different speeds of memory on the same motherboard, all chips will run at the

slower speed.

Quantity (as measured in megabytes): Unlike medication, with memory, more is better,

although you can reach a point of diminishing return. The motherboard documentation will

specify the maximum amount of memory it can accept.

Quality: Memory rarely fails unless it is exposed to static electricity. The most common

problem with memory is when brand new modules are bad—so a good warranty is essential.

Error detection: Memory comes in ECC or parity, or non-ECC or non-parity. ECC and

parity are systems for detecting and correcting memory errors. Parity memory can

compensate for single-bit errors. This parameter is specified by the motherboard

manufacturer, but is changeable in some BIOSs. If the BIOS is set for ECC/parity memory,

only ECC/parity memory will work. Again, even if the motherboard will accept either, it

likely will accept only one of these at a time. If you have a DIMM (see the next item in this

list) and you want to determine if it is ECC/parity or non-ECC/non-parity, simply count the

number of black chips soldered to the module. If the number of chips is evenly divisible by

three or five, then the module contains ECC or parity memory. If the number of chips is not

evenly divisible by three or five, you have non-ECC/non-parity memory.

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Physical module size and pin layout: Almost all currently used memory comes in Dual

Inline Memory Module (DIMM) form. However, size and number of pins vary. This

parameter must match that of the motherboard. Rambus

®

 memory comes in RIMM™ form,

which is Rambus’ version of DIMMs. Prior to DIMMs, Single Inline Memory Modules

(SIMMs) were prevalent. Notebook computers take very small DIMMs, called SODIMMs.

CAS Latency: Measured in numbers such as CL2 and CL3; make sure these match the

requirements of the motherboard.

Serial Presence Detect (SPD): This is memory with an additional chip that contains

information used by some motherboards to set certain memory parameters. This can be used

on any motherboard, but if the motherboard requires this type of memory and non-SPD

memory is used, the computer will display an error message when attempting to boot.

Single- or double-sided module: Some motherboards take either kind, but with a restriction.

The Intel D815EEA motherboard, for example, has four memory slots. You can use up to

four single-sided modules, but the motherboard will recognize only two double-sided

modules. If you install two double-sided modules on slots 0 and 1, any modules in slots 2 or 3

will be ignored. Interestingly, you might not be able to determine this parameter by looking at

the module. For more information, go to kingston.com and use the Memory Search function

to search for memory for the Intel D815EEA motherboard.

Windows Virtual Memory Settings

Computers often temporarily need more memory than they have. For this reason, Windows

manages virtual memory. Virtual memory is the use of a swap file on the hard drive for extra

memory when needed. The action of moving data between physical RAM and the swap file is

called paging. Therefore, if you run into a situation in which the hard drive is thrashing or

churning, which is visible when the hard drive indicator light flashes continuously for

extended periods of time, it is possible that the computer needs more memory.

Other terms:

Conventional Memory - The first 640K of memory addresses used to run applications. Also

referred to as Lower Memory.

Reserved Memory - The next 384K of memory address reserved for use by different types

of ROM BIOS and Video RAM. Also referred to as Upper Memory.

Extended Memory (XMS) - Any memory addresses above Reserved Memory.

High Memory Area (HMA) - The first 64K of Extended Memory

Expanded Memory (EMS) - Reserved or Extended Memory which is made to act like

Conventional Memory. Also known as LIM Memory.

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Limulation - The act of converting Extended Memory into Expanded Memory.

Shadowing - The process of copying ROM BIOS information into the Reserved Memory

area of RAM.

HIMEM.SYS - A device driver created to open up the HMA by unmasking the A20 wire,

and act as a gateway to Extended Memory. Directs all applications where to go in Extended

Memory to avoid conflicts with other applications using Extended Memory. There is a

version for both DOS and Windows. Extended memory cannot be accessed unless

HIMEM.SYS is running.

Upper Memory Blocks (UMB) - Unused memory addresses in the Reserved Memory area.

EMM386.EXE - DOS’ Memory Manager program. Performs Limulation and opens up the

UMBs so that device drivers and programs can be loaded via CONFIG.SYS or

AUTOEXEC.BAT.

EMS Page Frame - A 64K block of memory addresses in the Reserved Memory area used to

perform Limulation.

MEMMAKER - DOS utility that will free up Conventional Memory by loading as many

devices into UMBs as possible.

MEM - DOS command that lets you view the status of all memory. The /C switch classifies

all your memory and shows all programs load in Conventional and Upper Memory.