Chapter 7 RAM

Seite 1

In this chapter, you will learn

how to

■ Identify the different types of

DRAM packaging

■ Explain the varieties of RAM

■ Select and install RAM

■ Perform basic RAM

troubleshooting

1. “How much RAM is in your computer?”

2. “What is RAM and why is it so important that every PC has some?”

3. What makes DRAM dynamic?

4. What does random

access mean?

CHAPTER 7 RAM The more common term

in the industry is RAM, for

random access memory. More

specifically, the primary system

RAM is dynamic random access

memory (DRAM). For the most

part, this book uses the terms

RAM and DRAM.

When not in use, programs and data are held in a mass storage device

such as a hard drive, USB thumb drive, optical drive, or some other

device that can hold data while the computer is off. When you load a program

in Windows, your PC copies the program from the mass storage

device to RAM and then runs it

Understanding DRAM DRAM has been the standard RAM used in all computers because :

low cost,

high speed &

capability to contain a lot of data in

a relatively small package.

8088 processor had an 8-bit

frontside bus.

Most of the 8088

machine language commands are 1 byte, but more-complex

commands need 2 bytes. Just did 1 byte at a time. Took twice as long.

16-bit frontside

bus----> Intel did. Intel

invented a CPU called the 8086. But too expensive and no MCC to handle it.

The Intel 80386 CPU, for example, had a 32-bit data bus and

thus the need for 32-bit-wide DRAM.

Having enough RAM

makes their systems snappy and stable; not enough RAM means their systems

run poorly

***TYPES OF RAM****

SDRAM: synchronous DRAM (SDRAM)-- tied to the system clock just like the CPU and MCC

SDRAM made its debut in 1996 on a stick called a dual inline memory

module (DIMM)

SDRAM was tied to the system clock, so its clock speed matched

the frontside bus.

Five clock speeds were commonly used on the

early SDRAM systems: 66, 75, 83, 100, and 133 MHz.

The RAM

speed had to match or exceed the system speed or the computer

would be unstable or wouldn’t work at all.

These speeds were prefixed

with a “PC” in the front, based on a standard forwarded by

Intel, so SDRAM speeds were PC66 through PC133. For a Pentium

III computer with a 100-MHz frontside bus, you needed to buy

SDRAM DIMMs rated to handle it, such as PC100 or PC133

DDR SDRAM

(COPy'd RAMBUS)

AMD made double data rate SDRAM (DDR SDRAM).

DDR SDRAM basically copied

Rambus, doubling the throughput of SDRAM by making two processes

for every clock cycle. This synchronized (pardon the pun) nicely with the

Athlon and later AMD processors’ double-pumped frontside bus. DDR

SDRAM could not run as fast as RDRAM—although relatively low frontside

bus speeds made that a moot point—but cost only slightly more than

regular SDRAM.

DDR SDRAM

not as fast as rambus.BUT only cost slightly more than sdram.

RDRAM When Intel was developing the Pentium 4, they knew SDRAM wasn’t fast enough to handle the quadpumped

400-MHz frontside bus.

Intel announced plans to replace SDRAM

with a very fast, new type of RAM developed by Rambus, Inc., called

Rambus DRAM, or simply RDRAM

Hailed by Intel as the

next great leap in DRAM technology, RDRAM could handle speeds up to

800 MHz, which gave Intel plenty of room to improve the Pentium 4.

A stick of RDRAM is called a RIMM. In this

case, however, the letters don’t actually stand for anything; they just rhyme:

RDRAM employed an

interesting dual-channel architecture. Each RIMM was

64 bits wide, but the Rambus MCC alternated between

two sticks to increase the speed of data retrieval. You

were required to install RIMMs in pairs to use this dualchannel

architecture.

RDRAM motherboards also required that all RIMM

slots be populated.

Unused pairs of slots needed a passive

device called a continuity RIMM (CRIMM) installed

in each slot to enable the RDRAM system to terminate

properly.

NOT POPULAR BECAUSE:::::

a) technology was owned wholly by Rambus;

if you wanted to make it, you had to pay the licensing fees they charged.

That led directly to the second problem,

b) expense. RDRAM cost substantially

more than SDRAM.

c) Rambus and Intel made a completely closed deal

for the technology. RDRAM worked only on Pentium 4 systems using Intelmade

MCCs. AMD was out of luck. Clearly, the rest of the industry had to

look for another high-speed RAM solution.

DDR2

DDR2 is DDR

RAM with some improvements in its electrical characteristics, enabling it to run even faster than DDR while using less power. The big speed increase from DDR2 comes by clock doubling the input/output circuits on the chips. This does not speed up the core RAM—the part that holds the data—but speeding up the input/output and adding special buffers (sort of like a cache) makes DDR2 run much faster than regular DDR.

The delay in

RAM’s response time is called its latency.

RAM with a lower latency—suchas CL2—is faster than RAM with a higher latency—such as CL3—because

it responds more quickly.

The CL refers to clock cycle delays. The 2 means

that the memory delays two clock cycles before delivering the requested

data; the 3 means a three-cycle delay.

From a tech’s standpoint, you need to get the proper RAM for the system

you’re working on. If you put a high-latency stick in a motherboard

set up for a low-latency stick, you’ll get an unstable or completely dead

PC.

page file,

swap file, or virtual memory. When the PC starts running

out of real RAM because you’ve loaded too many

programs, the system swaps programs from RAM to

the page file, opening more space for programs currently

active.

If Windows needs to access the page file too frequently, you will notice

the hard drive access LED going crazy as Windows rushes to move programs

between RAM and the page file in a process called disk thrashing.

Windows uses the page file all the time.

Windows Vista and Windows 7 offer a feature called ReadyBoost that enables

you to use flash media devices—removable USB thumb drives or memory

cards—as super fast, dedicated virtual memory.

Windows 7 can handle up to eight flash devices, whereas

Windows Vista can benefit from only one device. Devices can be

between 1–32 GB in capacity

serial presence detect (SPD): Your motherboard should detect and automatically

set up any DIMM or RIMM. The SPD

chip stores all the information about your DRAM,

including size, speed, ECC or non-ECC, registered

or unregistered, and a number of other more technical

bits of information.

When a PC boots, it queries the SPD chip so

that the MCC knows how much RAM is on the stick, how

fast it runs, and other information

Every once in a while, something potentially catastrophic happens

within the PC, some little electron hits the big red panic button, and the

operating system has to shut down certain functions before it can save data.

This panic button inside the PC is called a non-maskable interrupt (NMI),

more simply defined as an interruption the CPU cannot ignore. An NMI

manifests to the user as what techs lovingly call the Blue Screen of Death

(BSoD)—a bright blue screen with a scary-sounding error message on it (see

Figure 7.42).