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A+ Guide to Managing and Troubleshooting PCs Chapter 4
12/24/2009
Chapter 4
RAM
Chapter 4
MEMORY
Lecture Notes
I. . Historical/Conceptual Understanding Dynamic Random Access Memory (DRAM)
A. Introduction
1) Special semi-conductor that temporarily stores 1s and 0s in microscopic capacitors and
transistors.2) Refers to byte-wide memory.3) Visualize like a spreadsheet:
a) A 1 MB8 RAM stick would be represented by 8 columns wide (8 bits = 1 byte) and
1,048,576 rows deep (1 MB).
B. Organizing DRAM
1. Intel Processors since the 8088 require chips to be stored in 8 bit (byte) chunks.
C. Practical DRAM.
1. Early RAM on the 8088 used an 8 bit wide data bus.
2. Some commands were more complex requiring two bytes. The Codebook (Instruction Set)
allowed this. They CPU would recognize commands that were two bytes wide and simply
read both commands.
3. 8086 chip would have worked with 16 bit data bus, but Intel made a marketing decision
using the cheaper and easier to implement 8088.
1. Address Bus. This is what addresses memory. The same as a house has an address, memory has
an address. We can store bytes of data in different addresses. The more address lines we have
on the address bus, the more bytes of data we can store.
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2. External Data Bus. This is where the data is passed back and forth between CPU and Memory
(via the MCC).
3. Frontside Bus. With the advent of cache onboard the chip and accessed directly from the CPU
(also known as the backside bus), the External Data Bus is now known as the frontside bus.
D. DRAM Sticks
1. 80386 required 32 bit wide data bus (as opposed to 8088 8 bit wide). If chips were still
only one bit wide, this would have required a minimum of 32 chips on the board.
2. DRAM manufacturers began creating wider chips X4 (4 bits wide), X8 (8 bits wide), etc.
3. Single Inline Memory Modules (SIMM): came in 30-pin and then 72 pin modules. SIMMs
inserted into special SIMM sockets.
4. Current RAM is 32 bits wide, 64 bits wide and sometimes 128 bits wide. The width of the
frontside bus is dictated by the MCC being used.
5. When replacing RAM, you need to know the width of the module being replaced. This is
different than the older method of identifying the width of the individual chips.
D. Consumer RAM
1. Even though RAM comes in packages wider than bytes, RAM is still described in bytes,
such as a 256 MB stick or a 512 MB stick.
II. Essentials – Types of RAM
A. Synchronous Dynamic RAM (SDRAM)
1. Synchronized with the system clock.
2. Came on stick called a Dual Inline Memory Module (DIMM).
3. Wide number of pins on DIMMs. 168 pins common on desktops. Laptops had 68 pins,
144 pins, 172 pin micro-DIMM. Laptops also had Small Outline-DIMM (SO-DIMM) of
72 pin, 144 pin, or 200 pins.
4. Most were 64 bits wide. (72 pin SO DIMM was 32 bits.)
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5. Instead of an access speed, it has a clock speed measured in MHz. Common speeds were
66, 75, 83, 100 and 133 MHz.
B. RAMBUS DRAM (RDRAM)
1. RDRAM could support speeds on the frontside bus of up to 800 MHz. This was needed
for the quad pumped CPUs.
2. Originally thought to be the next best thing, it suffered delays in manufacturing and was
significantly more expensive than SDRAM.
3. Commonly called a RIMM (though RIMM doesn’t specifically stand for anything).
4. 184 pin sticks for desktops. 160 pin SO-RIMM for laptops.
5. Requires all RIMM slots (in a pair) to be populated. Continuity RIMM (CRIMM) sticks
were essentially RIMM terminators.
6. RDRAM was proprietary and licensed to only work with Intel systems for several years.
The goal was to push AMD out of the market place, but the result was growth of other
types of memory and memory chips.
C. DDR SDRAM
1. Fast RAM supported by AMD and other manufacturers.
2. Doubles the throughput over SDRAM (makes two processes for each clock cycle.)
3. Commonly referred to as DDR, DDR RAM, and DDRAM.
4. Comes in 184-pin DIMMs.
5. Slots are keyed with guide notches, so you cannot insert RAM in incorrect slots.
6. Base clock is 100 MHZ to 300 MHz with the DDR Speed rating double the Clock Speed.
To determine the PC Speed Rating (bytes per second) multiply the DDR speed in MHz by
8. For example, a chip with a clock speed of 200 and doubled DDR Speed Rating of 400
would be referred to as a PC 3200 (400 MHz * 8).
7. Supported as standard system RAM, even by Intel for the Pentium 4..
8. Can make use of dual channel architecture.
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a) Similar to RAMBUS in how it improves performance.
b) Only works when two slots are populated.
c) Dual slots are often blue, third slot often black.
8. Dual slots are blue. If third slot exists, it is black.
D. DDR2
1. Doubled the clock to increase buffering and overall performance.
2. Uses 240 pin DIMM (not compatible with DDR DIMM).
3. PC Rating determined similar to how DDR is determined (with another doubling), and
prefixed by a 2. For example, for DDR2 with a clock speed of 200 MHz, the DDR I/O
speed would be 400 MHz, the DDR2 Speed Rating would be DDR2-800, and the PC
Speed Rating would be PC2-6400
E. RAM Variations
1. Double-Sided DIMMS. Many sticks come in single sided versions and double sided
versions. Some double sided sticks simply can’t go into all motherboards.
2. Latency – refers to how quickly the RAM responds to the electrical signal.
a) High latency – high wait time. Takes longer for RAM to respond. CL3 is high latency
b) Lower latency – lower wait time. RAM can get the signal out in fewer clock ticks.
CL2 is low latency.
c) The goal is to match the RAM with the motherboard. If necessary when swapping
RAM, the RAM timings can be adjusted in BIOS as discussed in chapter 5.
2. Parity and ECC – Detect errors in memory.
a) Parity allows the computer to detect if an error occurred in the reading or writing of
data in memory.
b) Error Correction Code (ECC) is an improvement over parity. Not only can errors be
detected, but they can also be corrected.
c) ECC is slower because of the extra calculations required
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d) Parity and ECC is only used on higher end systems. Often applications will check
and/or correct errors on the fly instead of depending on the hardware to do so.
2. Buffered Registered DRAM.
a) Typical motherboards only have four memory slots and can only accept four memory
sticks.
b) More than four sticks of DRAM causes electrical challenges that can be overcome
with “buffered registered DRAM”
c) If the motherboard accepts more than four sticks of RAM, it may require buffered
DRAM.
III. Working with RAM
A. Adding more RAM almost always improves overall system performance, processing speed, and
stability.
1. Determine if insufficient RAM is the cause of system problems.
2. Pick the proper RAM for the system.
3. Use good installation practices such as keeping RAM sticks in antistatic packaging and
following strict ESD practices.
4. Two symptoms show need for more RAM:
a) General system sluggishness.
b) Disk thrashing or excess hard drive accessing, generally indicates using excessive
movement of files between RAM and swap file or virtual memory space on hard drive.
B. Do You Need More RAM
1. Is your system sluggish?
a) Slow running system, especially with more programs running indicates RAM may be a
bottleneck.
2. Are you experiencing disk thrashing? Disk thrashing occurs when data is swapped
between physical RAM and virtual RAM (the paging file). The symptom of disk thrashing
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is that the disk drive LED is blinking feverously and you can hear it almost constantly
moving.
3. System RAM Recommendations. Microsoft’s system RAM recommendations for various
Windows operating systems are very low. As the old joke goes, “don’t try this at home.”
Instead, consider adding additional memory to reach the reasonable minimum, for solid
performance or if you are a power user.
a) Windows 2000. Reasonable minimum: 128 MB. Solid performance: 256 MB, Power
User: 512 MB.
b) Windows XP. Reasonable minimum: 256 MB. Solid performance: 512 MB, Power
User: 1 GB.
c) Windows Vista. Reasonable minimum: 512 MB. Solid performance: 1 GB, Power
User: 2 GB.
4. Determining Current RAM Capacity.a) Use Properties for My Computer to see the
RAM configuration.
C. Getting the Right RAM
1. We’ve talked about a lot of different types of RAM in the preceding pages. This is why.
You can only put the same type of RAM into a motherboard that it accepts. The first step
is to determine what the motherboard will accept. You will typically check the manual for
this answer.
2. Open the case and see what’s installed. For example, if you system can accept 2 GB and the
tools tell you that you have 1 GB installed, you might jump to the conclusion that you can
add 1 GB more. However, opening the case might show you that you have four 256 MB
sticks. There’s no room for new RAM unless you replace the existing RAM.
3. Mix and Match at your Peril. Different RAM sizes aren’t always handled well in
motherboards. It’s best to choose RAM sticks that match technology, capacity and speed.
4) Mixing Speeds.
a) its best not to mix speeds.
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b) Worst case you may cause random lockups and data corruption.
c) Best case, it’ll work at the slowest speed.
d) You can place higher rated RAM into a slower rated motherboard. The RAM will just
work at the slower rate of the motherboard.
D. Installing DIMMs and RIMMs
1. The sticks are keyed so they can only go in one way.
2. Line up the keys and place in the slot.
3. Push down and the tabs will lock into position.
4. SPD – Serial Presence Detect. Upon power up, the SPD chip detects the memory in the
system and reports it to the BIOS.
a) Any program can query the SPD. One of the Cool Tools we’ve explored is CPU-Z
which displays some of the SPD information.
5. The RAM Count.
a) Upon power up, BIOS will count and report the amount of RAM detected.
b) This is often a binary count so 256 MB wouldn’t be exactly 256 million.
c) Usually a ball park number tells you you’ve been successful.
E. Installing SO-DIMMs Laptops
1. For years, laptops had proprietary RAM packages making this difficult. However, the
acceptance of SO-DIMMs over the years has made it much easier.
2. First, power off, unplug and remove the battery pack. Follow ESD procedures.
3. Identify the access point for the SO-DIMM. This is usually either under the keyboard or
via an access panel on the back.
4. Replace the SO-DIMM.
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IV. IT Technician Troubleshooting RAM
A. Memory errors can include parity errors, ECC error messages, system lockups, page faults, and
other error screens in Windows.
B. Write down memory error addresses. If the error returns with the same address each time, it is
likely a RAM stick failure.
1. If the error address changes, it is likely not a RAM failure.
2. System lockups and page faults often indicate RAM problem.
3. RAM and parity errors can trigger non-maskable interrupt (NMI) that results in Blue
Screen of Death (BSoD).
4. Not all intermittent errors are caused by bad RAM. Dying power supply, electrical
interference, buggy applications or hardware can also produce intermittent errors.
C. Testing RAM
1. Number of RAM testing devices are available but many are expensive.
2. Can replace one stick at a time until problems disappear.
3. Use a software based tester such as MemTest86
Homework Questions
Questions
1. What bus determines how much RAM we can have in a system?
2. What does ECC DRAM provide that regular DRAM does not?
3. After adding a second 256 MB SDRAM, only 256 MB of RAM shows up during the RAM
count. What is the most likely cause?
4. Why was RAMBUS (RDRAM) not widely accepted?
5. What are the standard pin sizes for SO-DIMMs?
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6. What RAM packages do SDRAM and DDR-SDRAM commonly come in?
7. How many 168-pin DIMMs will fill a single bank of RAM for a Pentium 4 computer?
8. What is Dual DDR-DRAM and what are its advantages?
9. What should you consider before installing RAM into a computer?
10. What four ESD precautions should you take before installing RAM?
11. What bus determines how wide any replacement RAM must be?
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