How Computer Ram Memory Works
How Computer Ram Memory Works
RAM is also used to store instructions about currently running applications.
For example, when you start a computer game, a large set of the game’s instructions
(for example, how it works, how the screen should look, which sounds must be
generated) is loaded into memory. The processor can retrieve these instructions
much faster from RAM than it can from the hard drive, where the game normally
resides until you start it. Within certain limits, the more information that’s stored
in memory, the faster the computer will run. In fact, one of the most common
computer upgrades is to increase the amount of RAM.
The information in RAM is continually being read, changed, and removed. It is
also volatile, meaning that it cannot work without a steady power supply. When a
computer is turned off, the information in RAM is lost.
Recall that when a user makes a request, it is intercepted by the processor, which then
organizes the request into component-specific tasks. Many of these tasks must occur in
a specific order, with each component reporting its results back to the processor before
the next task can be completed. The processor uses RAM to store these results until
they can be compiled into the final result(s).
Many desktop components, such as the processor, power supply, and RAM, are installed
through simple physical attachment to the computer. That is, physical installation is
all that is required to make the component functional. Other devices, such as hard
drives and keyboards, require the additional assignment of system resources. This section
focuses on the physical installation of common components; resource assignment is
discussed in the next section, “IRQs, DMAs, and I/O Addresses.” Special hardware
configurations are discussed in Chapter 2.
Memory
The first RAM chips were dual inline packages (DIP) that attached directly into sockets
on the system board. However, their design made them prone to loosening due to the
alternating heating and cooling of the system board. Newer memory modules are actually
small cards with DIP chips on one or both sides. These cards fit upright into slots on the
system board and are held in place by clips that prevent “chip creep” (loosening).
RAM is automatically detected and counted on startup, so its installation is limited
to physical placement in the computer. That is, once RAM is physically installed,
no additional configuration is required. When installing memory in a motherboard,
verify the types and amounts of memory that the motherboard can accept.
SIMM Memory Single inline memory module (SIMM) memory is available
in 30- and 72-connector configurations. Most 80386, 80486, and Pentium
computers include slots for both SIMM types. Follow the steps in Exercise 1-5
to install SIMM.
However, newer drives are able to access RAM
directly using a protocol called Ultra DMA (UDMA). UDMA is a protocol used only
by hard drives and is not functionally associated with a computer’s standard DMA
channels.
As you know Memory, one function of RAM is to provide the processor with faster access to the
information it needs. Within limits, the more memory a computer has, the faster it will
run. One of the most common computer upgrades is the installation of more RAM.
Recall that most computers can use another type of RAM, called cache memory.
Cache memory chips can be accessed even faster than regular RAM, so their presence
can help speed up the computer. Generally, the more cache a computer has, the faster
it will run. The type of cache that can be added to the computer is called Level 2 (L2)
cache, and it can be installed in available slots on the motherboard.
Additional RAM Memory can be added to a portable system in a number of ways. Some systems
include extra RAM slots within the chassis. This type requires you to open the
computer’s case and place the RAM module in an available slot (see Figure 2-14).
Because RAM modules for portables are proprietary, you cannot use them in desktop
computers or in other portables.
An easier way to add more RAM to your portable is to use a memory PC Card.
PC Cards were described earlier as being small cards that can be easily inserted in a
portable to enhance or expand its abilities. In fact, PC Cards originated as PCMCIA
cards specifically for the purpose of adding more memory. PCMCIA stands for Personal
Computer Memory Card International Association, a bit of a misnomer because these
cards are usually used in laptops, not in PCs (desktops).
Processor and Memory Symptoms
In most cases, processor and memory problems are fatal, meaning that when there is
such a problem, the computer will not boot at all. However, you should be aware of
some nonfatal error indicators. As described, 1** error codes are typical of processor
problems, and 2** error codes are typical of memory problems.
If you turn on the computer and it does not even complete the POST or it does
nothing at all, and you have eliminated power problems, there might be a problem
with the processor or memory. The solution to a processor or memory problem is to
remove the offending component and replace it with a new one. If the error persists,
there might be a problem with the slot or socket that the memory or processor uses
to connect to the motherboard. In this case, the motherboard needs to be replaced.
On a final note: Some RAM errors are not reported by the computer at all. That
is, if an entire memory module does not work, the computer might just ignore it and
continue to function normally without it.Watch as the RAM is counted on the screen
at startup to ensure that the total amount matches the capacity installed in the
machine. If this amount comes up significantly short, you probably have to replace
the memory module.
Memory failures may not cause a system to appear to malfunction at all.
Most modern systems will simply ignore a memory card that has malfunctioned
and normal operations will continue. The user may note performance loss,
which is a key symptom of a memory card failure.
Random Access Memory
The primary function of RAM is to provide a temporary storage place for information
about devices and applications. However, there are many types of RAM with which
you should be familiar. This section discusses the many incarnations of RAM as it has
been developed and refined over time. This section also discusses important factors
to consider when installing or upgrading the RAM in a computer system: there are
guidelines you must follow about the type of RAM, the type of package, and the amounts
of RAM that you install in a particular system.
Types of RAM
RAM is not all the same. Over time, RAM technology has improved, changed form, and
been used for specialized components. The most common types of RAM are discussed here.
SRAM
Static RAM (SRAM) was the first type of RAM available. SRAM can be accessed at
approximately 10 nanoseconds (ns), meaning that it takes about 10ns for the processor
to receive requested information from SRAM. The structure of SRAM chips limits
them to a maximum data capacity of 256KB. Although SRAM is very fast compared
with DRAM, it is also very expensive. For this reason, SRAM is typically used only for
system cache.
DRAM
Dynamic RAM (DRAM) was developed to combat the restrictive expense of using
SRAM. DRAM chips provide much slower access than SRAM chips but can store
several megabytes of data on a single chip (or hundreds of megabytes if they are packaged
together on a module). Every “cell” in a DRAM chip contains one transistor and one
capacitor to store a single bit of information. This design makes it necessary for the
DRAM chip to receive a constant power refresh from the computer to prevent the
capacitors from losing their charge. This constant refresh can make access even slower
and causes the DRAM chip to draw more power from the computer than an SRAM
chip. Because of its low cost and high capacity, DRAM is used as “main” memory in
the computer.
The term DRAM is typically used to describe any type of memory that uses the
technology just described. However, the first DRAM chips were very slow (~80–90ns),
so faster variants have been developed. The list is quite large and includes fast-paged
RAM, EDO RAM, SDRAM, RDRAM, SDLRAM, and BEDO RAM. As computer
systems improve, the list of DRAM technologies continues to grow. However, EDO,
SDRAM, RDRAM, and DDR RAM are currently the most common, so they are
described here.
EDO RAM
Extended data out (EDO) RAM improves on traditional DRAM by performing more
than one task at a time. When one piece of data is being sent to the processor, another
is being retrieved from the RAM module. While that piece of data is being transferred,
the EDO RAM is looking for the next piece to retrieve for the processor. This process
enables the chip’s data to be accessed at about 60ns. EDO RAM chips can be used
only in a computer system whose processor and motherboard support its use.
SDRAM
Synchronous dynamic RAM, or SDRAM, is about twice as fast as
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