Present-day computers actually use a variety of storage
technologies. Each technology is geared toward a specific function,
with speeds and capacities to match.
These technologies are:
In terms of capabilities and cost, these technologies form a
spectrum. For example, CPU registers are:
-
Very fast (access times of a few nanoseconds)
-
Low capacity (usually less than 200 bytes)
-
Very limited expansion capabilities (a change in CPU
architecture would be required)
-
Expensive (more than one dollar/byte)
However, at the other end of the spectrum, off-line backup
storage is:
-
Very slow (access times may be measured in days, if the backup
media must be shipped long distances)
-
Very high capacity (10s - 100s of gigabytes)
-
Essentially unlimited expansion capabilities (limited only by
the floorspace needed to house the backup media)
-
Very inexpensive (fractional cents/byte)
By using different technologies with different capabilities, it
is possible to fine-tune system design for maximum performance at
the lowest possible cost. The following sections explore each
technology in the storage spectrum.
Every present-day CPU design includes registers for a variety of
purposes, from storing the address of the currently-executed
instruction to more general-purpose data storage and manipulation.
CPU registers run at the same speed as the rest of the CPU;
otherwise, they would be a serious bottleneck to overall system
performance. The reason for this is that nearly all operations
performed by the CPU involve the registers in one way or
another.
The number of CPU registers (and their uses) are strictly
dependent on the architectural design of the CPU itself. There is
no way to change the number of CPU registers, short of migrating to
a CPU with a different architecture. For these reasons, the number
of CPU registers can be considered a constant, as they are
changeable only with great pain and expense.
The purpose of cache memory is to act as a buffer between the
very limited, very high-speed CPU registers and the relatively
slower and much larger main system memory — usually referred
to as RAM. Cache memory has an operating speed
similar to the CPU itself so, when the CPU accesses data in cache,
the CPU is not kept waiting for the data.
Cache memory is configured such that, whenever data is to be
read from RAM, the system hardware first checks to determine if the
desired data is in cache. If the data is in cache, it is quickly
retrieved, and used by the CPU. However, if the data is not in
cache, the data is read from RAM and, while being transferred to
the CPU, is also placed in cache (in case it is needed again
later). From the perspective of the CPU, all this is done
transparently, so that the only difference between accessing data
in cache and accessing data in RAM is the amount of time it takes
for the data to be returned.
In terms of storage capacity, cache is much smaller than RAM.
Therefore, not every byte in RAM can have its own unique location
in cache. As such, it is necessary to split cache up into sections
that can be used to cache different areas of RAM, and to have a
mechanism that allows each area of cache to cache different areas
of RAM at different times. Even with the difference in size between
cache and RAM, given the sequential and localized nature of storage
access, a small amount of cache can effectively speed access to a
large amount of RAM.
When writing data from the CPU, things get a bit more
complicated. There are two different approaches that can be used.
In both cases, the data is first written to cache. However, since
the purpose of cache is to function as a very fast copy of the
contents of selected portions of RAM, any time a piece of data
changes its value, that new value must be written to both cache
memory and RAM. Otherwise, the data in cache and the data in RAM
would no longer match.
The two approaches differ in how this is done. One approach,
known as write-through caching,
immediately writes the modified data to RAM. Write-back caching, however, delays the writing of
modified data back to RAM. The reason for doing this is to reduce
the number of times a frequently-modified piece of data must be
written back to RAM.
Write-through cache is a bit simpler to implement; for this
reason it is most common. Write-back cache is a bit trickier to
implement; in addition to storing the actual data, it is necessary
to maintain some sort of mechanism capable of flagging the cached
data as clean (the data in cache is the same as the data in RAM),
or dirty (the data in cache has been modified, meaning that the
data in RAM is no longer current). It is also necessary to
implement a way of periodically flushing dirty cache entries back
to RAM.
Cache subsystems in present-day computer designs may be
multi-level; that is, there might be more than one set of cache
between the CPU and main memory. The cache levels are often
numbered, with lower numbers being closer to the CPU. Many systems
have two cache levels:
-
L1 cache is often located directly on the CPU chip itself and
runs at the same speed as the CPU
-
L2 cache is often part of the CPU module, runs at CPU speeds (or
nearly so), and is usually a bit larger and slower than L1
cache
Some systems (normally high-performance servers) also have L3
cache, which is usually part of the system motherboard. As might be
expected, L3 cache would be larger (and most likely slower) than L2
cache.
In either case, the goal of all cache subsystems — whether
single- or multi-level — is to reduce the average access time
to the RAM.
RAM makes up the bulk of electronic storage on present-day
computers. It is used as storage for both data and programs while
those data and programs are in use. The speed of RAM in most
systems today lies between the speed of cache memory and that of
hard drives, and is much closer to the former than the latter.
The basic operation of RAM is actually quite straightforward. At
the lowest level, there are the RAM chips — integrated
circuits that do the actual "remembering." These chips have four
types of connections to the outside world:
-
Power connections (to operate the circuitry within the chip)
-
Data connections (to enable the transfer of data into or out of
the chip)
-
Read/Write connections (to control whether data is to be stored
into or retrieved from the chip)
-
Address connections (to determine where in the chip the data
should be read/written)
Here are the steps required to store data in RAM:
-
The data to be stored is presented to the data connections.
-
The address at which the data is to be stored is presented to
the address connections.
-
The read/write connection is set to write mode.
Retrieving data is just as straightforward:
-
The address of the desired data is presented to the address
connections.
-
The read/write connection is set to read mode.
-
The desired data is read from the data connections.
While these steps seem simple, they take place at very high
speeds, with the time spent on each step measured in
nanoseconds.
Nearly all RAM chips created today are sold as modules. Each module consists of a number of
individual RAM chips attached to a small circuit board. The
mechanical and electrical layout of the module adheres to various
industry standards, making it possible to purchase memory from a
variety of vendors.
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Note |
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The main benefit to a system using industry-standard RAM modules
is that it tends to keep the cost of RAM low, due to the ability to
purchase the modules from more than just the system
manufacturer.
Although most computers use industry-standard RAM modules, there
are exceptions. Most notable are laptops (and even here some
standardization is starting to take hold) and high-end servers.
However, even in these instances, it is likely that third-party RAM
modules are available, assuming the system is relatively popular
and is not a completely new design.
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All the technologies discussed so far are volatile in nature. In other words, data contained
in volatile storage is lost when the power is turned off.
Hard drives, on the other hand, are non-volatile — the data they contain remains
there, even after the power is removed. Because of this, hard
drives occupy a special place in the storage spectrum. Their
non-volatile nature makes them ideal for storing programs and data
for longer-term use. Another unique aspect to hard drives is that,
unlike RAM and cache memory, it is not possible to execute programs
directly when they are stored on hard drives; instead, they must
first be read into RAM.
Also different from cache and RAM is the speed of data storage
and retrieval; hard drives are at least an order of magnitude
slower than the all-electronic technologies used for cache and RAM.
The difference in speed is due mainly to their electromechanical
nature. There are four distinct phases taking place during each
data transfer to or from a hard drive. The following list
illustrates these phases, along with the time it would take a
typical high-performance drive, on average, to complete each:
-
Access arm movement (5.5 milliseconds)
-
Disk rotation (.1 milliseconds)
-
Heads reading/writing data (.00014 milliseconds)
-
Data transfer to/from the drive's electronics (.003
Milliseconds)
Of these, only the last phase is not dependent on any mechanical
operation.
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Note |
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Although there is much more to learn about hard drives, disk
storage technologies are discussed in more depth in Chapter 5 Managing Storage. For the
time being, it is only necessary to keep in mind the huge speed
difference between RAM and disk-based technologies and that their
storage capacity usually exceeds that of RAM by a factor of at
least 10, and often by 100 or more.
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Off-line backup storage takes a step beyond hard drive storage
in terms of capacity (higher) and speed (slower). Here, capacities
are effectively limited only by your ability to procure and store
the removable media.
The actual technologies used in these devices varies widely.
Here are the more popular types:
-
Magnetic tape
-
Optical disk
Of course, having removable media means that access times become
even longer, particularly when the desired data is on media not
currently loaded in the storage device. This situation is
alleviated somewhat by the use of robotic devices capable of
automatically loading and unloading media, but the media storage
capacities of such devices are still finite. Even in the best of
cases, access times are measured in seconds, which is much longer
than the relatively slow multi-millisecond access times typical for
a high-performance hard drive.
Now that we have briefly studied the various storage
technologies in use today, let us explore basic virtual memory
concepts.
