The Storage Spectrum
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 each technologies 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.
CPU Registers
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 (unchangeable without great
pain).
Cache Memory
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[1]. Cache memory has an operating speed
similar to the CPU itself, so that 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 see 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 will be needed again). 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 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. However, 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 will no longer match.
The two approaches differ in how this is done. One approach,
known as write-through cache, immediately
writes the modified data to RAM. Write-back
cache, however, delays the writing of modified data back to RAM; in
this way, should the data be modified again it will not be necessary
to undergo several slow data transfers to RAM.
Write-through cache is a bit simpler to implement; for this reason
it is often seen. Write-back cache is a bit trickier to implement, as
in addition to storing the actual data, it is necessary to maintain
some sort of flag that denotes that the cached data is clean (the data
in RAM is the same as the data in cache), or dirty (the data in RAM is
not the same as the data in cache). It is also necessary to implement
a way of periodically flushing dirty cache entries back to RAM.
Cache Levels
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 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.
Main Memory — 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 speeds 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 to set to write mode.
Retrieving data is just as simple:
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 are simple, they take place at very high speeds,
with the time spent at 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 adhere to various
industry standards, making it possible to purchase memory from a
variety of vendors.
 | Note |
|---|
| | The main benefit to a system that uses 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 you will be able
to find third-party RAM modules, assuming the system is not a
completely new design and is relatively popular. |
Hard Drives
All the technologies that have been discussed so far are
volatile in nature. In other words, data
contained in volatile storage technologies will be lost when the power
is turned off.
Hard drives, on the other hand, are
non-volatile — the data they contain will
remain 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; 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.
Here are the four distinct phases during data transfer to/from a hard
drive. The times shown reflect how long it would take a typical
high-performance drive, on average, to complete each phase:
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.
| Note |
|---|
| | Although there is much more to learn about hard drives, we will
discuss disk storage technologies in more depth in Chapter 5. For the time being, it is only necessary to
realize 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. |
Off-Line Backup Storage
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 store the removable
media.
The actual technologies used in these devices can vary widely.
Here are the more popular:
Magnetic tape
Optical disk
Of course, having removable media means that access times become
even longer, particularly when the desired data is on media that is
not currently in the storage device. This situation is alleviated
somewhat by the use of robotic devices to automatically load and
unload media, but the media storage capacities of such devices are
finite, and even in the best of cases access times are measured in
seconds — a far cry even from the slow multi-millisecond access
times 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.
