Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (7 page)

2.4.3 Memory Map

Another important concept to understand with respect to memory is the memory
map. Since the CPU only communicates with the overall system via the memory
chips, there must be a clear specification in the design of the computer as to
how these two electronic components will communicate. This is the memory
map. It indicates the predetermined allocation and meaning of memory ad-
dresses (Figure 2.2).

FIGURE
2.2
A very simple memory map for 64 kB of memory (65,536 addresses with
0 counting as an address).

14 Chapter 2

The design of a computer may specify that at address number 640,000,
there should be one byte of data (8 ones and/or zeroes) that indicates what color
should be shown on the first “dot” or pixel of a computer monitor. Of course,
since different computer systems (even from the same manufacturer) may have a
variety of options for memory (such as buying a computer with 1 gigabyte of
RAM versus one with 2 gigabytes), the memory map must allow for this varia-
tion. This is usually accommodated by having all the essentials of running the
computer system at the very “low” addresses (perhaps the first few thousand
bytes out of the millions or billions of available addresses). At these low ad-
dresses spaces you would find data or programming for such things as the key-
board and mouse, since all systems are going to have these devices. However, the
memory map would eventually have regions or lists of addresses that are just for
applications (such as the CAD program), and the memory map is somewhat
“open” at that point. If there is physically enough memory chips on the system,
then the addresses in that region would be available. If not, then these addresses
would simply not be used (and swapping may start).

The maximum number of addresses possible for the map is set by the num-
ber of bits in the architecture. A 32-bit system has a highest possible address num-
ber of 4 gigabytes (232
or about 4 billion addresses). So the memory map for the
32-bit system would stop at that point. However, even if the CPU can handle 64
bits, the memory chips or the CAD software may still be meant for 32 bits only.
Of course, in time, all the components and software will be able to take advantage
of 64 bits, and so the memory map could then extend to 18 billion gigabytes (264).

It may seem ridiculous to have this much data loaded into Memory for the
CPU to manipulate, but CAD software could actually create this amount of de-
mand. It would not be unusual for a mechanical assembly (such as the crane shown
in Figure 2.3) to require billions of 3-D surfaces to be totally accurate to the physi-
cal product. Since it may take thousands of bytes to describe each surface (color,
texture, boundaries, etc.), it would not be difficult to require trillions of bytes to
fully realize that 3-D model down to each tiny component (nut, bolt, wire, etc.).

2.4.4 Memory Configurations

In terms of the physical memory chips, they are often found in a wide variety of
configurations. Usually, the main circuit board or motherboard contains the CPU
as well as the memory chips. These chips are arranged in banks. These banks
have a given capacity to hold a certain number of memory chip modules. These
modules may be called SIMMs, although there are other types of configurations
with different names. The number of these modules in combination with the ca-
pacity of a given memory chip will create the total “physical” memory or RAM
for the given computer system.

A typical situation would be a SIMM module with 4 chips on it. The chips
could have a capacity of 32 Megabytes each, so when they are combined into a

Computer Hardware Basics 15

FIGURE
2.3

Example of large 3-D model.

single SIMM, there would be 128 Megabytes. Then 4 of these SIMMs could be
combined on the motherboard to give a total of 512 megabytes of physical mem-
ory. Recall that this then becomes the maximum “address space” that the CPU
could specifically utilize to run the system, and then depending on the operating
system, this could be expanded to a certain higher number as “virtual” memory.
For instance, this 512 megabyte physical memory could be configured to mimic 4

gigabytes of memory as virtual memory.

Table 2.1 summarizes the contrasts between the memory system and the
disk system (which is explained in more detail in the next section).

TABLE
2.1

A Comparison Between the Memory System and the Storage System

Memory System Storage System

Physical device type

Integrated circuits (chips) Disk drives

Data behavior

Data speed

Volatile (data is lost when
the system is shutdown)
Fastest device for the com-
puter (data rates mea-
sured in nanoseconds)

Terminology

RAM
Main memory
System memory
Core memory

Floppy drives

Nonvolatile (data is not lost
when the system is shutdown)
Slow devices for the computer
(data rates in milliseconds)

Disk

Drive

Hard drive

Floppy drive

DASD (archaic mainframe
reference)

16 Chapter 2

2.5 STORAGE

As already mentioned, the storage system is made up of a device or devices that
can permanently store data (it is “nonvolatile”). This is in contrast to the memory
system which is erased or empty when the computer is turned off or shut down.
In addition, the capacity or “size” of the data that can be stored on a storage de-
vice is usually much larger.

In terms of actual devices, the storage system is generally made up of one
or more disk drive. These drives are devices that have a spinning a disk within
them. Thus, these devices are often also referred to as disks or hard disks. These
devices may also be referred to as hard drives, disk drives, or C drives. These
terms basically all refer to the same thing. The spinning disk has magnetic mate-
rial on it that can be altered by recording or playback “heads” (similar to a mag-
netic tape). These heads are on a electronically controlled arm that can swing
over the spinning disk. The whole mechanism is in contained in a vacuum-sealed
enclosure that fits into the computer system enclosure. Another type of storage
device would be a floppy drive or a diskette drive. These work on the same prin-
ciple as the hard disk, but they are generally used for transporting smaller
amounts of data between computers.

Although the memory system is the only way that the CPU really commu-
nicates with the outside world, the storage system is the way that a user really
communicates with the computer. Users do not save what the CPU has done, un-
less it is written to the storage system. This is true for CAD programs, as well.
The user does need to be aware of what data is currently stored or saved on the
storage system, and the user must be aware of how the CAD program is going to
read and/or write new forms of that data. The way that the user generally recog-
nizes the data on the storage devices is through files.

2.5.1 Files

The files that computers work with via the storage system can be thought of as
electronic versions of paper files. A paper file is a document, a piece of paper
with information written on it, or a set of pages. On the paper file, there will be
letters, numbers, and graphics written on it. The computer’s files contain the
same thing, only in an electronic form. The electronic form really is just a se-
quence of ones and zeros, and something called the ASCII table dictates which
pattern of ones and zeros represent the letters of the alphabet or the numerals 0 to
9, etc. The precise meaning and behavior of computer files on a specific com-
puter system is controlled by the operating system (this is covered in more detail
in Chapter 3 on computer software).

The size of files refers to how many bytes (a pattern of 8 ones and zeros)
are stored on the storage device for the given file. A typical word processing doc-
ument could be 50,000 bytes in size. A typical CAD 2-D drawing file could have

Computer Hardware Basics 17

a size of 150,000 bytes. A typical 3-D model of a single part could use 100,000
bytes. A typical model of an assembly of a few hundred parts (such as Figure 2.3)
could have a file size of 50,000,000 bytes.

Since the size of these files can be difficult to express in terms of the actual
number of bytes, there are metric system “short cuts” for the sizes by the
“1000’s” (refer to Table 2.2). So, approximately 1,000 bytes is a kilobyte (in the
metric system, the prefix for one thousand is kilo). Approximately 1,000,000
bytes is a megabyte, and approximately 1,000,000,000 bytes is a gigabyte. These
various short cuts are abbreviated as KB, MB, and GB respectively. In each case,
the conversion is approximate. This is because the actual conversion is not based
on 1000s, but on 1024. 1024 is 210, and most of the operations of the computer
are based on the logic represented by 2 numbers (ones and zeros), so it is easier to
base capacities on the 1024 value. In practice, the capacity can just be assumed to
be based on 1000, not 1024, since it only introduces a small amount of error (Ta-
ble 2.2 shows the actual numbers).

2.5.2 Disk Drive Functioning

It is important to understand how disk drives work and behave since the files that
CAD programs generate can be very large, and performance will suffer if the
storage system is not utilized properly.

As mentioned earlier, the actual information on the storage system is made
up of extremely small regions of magnetism deposited or removed on a spinning
disk (referred to as the disk drive). These magnetic regions are located based on a
radial and an angular position (see Figure 2.4). In other words, each position can
be located by a radius (or track) from the center of the disk and an angular posi-
tion (or sector) within that circular radius. These locations are grouped and often
referred to as cylinders. When data needs to be stored or “written” on the disk
drive, the computer finds an “unused” region of the disk and the heads are posi-

TABLE
2.2

Large Byte Count “Shortcut” Terminology

Shortcut Approximate size (bytes) Actual size (bytes)

Kilobyte (thousands) or KB 1,000 1,024
Megabyte (millions) or MB 1,000,000 1,048,576
Gigabyte (billions) or GB 1,000,000,000 1,073,741,824
Terabyte (trillions) or TB 1,000,000,000,000 1,099,511,627,776
Petabyte (quadrillions) or
1,000,000,000,000,000 1,125,899,906,842,624
PB

Quitealotabytes (quintil-
1,000,000,000,000,000,000 1,152,921,504,606,846,976
lions) or QB?

18 Chapter 2

FIGURE
2.4

Disk drive anatomy.

tioned at the region. Then the heads produce a magnetic field at the exact right
time to alter the magnetic medium on the disk. This event corresponds to the data
in the file being created. When
data needs to be retrieved or “read” from the disk
drive, the heads are positioned to where the desired data is supposed to be lo-
cated, and the magnetism on the disk is sensed by the heads. This generates the
signals that are then translated back into the data. Believe it or not, this process
proceeds at the rate of millions of signals each second. Each second, the heads
are sent to the proper location on the disk within very tiny distances, the signals
are sensed as the disk spins by at perhaps 10,000 RPM, and then the heads move
on again. In that one second, all the text of this book could be read or written, and
there are disk drives that can go much faster than that.

A number of issues are raised by the disk drive’s behavior. First of all, it is
important to understand the idea behind “reading” and “writing.” It should be
clear that this would be closely linked to data security (i.e. setting up who is al-
lowed to do what with the computer and/or its data). If someone is able to read
data from the disk drive, then they would be allowed to see the data stored there.
And, if this person is able to write data to the disk drive, then they could create or
change data there. A person who is able to do both these operations is said to
have “Read/Write” or “R/W” access. On the other hand, if someone is only able
to read the data, but not create or alter it on the disk drive, then this person is said
to have “Read/Only” or “R/O” access. For CAD programs, this capability corre-
sponds to active or released files or drawings, versus ones that are preliminary or
under revision. For instance, if drawings are created and being used in manufac-