In order for computers to manipulate data, they must be
able to store and retrieve it on demand. This requires a way
to specify the location and extent of a data item in memory.
These locations are represented by sequential numbers, or
addresses.
Physically, a modern RAM (random access memory)
can be visualized as a grid of address lines that crisscross
with data lines. Each line carries one bit of the address,
and together, they specify a particular location in memory
(see memory). Thus a machine with 32 address lines can
handle up to 32 bits, or 4 gigabytes (billions of bytes) worth
of addresses. However the amount of memory that can be
addressed can be extended through indirect addressing,
where the data stored at an address is itself the address of
another location where the actual data can be found. This
allows a limited amount of fast memory to be used to point
to data stored in auxiliary memory or mass storage thus
extending addressing to the space on a hard disk drive.
Some of the data stored in memory contains the actual
program instructions to be executed. As the processor
executes program instructions, an instruction pointer
accesses the location of the next instruction. An instruc-
tion can also specify that if a certain condition is met the
processor will jump over intervening locations to fetch
the next instruction. This implements such control struc-
tures as branching statements and loops.
Addressing in Programs
A variable name in a program language actually references
an address (or often, a range of successive addresses, since
most data items require more than one byte of storage). For
example, if a program includes the declaration
Int Old_Total, New_Total;
when the program is compiled, storage for the variables
Old_Total and New_Total is set aside at the next available
addresses. A statement such as
New_Total = 0;
is compiled as an instruction to store the value 0 in the
address represented by New_Total. When the program later
performs a calculation such as:
New_Total = Old_Total + 1;
the data is retrieved from the memory location designated
by Old_Total and stored in a register in the CPU, where 1 is
added to it, and the result is stored in the memory location
designated by New_Total.
Although programmers don’t have to work directly with
address locations, programs can also use a special type of
variable to hold and manipulate memory addresses for more
efficient access to data.
Further Reading
“Computer Architecture Tutorial.” Available online. URL: http://
www.cs.iastate.edu/~prabhu/Tutorial/title.html. Accessed April
10, 2007.
Murdocca, Miles J., and Vincent P. Heuring. Principles of Computer
Architecture. Upper Saddle River, N.J.: Prentice Hall, 2000.
Adobe Systems
Adobe Systems (NASDAQ symbol ADBE) is best known for
products relating to the formatting, printing, and display of
documents. Founded in 1982 by John Warnock and Charles
Geschke, the company is named for a creek near one of their
homes.
Adobe’s first major product was a language that describes
the font sizes, styles, and other formatting needed to print
pages in near-typeset quality.this was a
significant contribution to the development of software for
document creation , particularly on
the Apple Macintosh, starting in the later 1980s. Building on
this foundation, Adobe developed high-quality digital fonts
(called Type 1). However, Apple’s TrueType fonts proved to
be superior in scaling to different sizes and in the precise
control over the pixels used to display them. With the licens-
ing of TrueType to Microsoft for use in Windows, TrueType
fonts took over the desktop, although Adobe Type 1 remained
popular in commercial typesetting applications. Finally, in
the late 1990s Adobe, together with Microsoft, established a
new font format called OpenType, and by 2003 Adobe had
converted all of its Type 1 fonts to the new format.
Adobe’s Portable Document Format has become
a ubiquitous standard for displaying print documents. Adobe
greatly contributed to this development by making a free
Adobe Acrobat PDF reader available for download.
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