All computers share the same
basic architecture, whether it be a multi-million dollar
mainframe or a Palm Pilot. All have memory, an I/O system,
and arithmetic/logic unit, and a control unit. This type of
architecture is named Von Neumann architecture after the
mathematician who conceived of the design.
Memory
Computer Memory is that
subsystem that serves as temporary storage for all program
instructions and data that are being executed by the
computer. It is typically called RAM. Memory is divided up
into cells, each cell having a unique address so that the
data can be fetched.
Input / Output
This is the subsystem that
allows the computer to interact with other devices and
communicate to the outside world. It also is responsible for
program storage, such as hard drive control. Again, I
discuss this in other sections of the site in more detail,
so I will not address it again here.
Arithmetic/Logic Unit
This is that subsystem that
performs all arithmetic operations and comparisons for
equality. In the Von Neumann design, this and the Control
Unit are separate components, but in modern systems they are
integrated into the processor. The ALU has 3 sections, the
register, the ALU circuitry, and the pathways in between.
The register is basically a storage cell that works like RAM
and holds the results of the calculations. It is mush faster
than RAM and is addresses differently. The ALU circuitry is
that actually performs the calculations. and it is designed
from AND, OR, and NOT gates just as any chip. The pathways
in between are self-explanatory – pathways for electrical
current within the ALU.
Control Unit
The control unit has the
responsibility of (1) fetching from memory the next program
instruction to be run, (2) decode it to determine what needs
to be done, then (3) issue the proper command to the ALU,
memory and I/O controllers to get the job done. These steps
are done continuously until the last line of a program is
done, which is usually QUIT or STOP.
At the machine level, the
instructions executed by the computer are expressed in machine
language. Machine Language is in binary code and is
organized by op code and address fields. Op codes are
special binary codes that tell the computer what operations
to carry out. The address fields are locations in memory on
which that particular op code will act. All machine language
instructions are organized with the op code first, then the
memory addresses following. Let me give an example: Let’s
assume we want to add two number together that are in memory
locations 99 and 100. Let’s assume the decimal 9 is the op
code for the ADD function. The format, then, for the command
would be 9-99-100. Of course, this is in decimal form and
not the way the computer sees it. Convert these to binary to
get:
0000100100000000011000110000000001100100
That’s a 9, a 99,
and a 100 put together with no dashes. Now, you get an idea of
just how a computer thinks.
The set of all
operations a processor can do is called its instruction set.
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