instructions provide the computer with the ability to
make decisions based on the results of previously
generated data.
Control instructions send commands to devices
not under direct control of the control unit, such as
input and output units. The address portion of the
control instruction does not specify a location in
memory, but is usually a coded group that specifies an
action be required of a particular piece of equipment.
In a single-address computer, where each
instruction refers to only one address or operand, the
instructions are normally taken from the memory in
sequential order.
If one instruction comes from a
certain location, such as X, the next instruction is
usually taken from location X + 1. However, the
execution of a logic instruction may produce a result
that dictates that the next instruction is to be taken
from an address as specified in a portion of the logic
instruction. For example, the logic instruction may
initiate certain operations in the computer to
determine if the content of a given register in the
arithmetic section is negative. If the answer is yes,
the location of the next instruction is specified in an
address section of the logic instruction. If the answer
is no, the next instruction would be taken from the
next sequential location in the memory.
Every computer provides circuitry for a variety of
logic instructions, thus providing the capability of
selecting alternate instruction sequences if certain
desirable or undesirable conditions exist. The ability
to branch at key points is the special feature of the
computer that makes it able to perform such diverse
tasks as missile control, accounting, and tactical air
plotting.
ARITHMETIC-LOGIC UNIT
The arithmetic-logic unit (ALU) is the section in
which arithmetic and logic operations are performed
on the input or stored data. The operations performed
in this unit include adding, subtracting, multiplying,
dividing, counting, shifting, complementing, and
comparing.
Generally, information delivered to the control
unit represents instructions, while information routed
to the arithmetic unit represents data. Frequently, it is
necessary to modify an instruction. This instruction
may have been used in one form in one step of the
program, but must be altered for a subsequent step. In
such cases, the instruction is delivered to the
arithmetic unit where it is altered by addition to or
subtraction from another number in the accumulator.
The resultant modified instruction is again stored in
the memory unit for use later in the program.
All arithmetic operations can be reduced to one of
four processes: addition, subtraction, multiplication,
or division. In most computers, multiplication
involves a series of additions, and division is a series
of subtractions.
The arithmetic unit contains several registers.
Each register is a unit that can store one “word” of
computer data. These registers generally include the
D, X, and Q register, and a unit called the accumulator
(A register).
The registers are so named for
identification purposes only. During an arithmetic
process, the D, X, and Q registers temporarily hold or
store the numbers being used in the operation. These
numbers are called the operands. The accumulator
stores the result of the operation. The control unit
instructs the arithmetic unit to perform the specified
arithmetic operation as requested by the program.
The control unit transfers the necessary information
into the D, X, and Q registers from memory and
controls the storage of the results in the accumulator
or in some specific location in memory.
The arithmetic unit also makes comparisons and
produces yes or no or GO/NO-GO outputs as a result.
The computer can be programmed so that ayes or GO
result causes the computer to perform the next step in
the program, while a no or NO-GO result causes the
computer to jump several steps in the program. A
computer can also be programmed so that a no result
at a certain point in the program will cause the
computer to stop and await instructions from a
keyboard or other input device.
INTERNAL DATA STORAGE UNIT
In some digital computers, the internal data
storage unit, or memory section, is constructed of
small, magnetic cores, each capable of representing
anon or off condition. An on condition is represented
by a 1 and an off condition is represented by a 0. A
system of these cores arranged in a matrix can store
any computer word that is represented in binary form.
All computers must contain facilities to store
computer words or instructions until these
instructions or words are needed in the performance
of the computer calculations.
Before the
stored-program type of computer can begin to operate
on its input data, it is first necessary to store, in
memory, a sequence of instructions and all numbers
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