l Up counter
l Down counter
l Updown counter
REGISTERS.— Registers are built simply by
combining groups of flip-flops to act as a unit. The
length of a register corresponds to the number of bits or
flip-flops within this grouping. Three aspects of
registers must be considered. A register must be able
to:
Receive information from one or more sources
Preserve the information without alteration until
it is needed
Deliver the information to one or more
destinations when it is required (command or
enable)
Registers can represent one bit or more than one bit.
Multiple bits can be represented in various sets such as
4, 8, 16, 32, and 64; the maximum is usually the
computer’s word size.
Registers take on different
names depending on their functional use in the
computer. They are used throughout the computer.
You will learn about some of the specific functional
names when you study the functional areas of the
computer: central processing unit (CPU), memory, and
input/output (I/O).
A register has two parts: the control and the actual
flip-flops. The control (enable) portion contains the
logic gates (AND, NAND, and the like) and any input
signals or control functions that are common to all the
flip-flops in the register.
Some registers can be accessed by programmers
and/or directly monitored and accessed on some
computers by the front panel. The front panel will
either have a display of numbers (some converted for
an octal, decimal, or hexadecimal display by LEDs).
Other front panels simply display the numbers in binary.
These binary numbers can be represented in
Figure 4-23.—Example of a register.
4-16
