Glossary
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data area An area in the PC’s memory that is designed to hold a specific type of data,
e.g., the SR area is designed to hold flags and control bits. Memory areas
that hold programs are not considered data areas.
data area boundary The highest address available in a data area. When designating an operand
that requires multiple words, it is necessary that the highest address in the
data area is not exceeded.
debug A process by which a draft program is corrected until it operates as intended.
Debugging includes both removal of syntax errors as well as fine-tuning of
timing and coordination of control operations.
decimal A number system where all numbers are expressed to the base 10. Although
in a PC all data is ultimately stored in binary form, four binary bits are often
used to represent one decimal digit, a system called binary-coded decimal.
decrement Decreasing a numeric value by 1.
default A value assumed and automatically set by the PC when a specific value is
not input by the user.
definer A number used as an operand for an instruction but that serves to define the
instruction itself rather that the data on which the instruction is to operate.
Definers include jump numbers, subroutine numbers, etc.
delay In tracing, a value that specifies where tracing to begin in relationship to the
trigger. A delay can be either positive or negative, i.e., can designate an off-
set on either side of the trigger.
destination The location where data of some sort in an instruction is to be placed as op-
posed to the location from which data is to be taken for use in the instruction.
The location from which data is to be taken is called the source.
differentiation instruction An instruction used to ensure that the operand bit is never turned ON for
more than one scan after the execution condition goes either from OFF to
ON for a Differentiate Up instruction or from ON to OFF for a Differentiate
Down instruction.
digit A unit of storage in memory that consists of four bits.
digit designator An operand that is used to designate the digit or digits of a word to be used
by an instruction.
distributed control An automation concept in which control of each portion of an automated sys-
tem is located near the devices actually being controlled, i.e., control is de-
centralized and ‘distributed’ over the system. Distributed control is a concept
basic to PC Systems.
DM area A data area used to hold word data. A word in the DM area cannot be ac-
cessed by bit.
download The process of transferring a program or data from a higher-level computer
to a lower-level computer or PC.
electrical noise Electrical ‘static’ that can disturb electronic communications. The ‘snow’ that
can appear on a TV screen is an example of the effects of electrical noise.
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