indirect word addressed by the instruction. In
computers with an 8-bit word, no special instruction is
needed because each character has its own address.
Double-length instructions consist of two adjacent
words stored in memory.
Multiple-word instructions can be used to process
two or more sequential words from memory. This
concept is commonly used in microcomputers where
the instruction word is 16 bits and the memory word
size is 8 bits (a byte). In this case two or more sequential
bytes from memory are transferred into two or more
16-bit registers for processing; or multiple word store
instructions are used to process 16-bit registers into
sequential bytes in memory (two bytes for each
register). Refer back to figure 8-4 for an example of a
multiple-word instruction format.
To use or maintain a computer, you must be able to
control the computers operation through some form of
a man/machine interface. The man/machine interface
is accomplished by the CPU and will vary with the type
of computer. However, there are no major differences
in the functions performed by the interfaces. You
studied the controlling devices in chapter 3. The
controlling devices allow you to interface with the
computer. The methods are discussed in this topic.
The controlling devices used by operator and
maintenance personnel vary with different types and
generations of computers. In some cases the particular
devices used are the same for both general system
operation and the more specific maintenance functions.
In many cases the man/machine interfaces have evolved
from large panels containing many pushbutton/
indicators, and pushbutton/toggle switches, and
switches (fig. 8-16) on a maintenance panel to more
Figure 8-16.Pushbutton/indicators, push button/toggle switches, and switches on a maintenance panel.