We now need a way
of storing information in the computer's memory so that the results of
expressions can be saved for later use. Earlier in the chapter we said that
memory can be thought of as if it were a large number of boxes, each of
which is capable of storing some information. To use them, we place
a particular piece of
information in a named location (box),
and then refer by name to the the location that contains the
information. A named location is called a
variable.
POP-11 carries out the work of allocating and
accessing particular
locations automatically.
Suppose that we want to store a list of words in memory. We
give that part of memory a name, say myinfo,
by means of the POP-11 instruction
vars myinfo;
It is preceded by vars because myinfo is the name of a
variable.
Any POP-11 word can be the name of a variable, and the
vars instruction
declares
the variable by entering the word in its list of variable names and by
allocating memory for it.
Once we have given the vars command, we can store a
data item in the memory set aside, and refer
to it again by the name myinfo.
We can have as many variables as we like in a POP-11 program and
we can declare more than one in a vars
command; they should be separated by commas.