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Forth also allows you to define your own
constant and variables using the words
CONSTANT
VARIABLE
When a constant is called up, this
causes its value to be pushed onto the stack. However when a variable is called
up this causes its address to be pushed onto the stack. The Forth words
!
and @
are used to modify the
contents of the variable.
A constant is defined by using the form:
value CONSTANT name
and any reference the the name will
cause the value to be put on the stack.
A variable is defined by using the form:
value VARIABLE
name
and any reference to the name will
result in the address of that variable to be put on the stack for further
manipulation using !
and @. It is essential
that you realize the difference between the constant and the address of a
variable. Now for some examples:
64 CONSTANT R <CR>
1000 CONSTANT Q <CR>
256 VARIABLE X <CR>
0 VARIABLE Y <CR>
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R Q + .
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will print the values of R
+ Qi.e. 1064
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X .
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will print the address of X
not its value
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X @ .
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will print the value of X
i.e. 256
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R Y !
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will store the value of R
in the variable Y
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Y X !
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will store the address of Y
in the variable X
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4 X !
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will store the value 4 in the
variable X
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BASIC command
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Forth equivalent
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LET X = Y
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Y @ X !
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LET X = R
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R X !
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LET X = 4
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4 X !
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LET X = X + 5
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5 X +!
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