Page 251 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

228 Chapter 8 Programming in C and C++

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Figure 83. Loop Statements

contents of the word pointed to by a, which is the contents of b. (Note that a*b is a
times b but *b is the contents of the word pointed to by b.) Whenever you see &,
read it as "address of," and whenever you see *, read it as "contents of thing pointed to
by." In a declaration statement, the statement char *p; means that the thing pointed to
by p is a character, and p points to (contains the address of) a character. In an
assignment statement, *p = 1; means that 1 is assigned to the value of the thing
pointed to by p, whereas p*l; means that the pointer p is given the value 1.
Similarly, a=*p; means that a is given the value of the thing pointed to by p, while
a^p; means a gets the value of the pointer p. C compilers can give an error message
when you assign an integer to a pointer. If that occurs, you have to use a cast. Write p
~ (int *) 0x4000; to tell the compiler 0x4000 is really a pointer value to an integer
and not an integer itself.
Constants can be defined by define or enum statements, put before any declarations
or statements, to equate names to values. The define statement begins with the
characters #def ine and does not end with a semicolon.
#define ALPHA 100
Thenceforth, we can use the label ALPHA throughout the program, and 100 will
effectively be put in place of ALPHA just before the program is actually compiled. This
permits the program to be better documented, using meaningful labels, and easier to
maintain, so that if the value of a label is changed, it is changed everywhere it occurs.