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2.7.1 Integer Promotions

ISO mandates that all arithmetic be performed at int precision or greater. By default, MPLAB C18 will perform arithmetic at the size of the largest operand, even if both operands are smaller than an int.
The ISO mandated behavior can be instated via the -Oi command-line option.
For example:
unsigned char a, b;
unsigned i;
a = b = 0x80;
i = a + b; /* ISO requires that i == 0x100, but in C18 i == 0 */
Note that this divergence also applies to constant literals. The chosen type for constant literals is the first one from the appropriate group that can represent the value of the constant without overflow. 
For example:
#define A 0x10 /* A will be considered a char unless -Oi specified */
#define B 0x10 /* B will be considered a char unless -Oi specified */
#define C (A) * (B)
unsigned i;
i = C; /* ISO requires that i == 0x100, but in C18 i == 0 */