No, the preprocessor does not do it!

Example:

#define CLOCK_RELOAD_H (CLOCK_RELOAD / 256)
#define CLOCK_RELOAD_L (CLOCK_RELOAD % 256)

TH = CLOCK_RELOAD_H;
TL = CLOCK_RELOAD_L;

after preprocessing becomes:

TH = (CLOCK_RELOAD / 256);
TL = (CLOCK_RELOAD % 256);

the preprocessor does text substition ; it does not evaluate the expressions

However, the Compiler may evaluate the expressions at compile-time, rather than generate code to do it at run-time.