Why use hex?

Question

Hey! I was looking at this code at http://www.gnu.org/software/m68hc11/examples/primes_8c-source.html

I noticed that in some situations they used hex numbers, like i

Answer 1

The single biggest use of hex is probably in embedded programming. Hex numbers are used to mask off individual bits in hardware registers, or split multiple numeric values packed into a single 8, 16, or 32-bit register.

When specifying individual bit masks, a lot of people start out by:

#define bit_0 1
#define bit_1 2
#define bit_2 4
#define bit_3 8
#define bit_4 16
etc...

After a while, they advance to:

#define bit_0 0x01
#define bit_1 0x02
#define bit_2 0x04
#define bit_3 0x08
#define bit_4 0x10
etc...

Then they learn to cheat, and let the compiler generate the values as part of compile time optimization:

#define bit_0 (1<<0)
#define bit_1 (1<<1)
#define bit_2 (1<<2)
#define bit_3 (1<<3)
#define bit_4 (1<<4)
etc...