hex

a hex is half a byte, 4 bits, 16 values 0...15 or 0... 9 A B C D E F (F=15)

int60 is a smart pointer 0xB0000000.00000000 in which the first hex 0xB is used to denote the type and 15 of the 16 hexes are used for the numeric value (15*4 bit = 60 bit bignum)