hex decimal start with 0x, && oxff, hex decimal numbering

 hexi decmial start with 0x

https://stackoverflow.com/questions/2670639/why-are-hexadecimal-numbers-prefixed-with-0x

hort story: The 0 tells the parser it's dealing with a constant (and not an identifier/reserved word). Something is still needed to specify the number base: the x is an arbitrary choice.

0h or 0x (H e X idecimal

& oxff

https://stackoverflow.com/questions/14713102/what-does-and-0xff-do

ff = 255 = 1111 1111 = 8 bits

& oxff means get last 8 bits:

   0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
 &  0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
    -------------------------------
    0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1

hex decimial numbering:

https://byjus.com/maths/hexadecimal-number-system/

Decimal Numbers
4-bit Binary Number
Hexadecimal Number
0
0000
0
1
0001
1
2
0010
2
3
0011
3
4
0100
4
5
0101
5
6
0110
6
7
0111
7
8
1000
8
9
1001
9
10
1010
A
11
1011
B
12
1100
C
13
1101
D
14
1110
E
15
1111
F