C中的位屏蔽和分离

Question

I am new to c programming and i need help in bit manipulation. I would like to separate the number from a register which have encoded numbers in BCD. for example;

the register got '29' as value two bits will denote 2 ='10' and four bits will denote 9='1001'. It is an 8 bit register and rest bits are zero. So shifting out the 4 bits will give me 2 at disposal.But what about getting the unit digit? I need some help regarding that I'm posting the code here:

  #include<stdio.h>
    main()
 {
     int x,y;
     y=0x29;
      x=y;
     x=x>>4;
    x=x*10;
    printf("%d",x);
    return(0);

 }

Answer 1

You need to mask it out with binary 00001111 , which is decimal 15 or hexadecimal 0x0f .

uint8_t reg = 41; // binary 00101001
uint8_t lo_nibble = (reg >> 0) & 0x0f;
uint8_t hi_nibble = (reg >> 4) & 0x0f;

Answer 2

To form a mask to capture the bottom n bits of a number, you can perform these steps (pen and paper at first, eventually in your head):

• start with the value 1.
  (1) // == 1 or 00000001

• shift the value 1 up by n bits.
  (1<<4) // == 16 or 00010000

• subtract 1.
  (1<<4)-1 // == 15 or 00001111

ANDing this mask with another value or variable will yield the bottom n bits of the number.

int in, hi, lo;

lo = in & ((1<<4)-1);
hi = (in>>4) & ((1<<4)-1);