What does least significant byte mean?

Question

I have to implement a method that writes a byte to an ostream object. Let's just called this ostream object strobj. I also have a bit buffer used to store a byte of data, let's call it:

char &bitter;

In my header file, I have this:

void writeThisByte(int sumInt);

The instructions says that I have to write the "less significant byte" of the int being passed in to an ostream object, i.e. strobj;

However, I am confused on exactly what the least significant byte means.

Does this mean you are checking to see whether sumInt == 1? And if it is, do you write it to the ostream like this?

strobj.write(&bitter, 1);

I'm not sure :(

Answer 1

Imagine a 32-bit integer, where each bit can be 0 or 1, say:

011101010101010101010101010110110
                         ^^^^^^^^

The least significant byte is the 8-bits at the right-hand side.

It's probably easier to understand if you take a decimal example: given the number 291023, the '3' is the least-significant digit, because if you change it you have the least effect on the overall number.

To get the least significant byte, just bitwise-AND the larger int with 0xFF hex or 255 decimal (1+2+4+8+16+32+64+128=255)- that will clear out all the more-signficant bits and preserve the 8 least significant bits...

int x = ...whatever...;
unsigned char least_significant_bits = x & 255;

The effect is like this:

011101010101010101010101010110110 // x
000000000000000000000000011111111  //255
result:
000000000000000000000000010110110 // x & 255