Declaring masks for bitwise operations

Question

I'm new to low level operations like this, I'm hoping someone can point out the obvious mistake I must be making here.

//Input value - 00111100
//I want to get the value of the bits at indexes 1-3 i.e 0111.

byte mask = (byte)0x00001111; // This gives 17 not the 15 I'd expect 

byte shifted = (byte)(headerByte >> 3);
//shifted is 7 as expected

byte frameSizeValue = (byte)(shifted & mask); //Gives 1 not 7

It looks like the problem lies with the way the mask is defined, but I can't see how to fix it.

Answer 1

First of all 0x00001111 is in hex, which is a larger number than 255 - 16^3 + 16^2 + 16 + 1 = 4369 and byte overflows. Look here how to represent binary numbers or just use shifted & 15.