Bit shifting not shifting correctly?

Question

I am programming the 8051 in C using the Si Labs IDE. I currently have three bytes: address_byte3, address_byte2, and address_byte1. I then initialized a variable address_sum to be an unsigned long int then did the following operation on it...

address_sum=(address_byte3<<16)+(address_byte2<<8)+(address_byte1);

This operation would lead me to believe that the value loaded into address_sum if address_byte3, address_byte2, & address_byte1 were 0x92, 0x56, & 0x78, respectively, would be 0xXX925678. Instead I am getting a value of 0xXX005678. My logic seems sound but then again I am the one writing the code so I'm biased and could be blinded by my own ignorance. Does anyone have a solution or an explanation as to why the value for address_byte is "lost"?

Thank you.

Answer 1

Variables shorter than int are promoted to int when doing calculations on them. It seems that your int type is 16-bit, so shifting it by 16 bits doesn't work right.

You should explicitly cast the variables to the result type (unsigned long):

address_sum = ((unsigned long)address_byte3<<16) +
              ((unsigned long)address_byte2<<8) +
              (unsigned long)address_byte1;

The last casting is superfluous but doesn't hurt.