NASM为注册错误添加值

I am trying to add several pixels together in order to do a blurr filter in NASM. I've managed to add three pixels with the value of 00 + d3 + d8 (0 + 211 + 216). When I try to add one more pixel, with the value of 0, the program cannot print the value of the variable I am trying to add several pixels together in o