Sorry, I'm actively editing my answer as I get a better sense of your questions and I can answer more of them. Please see my edits; also, the specific lines you pointed out are not related to reading in argv[1]. They're just zeroing out eax and ecx; eax because we will only be writing to the lower bits with the lodsb and don't want out arithmetic to be fouled by anything left over in the higher bits, and ecx because we are continuously adding to it to in atoi, so it needs to start off at zero.