xor'ing a value with itself produces 0, so XOR AX,AX is equivalent to "AX = 0". It's a common trick in assembly code because it's faster (and/or maybe more comp

xor'ing a value with itself produces 0, so XOR AX,AX is equivalent to "AX = 0". It's a common trick in assembly code because it's faster (and/or maybe more compact, can't remember) than the more obvious MOV AX,$0.