x86, difference between BYTE and BYTE PTR

Question

What is the difference between these two lines? What PTR changes here?

;first
mov BYTE [ecx], 0  
;second
mov BYTE PTR [ecx], 0

Answer 1

You are using a permissive assembler, it seems, my C compiler's support for in-line assembly sure isn't happy with it. The proper syntax is BYTE PTR to tell the assembler that the value in the ECX register should be treated like a pointer. PTR. But that's syntax that's over specified, it could already tell that you meant to use it as a pointer by you putting [brackets] around the register name. Using [ecx] already made it clear that you meant to store zero into the address provided by the ECX register.

So it knows how to use the ECX register, the only other thing it doesn't know is how many bytes need to be set to zero. Choices are 1, 2 or 4. You made it clear, 1. BYTE.