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I'm specifically talking about the intel x86 processor. For the MOV function I am very confused on when I should use byte ptr when working with immediate values. For example, mov esi, 5000 ... Would I have to include ptr in this case??? I know for this operation particularly you have to: mov byte ptr (esi), 15 .... Other examples online people say an operation like mov ax, 7 is valid. I am confused when to include the ptr when handling immediate values with mov functions as some people are writing mov functions with immediate value without using ptr like I stated above. Can someone please explain?? Thank you. BTW this is regarding intel x86.

Safoorah Siddiqui
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  • `byte ptr` is for memory operands, not immediates. – fuz Feb 14 '21 at 11:49
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    You only ever use `ptr` to indicate a memory operand, e.g. `mov mem, imm` such as `mov dword ptr [esi], 5000` to store `5000` to memory. And then the ptr is because of the memory destination, not the immediate source. – Peter Cordes Feb 14 '21 at 11:50
  • @PeterCordes thank you so much!! So to clarify, anytime you're using a memory operand that has an immediate source that is when you have to use such things like ptr/dword/etc ? – Safoorah Siddiqui Feb 14 '21 at 12:09
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    Yup, otherwise the operand-size is ambiguous (with neither operand being a register to imply it). Or in MASM to override the default size of an array, e.g. to load 4 bytes from an array like `foo db 1, 2, 3, 4, 5, 6` into EAX you might need `mov eax, dword ptr [foo+1]`, because MASM tries to be helpful and would actually complain about size mismatch if you didn't override. – Peter Cordes Feb 14 '21 at 12:24

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