Difference between MOV and MOV ptr

Question

Maybe it’s not the ideal place to ask this, but I’m having a test and I don’t understand the difference between MOV and MOV ptr. for example, if we have in c language "unsigned char x,y", in Assembly if we want "X=2" we use MOV x,2

but if we have "unsigned char tabbyte[4]" and we want "tabbyte[0]=15" we already use
MOV byte ptr tabbyte [0],15.

Can anyone explain the difference to me, and when we should simply use MOV and when we should use MOV ptr? Thank you

Answer 1

It seems you’ve seen this here.

The explanation is what is there on the link.

In an instruction of the kind

 mov [ESI], al ; Store a byte-size value

the Assembler (masm, tasm, etc.) knows which machine instruction to use, because the origin is the AL register that has 8 bits (1 byte).

In an instruction of the kind

mov [ESI], 5   ; Error: operand must have the size specified

the Assembler does not know which machine instruction to use, because "5" can be a byte with value 5, or a word with value 5, etc.

Then you need to give the Assembler more information so it can generate the right machine instruction:

; move o valor 0x05 (8 bits) para o byte cujo endereço está em ESI
mov  BYTE PTR [ESI], 5

; move o valor 0x0005 (16 bits) para a word cujo endereço está em ESI
mov  WORD PTR [ESI], 5

; move o valor 0x00000005 (32 bits) para a double-word cujo endereço está em ESI
mov  DWORD PTR [ESI], 5

Note that although the Assembly instruction is the same in these 3 cases (MOV) actually the machine code is different, so the Assembler program (masm, tasm, etc) needs this additional information: byte ptr, word ptr, dword ptr.

PS. in my previous answer on this subject I did not worry much about it, by guarantee I used byte ptr, word ptr, maybe even in places where it was not strictly necessary.