CODEWITHSUNDEEP
assemblyx86
Artur Pyśk
Artur Pyśk

Reputation: 51

Assembly byte ptr meaning

In ecx I have some string, like "abc".

mov ah, BYTE PTR [ecx+0]
mov al, BYTE PTR [ecx+1]

What does it exactly do? It's like in ah I have "a" and in al I have "b"?

Upvotes: 1

Views: 34799

Answers (2)

Zahid Khan
Zahid Khan

Reputation: 3233

There are times when we need to assist assembler in translating references to data in memory.

byte ptr -> it simply means that you want to fetch a byte from the address. if it said word ptr or dword ptr, you would get a word or dword from the address in source index.

When you need something like byte ptr example you move an immediate value to an indirect address:

mov ebx, OFFSET some_symbol    ; or a pointer from another register
mov [ebx], 10

This won't normally be allowed -- the assembler doesn't know whether you want the 10 as a byte, a word, a double-word, or (in 64-bit code) a quad-word. You need to make it explicit with a size specification:

mov byte ptr [ebx], 10  ; write 10 into a byte
mov word ptr [ebx], 10  ; write 10 into a word
mov dword ptr [ebx], 10 ; write 10 into a dword

Upvotes: 4

fuz
fuz

Reputation: 93014

byte ptr indicates that the memory operand refers to a byte in memory as opposed to a word or dword. Usually, this can be omitted as the assembler can infer the operand size from registers used but there are some instructions like mov [eax],0 where the size cannot be inferred, so a byte ptr, word ptr or dword ptr prefix is needed.

Upvotes: 5

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