CODEWITHSUNDEEP
assemblymasmaddressing-mode
geo10
geo10

Reputation: 407

difference between memory at and ptr in assembly

Back at it with another assembly question. I am having a hard time seeing what is the difference between square brackets and ptr in assembly.

mov dword ptr eax, 1234 ; this should write 1234 to the memory address stored at eax right?
mov dword ptr [eax], 1234; this writes to the memory address that is stored at the memory address that is stored in eax?
mov [eax], 1234 ; this does the same as the first one right? it writes 1234 to the memory address that is stored in eax?

Can anyone please shed some light on this topic?

Upvotes: 0

Views: 327

Answers (1)

Michael
Michael

Reputation: 58467

dword PTR is just a size specifier for what follows. Since the size of eax is known, the dword PTR is redundant in the first case.

That is, mov dword ptr eax, 1234 is the same as just writing mov eax, 1234.

mov dword ptr [eax], 1234 means write 1234 to the doubleword in memory at the address given by eax.

mov [eax], 1234 is ambigous and shouldn't even assemble. The assembler has no way of knowing if you intended to store a word or a doubleword.

Upvotes: 4

Related Questions