CODEWITHSUNDEEP
assemblyx86masmpointers
sharon.biren
sharon.biren

Reputation: 588

when should i use PTR with indirect operands ?

I'm little confused with the rules of using the "xxxx PTR" with indirect operand. Can anybody make it clear ? Thanks

Upvotes: 0

Views: 307

Answers (1)

Kerrek SB
Kerrek SB

Reputation: 477680

Perhaps you refer to this mnemonic syntax (e.g. objdump -Mintel):

add DWORD PTR [eax],0x5

The point here is that [eax] is just a location in memory, but it doesn't carry size information. So we don't know whether to add 5 to a byte, a word, a dword or a qword. It would be clear if we used the annotated opcode names (addb, addw, addl, addq), but this particular dialect of assember chooses instead to annotate the memory operand. In this case, it says, "treat eax is a pointer to a dword".

By contrast, register operands need no such annotation, since the register name implies its size:

add  al,0x5    ; addb
add  ax,0x5    ; addw
add eax,0x5    ; addl
add rax,0x5    ; addq

Upvotes: 1

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