CODEWITHSUNDEEP
assembly
assem
assem

Reputation: 3221

How to interpret this assembly?

lea    (%edx,%eax,1),%eax

Anyone knows the usage of (%edx,%eax,1)?

Upvotes: -1

Views: 85

Answers (3)

querist
querist

Reputation: 654

The lea instruction (Load Effective Address) is used to calculate an address in the same manner as indirect addressing and to save the resulting address instead of reading what is in the address. In the case of AT&T syntax, (%edx, %eax, 1) means (%edx + %eax * 1). In this case it is, as Laurent G stated, the equivalent of add %edx, %eax. However, by using other factors (a displacement before the parenthesis and a number other than 1) you can do slightly more complicated math.

This type of addressing is typically used to handle arrays, but the lea instruction does not validate that the resulting address is valid, so you can use this as a short circuit way to do a calculation that would take multiple instructions.

Upvotes: 0

Laurent G
Laurent G

Reputation: 3184

(%edx,%eax,1) is an operand address corresponding to edx+eax*1

In other words, the instruction being lea, this is simply an add statement equivalent to eax += edx

Upvotes: 1

t.dubrownik
t.dubrownik

Reputation: 13432

Take a look at GAS address operand syntax, it seems to be what you're looking for.

Upvotes: 0

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