Question about lea instruction, IA32

If I have an IA32 instruction like开发者_运维百科 the following:

lea 0x4(x1, x2, x3), %eax

What do x1, x2, and x3 represent? Thanks.

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If you mean "what is lea for," follow the link in Greg's comment.

If your question is about the syntax and are familiar with Intel syntax (but not AT&T syntax), x1 is the base, x2 is the index, and x3 is the scale (with 0x4 being the displacement). Thus,

lea 0x4(x1,x2,x3),%eax

is equivalent to

lea eax,[x1+x3*x2+4]

See this article on SourceForge for more.

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They are a base register, an index register, and a scale value. So, it's useful for calculating the address of an element of an array base of elements of size scale at position index, i.e. in C:

base[index]

where scale will be 1 for a char array, 2 for a short array, ... on typical C compilers for x86.