LEA is the equivalent of & in C. It gives you the address of something.
Fun question: what does the last line of this do?
MOV BP,12 LEA AX,[BP] MOV BX,34 LEA AX,BX
Fun question: what does the last line of this do?
MOV BP,12 LEA AX,[BP] MOV BX,34 LEA AX,BX
Why is this written
lea eax, [rdi + rsi]
lea eax, rdi + rsi
?LEA would normally be used for things like calculating address of an array element, or doing pointer math.
[1] On a hardware level, the ModR/M encoding of most x86 instructions allows you to specify a register operand and either a memory or a register operand. The LEA instruction only allows a register and a memory operand to be specified; if you try to use a register and register operand, it is instead decoded as an illegal instruction.
Not quite unique: the now-deprecated Intel MPX instructions had similar semantics, e.g. BNDCU or BNDMK. BNDLDX/BNDSTX are even weirder as they don't compute the address as specified but treat the index part of the memory operand separately.
In `op reg1, reg2`, the two registers are encoded as 3 bits each the ModRM byte which follows the opcode. Obviously, we can't fit 3 registers in the ModRM byte because it's only 8-bits.
In `op reg1, [reg2 + reg3]`, reg1 is encoded in the ModRM byte. The 3 bits that were previously used for reg2 are instead `0b100`, which indicates a SIB byte follows the ModRM byte. The SIB (Scale-Index-Base) byte uses 3 bits each for reg2 and reg3 as the base and index registers.
In any other instruction, the SIB byte is used for addressing, so syntax of `lea` is consistent with the way it is encoded.
Encoding details of ModRM/SIB are in Volume2, Section 2.1.5 of the ISA manual: https://www.intel.com/content/www/us/en/developer/articles/t...