Comment by sparkie

It's due to the way the instruction is encoded. `lea` would've needed special treatment in syntax to remove the brackets.

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...