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by rep_lodsb
1237 days ago
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The addressing logic on the 80386+ can add an index shifted left by 0..3 bits (unscaled, x2, x4, x8) with a base register plus an immediate offset. By using the same register for base and index you can also multiply one register by 3, 5 or 9. Earlier (16 bit) x86 chips did not have the scaling feature and were limited to certain combinations of base and index (BX/BP as base, SI/DI as index), so LEA was less useful. If the registers are carefully assigned, it could still be used to do an addition and put the result into another register. Normal ALU operations always use one of the operands as their destination. |
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