Question

LEA CX,[BX][SI] instruction of 8086 microprocessor indicates \{where (BX) and (SI) represent the content of BX and SI respectively\}

• A. Load CX with the value equal to (BX)+(SI)
• B. Load CX with the value equal to (BX)-(SI)
• C. Less the content of CX by the (SI)
• D. Less the content of CX by the (BX)

Hint:

`LEA destination, source_memory_operand` calculates the offset address of `source_memory_operand` and stores it in `destination`.
It does *not* access the memory content at that address.
Addressing mode `[BX][SI]` (or `[BX+SI]`) means Effective Address = Content of BX + Content of SI.
Contrast with `MOV CX, [BX][SI]`, which would load CX with the *content* of the memory location whose address is (BX)+(SI).

Answer 1

The Correct Option is A

The `LEA` (Load Effective Address) instruction in 8086 microprocessor calculates the effective address of the source operand (which is a memory location specified by its addressing mode) and loads this calculated address (offset) into the destination register. It does not load the content of the memory location, but rather the address itself. The source operand is `[BX][SI]`. This is a based-indexed addressing mode. The effective address (EA) calculated for `[BX][SI]` is the sum of the contents of the BX register and the SI register: EA = (BX) + (SI). The instruction `LEA CX, [BX][SI]` will therefore: Calculate the effective address EA = (BX) + (SI). Load this calculated effective address EA into the CX register. So, CX will contain the value (BX) + (SI). This matches option (a). Option (c) and (d) "Less the content" are incorrect; LEA loads, it doesn't subtract or compare in this context. \[ \boxed{\text{Load CX with the value equal to (BX)+(SI)}} \]