Question

Assume that EA = (X) + is the effective address equal to the contents of location X, with X incremented by one word length after the effective address is calculated; EA = -(X) is the effective address equal to the contents of location X, with X decremented by one word length before the effective address is calculated; EA = (X) - is the effective address equal to the contents of location X, with X decremented by one word length after the effective address is calculated. The format of the instruction is (opcode, source, destination), which means (destination $\leftarrow$ source opdestination). Using X as a stack pointer, which of the following instructions can pop the top two elements from the stack, perform the addition operation and push the result back to the stack.

• A. ADD -(X), (X)+
• B. ADD (X), (X)-
• C. ADD -(X), (X)
• D. ADD (X)-, (X)

Answer 1

The correct option is D ADD (X)-, (X)

EA = (X) + works as post increment

EA = -(X) works as pre-decrement

EA = (X) - works as post decrement

To add top 2 elements:

1. Pop (Top) $\Rightarrow$ Pop (X) $\Rightarrow$ a


2. Then decrement X i.e., X = X - 1


3. Pop (Top) = Pop (X) $\Rightarrow$ b



Add a + b and push at address of X.

Which is implemented by ADD (X)-, X.