Question

Consider a processor with byte-addressable memory. Assume that all registers, including Program Counter (PC) and Program Status Word (PSW), are of size 2 bytes. A stack in the main memory is implemented from memory location $(0100{)}_{16}$ and it grows upward. The stack pointer (SP) points to the top element of the stack. The current value of SP is $(016E{)}_{16}$. The CALL instruction is of two words, the first word is the op-code and the second word is the starting address of the subroutine (one word = 2 bytes).

The CALL instruction is implemented as follows:

The content of PC just before the fetch of a CALL instruction is $(5FA0{)}_{16}$. After execution of the CALL instruction, the value of the stack pointer is

• A. $(016C{)}_{16}$
• B. $(0170{)}_{16}$
• C. $(0172{)}_{16}$
• D. $(016A{)}_{16}$

Answer 1

The correct option is C $(0172{)}_{16}$


Just before CALL instruction execution, SP contains 016E

While CALL execution:

(i) PC contents are pushed i.e., SP incremented by 2 $\Rightarrow$ SP = 0170

(ii) PSW contents are pushed i.e., SP incremented by 2 $\Rightarrow$ SP = 0172

$\therefore$ The value of stack pointers is $(0172{)}_{16}$