GATE CS 2015 Set 2 — Question 54
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Computer Organization & Architecture → Instructions & Addressing Modes → Subroutine Call & Return
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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 and it grows upward. The stack pointer (SP) points to the top element of the stack. The current value of SP is . 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:
- Store the current value of PC in the stack
- Store the value of PSW register in the stack
- Load the starting address of the subroutine in PC
Correct answer
(D) (0172)₁₆
Solution
1.Analyze the Stack Properties:
- The memory is byte-addressable.
- The stack grows upward (towards higher memory addresses).
- The Stack Pointer (SP) points to the top element of the stack.
- Current SP = .
- The instruction pushes the Program Counter (PC) and the Program Status Word (PSW) onto the stack.
- Size of PC = 2 bytes.
- Size of PSW = 2 bytes.
- Total data to be pushed = bytes.
- Since the stack grows upward, pushing data increases the SP value.
- New SP = Old SP + Total bytes pushed.
- Old SP = .
- Bytes pushed = .
- Perform Hexadecimal addition:
- (which is , so carry 1, remainder 2).
- .
Therefore, the new value of the stack pointer is .
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