In an Intel 8085 microprocessor system, what is the purpose of the stack pointer register and what does it contain at any given time?

Introduction / Context:
This question focuses on the role of the stack pointer in the Intel 8085 microprocessor. The stack pointer is a special purpose register that works closely with the stack, a last in first out memory area used for function calls, interrupts, and temporary storage. Understanding what the stack pointer holds and how it changes during PUSH, POP, CALL, and RET operations is essential for debugging assembly programs and for reasoning about nested procedure calls.

Given Data / Assumptions:

We are discussing the stack pointer in the context of the 8085 microprocessor.

The stack resides in read or write memory and grows downward towards lower addresses.

The stack pointer SP is a 16 bit register used to address this memory area.

Stack operations such as PUSH and CALL store data and return addresses on the stack.

Stack pointer should always reflect the location of the current top of the stack.

Concept / Approach:
In the 8085, the stack pointer SP holds the address of the top of the stack, which is the memory location containing the most recently pushed data. When the CPU executes a PUSH instruction, it first decrements SP and then writes data to the memory location pointed to by SP, so the top of the stack moves to a lower address. When a POP instruction executes, data is read from the address in SP and then SP is incremented, effectively removing the top element. During CALL and RET, the stack pointer is used to save and restore the program counter. At all times, SP provides the link between the CPU and the current top of the stack.

Step-by-Step Solution:
Step 1: Remember that the stack pointer is a 16 bit register that points into RAM, not a data register like the accumulator.Step 2: Recall that the stack pointer always holds the address of the top of the stack, which contains the most recent item stored by PUSH, CALL, or interrupt handling.Step 3: Look for an option that explicitly mentions that the stack pointer keeps the 16 bit memory address of the top of the stack.Step 4: Verify that option C describes this behavior accurately.Step 5: Select option C and reject options that confuse SP with the program counter, accumulator, or a general purpose register.

Verification / Alternative check:
Diagrams of the 8085 architecture show SP alongside PC and general purpose registers but clearly label SP as pointing to the top of stack in memory. Timing diagrams for PUSH and POP also show SP being decremented or incremented around memory accesses. These resources confirm that SP contains the address of the top of the stack, not instruction addresses or accumulator contents. This matches option C exactly.

Why Other Options Are Wrong:
Option A describes the program counter, not the stack pointer. Option B refers to the accumulator, which holds data for arithmetic and logical operations. Option D imagines SP as storing the base address of the interrupt vector table, which is not how the 8085 handles vectors. Option E describes a general purpose register, but the stack pointer is special and should not be used arbitrarily in arithmetic if the program depends on the stack for subroutines or interrupts.

Common Pitfalls:
Students sometimes confuse PC and SP because both hold addresses, but PC is for the next instruction while SP is for the top of the stack. Another pitfall is to treat SP as a spare general purpose register and overwrite it, which leads to stack corruption. To avoid both conceptual and practical errors, always associate the stack pointer with the top of the stack memory and treat it with care in assembly programs.

Final Answer:
The stack pointer in the 8085 keeps the 16 bit memory address of the top of the stack where the most recent data item was stored.