In the Intel 8085 microprocessor, what is the role of the program counter and what information does it hold during normal execution?

Introduction / Context:
This question tests understanding of the program counter in the Intel 8085 microprocessor. The program counter is one of the most important registers in any CPU because it dictates the sequence of instruction execution. Knowing exactly what the program counter holds and how it changes during instruction fetch and branch operations is essential for understanding how programs flow at the machine level.

Given Data / Assumptions:

We are working with the 8085 microprocessor register set.

The program counter is a 16 bit register often abbreviated as PC.

The question asks what the program counter holds during normal execution.

Normal execution refers to the usual fetch decode execute cycle between jumps, calls, or interrupts.

The program counter interacts closely with the memory address bus during instruction fetch cycles.

Concept / Approach:
The program counter holds the address in memory from which the next instruction byte will be fetched. During the fetch cycle, the contents of the PC are placed on the address bus, the memory at that address is read, and the instruction opcode is loaded into the instruction register. After fetching, the program counter is automatically incremented to point to the next instruction byte, unless a branch instruction modifies it. This mechanism ensures sequential execution of instructions stored in memory until a jump, call, return, or interrupt changes the control flow by loading a new value into the program counter.

Step-by-Step Solution:
Step 1: Recall that the program counter is responsible for tracking the address of the instruction that should be fetched next.Step 2: Remember that the 8085 uses a 16 bit program counter to address up to 64 kilobytes of memory.Step 3: Identify the option that states the program counter keeps the address of the next instruction in memory that the processor will fetch and execute.Step 4: Check that this description matches the standard textbook explanation of the program counter.Step 5: Select this option and discard others that confuse the program counter with the stack pointer, accumulator, or flag register.

Verification / Alternative check:
Block diagrams of the 8085 show the program counter connected to the internal address bus and to the control unit responsible for fetching instructions. Instruction timing diagrams also show the value of PC appearing on the address bus during opcode fetch. This confirms that the primary job of the program counter is to hold the address of the next instruction to be executed. Other registers, such as SP for the stack pointer and the flag register for conditions, have different roles and do not match this behavior.

Why Other Options Are Wrong:
Option A describes the stack pointer, not the program counter. Option B refers to the accumulator, which stores data for arithmetic and logical operations. Option D describes the flag register, which holds status bits like zero and carry but not instruction addresses. Option E suggests that the program counter permanently stores the starting address of the interrupt vector table, which is incorrect because the program counter changes dynamically as the program executes.

Common Pitfalls:
Students sometimes mix up the names and roles of CPU registers, especially SP, PC, and the flag register, because they all hold important values for program execution. Another pitfall is to think that the program counter holds the current instruction rather than the address of the next instruction. To avoid confusion, always associate the program counter with memory addresses and sequential instruction fetching.

Final Answer:
In the 8085, the program counter keeps the address of the next instruction in memory that the microprocessor will fetch and execute.