In the 8085 microprocessor, what is the primary purpose of the internal temporary registers W and Z?

Introduction / Context:
The Intel 8085 microprocessor exposes several registers to the programmer, such as the accumulator, general purpose registers, and the program counter. In addition to these visible registers, the 8085 also contains internal temporary registers that are used by the hardware to implement complex operations. Among these are the W and Z registers. Understanding their purpose helps you appreciate how the processor manages 16 bit operations internally, even though programmers do not manipulate these registers directly.

Given Data / Assumptions:

• The microprocessor is the Intel 8085.
• W and Z are documented as temporary registers.
• Programmers cannot directly address W and Z in assembly language instructions.
• These registers assist internal operations such as jump, call, and stack related instructions.

Concept / Approach:
Because the 8085 has an 8 bit data bus but frequently works with 16 bit addresses and data, it needs internal storage for intermediate values. W and Z form an internal 16 bit register pair used by the control unit for temporary holding of addresses and intermediate results during certain operations. For example, when executing instructions like CALL, PUSH, or branch instructions that involve memory addresses, the processor may use W and Z to assemble or manipulate the 16 bit values. They help reduce complexity in the visible register set while still supporting 16 bit behaviour.

Step-by-Step Solution:
Step 1: Recall that the visible general purpose registers in the 8085 are B, C, D, E, H, L, and the accumulator A.Step 2: Remember that W and Z are not listed among these programmer accessible registers and do not appear as operands in instructions.Step 3: Understand that the microprocessor needs scratch registers internally to hold intermediate values during complex instructions, especially those involving 16 bit addresses.Step 4: Recognize that W and Z are used as such internal scratch registers and are treated as a hidden register pair by the control logic.Step 5: Conclude that their primary purpose is to support internal 16 bit operations, not to serve as extra general purpose registers for programmers.

Verification / Alternative check:
Microprocessor documentation and detailed timing diagrams often show W and Z in internal block diagrams or microinstruction sequences. For example, when the processor fetches a 16 bit address from memory, it may first load one byte into W and the next into Z, then combine them to form a full address. Assembly language programmers do not refer to W and Z, but their presence allows the processor to perform necessary operations without exposing additional complexity in the instruction set.

Why Other Options Are Wrong:
Option B is incorrect because there are no instructions that directly load or store W and Z; they are not general purpose registers. Option C is wrong because interrupt vector handling uses fixed addresses and internal logic, but W and Z are not reserved exclusively for that purpose. Option D is incorrect because input and output port addressing uses the accumulator and other registers, not dedicated W and Z registers.

Common Pitfalls:
Learners sometimes assume that every named register must be directly accessible in assembly, which is not always true. Another pitfall is to overemphasize the alphabetic sequence of register names and try to infer meaning from letters alone. The important point is functional: W and Z are internal temporary registers that the hardware uses autonomously. Understanding this helps clarify which resources a programmer can control and which are managed internally by the processor.

Final Answer:
In the 8085 microprocessor, W and Z are internal working registers used during certain 16 bit operations such as address calculations and stack manipulation, and they are not directly accessible to the programmer, which is option A.