In the Intel 8085 microprocessor, which register pair is commonly referred to as the high order and low order register pair used together as a 16 bit register?

Introduction / Context:
Understanding register organisation is essential when learning any microprocessor architecture. The Intel 8085 uses several 8 bit registers that can also be paired to form 16 bit register pairs. This question focuses on which pair is often called the high order and low order register pair, a term commonly used when addressing memory.

Given Data / Assumptions:

- The microprocessor in question is the Intel 8085.- The phrase high order and low order register pair refers to two 8 bit registers used together.- The options list different possible combinations of registers.

Concept / Approach:
In the 8085 microprocessor, several register pairs exist, such as BC, DE, and HL. The HL pair is special because it is often used as a 16 bit memory pointer, with H holding the high order 8 bits of an address and L holding the low order 8 bits. This is why these two registers are frequently referred to as the high order and low order registers. Other registers like A, B, D, and E are important for operations but are not specifically given this high order and low order terminology in standard teaching materials.

Step-by-Step Solution:
Step 1: Recall that the 8085 has six general purpose 8 bit registers: B, C, D, E, H, and L, as well as the accumulator A.Step 2: These can be paired as BC, DE, and HL to form 16 bit registers for certain instructions.Step 3: The HL pair is heavily used for memory addressing, where H stores the higher 8 bits of the memory address and L stores the lower 8 bits.Step 4: Due to this usage, many texts describe H as the high order register and L as the low order register.Step 5: Option A explicitly mentions the H and L register pair as a 16 bit memory pointer, which fits the description.Step 6: Options B, C, D, and E either refer to different register combinations or to concepts that are not labelled high order and low order in standard 8085 documentation.

Verification / Alternative check:
When examining 8085 instruction examples, you will often see the HL pair used with instructions like MOV M, A or LXI H, address. These show that HL points to memory, with H forming the high part and L forming the low part of the address. Reference books on 8085 microprocessor architecture also refer to the HL pair as the high order and low order pair, confirming the correctness of option A.

Why Other Options Are Wrong:
Option B pairs A and B, but A is the accumulator and not normally paired with B as a 16 bit register. Option C refers to DE but incorrectly restricts its use to input output, which is not accurate. Option D pairs the program counter and stack pointer, which are separate 16 bit registers and not referred to as a single high order low order pair. Option E combines the accumulator and flag register, which together form the program status word but are not described as high and low order registers in the same sense.

Common Pitfalls:
Some learners mix up the various register pairs or think that any two registers used together can be called high and low order. It is helpful to remember that this specific phrase is typically reserved for the HL pair because of its special role in memory addressing. Drawing diagrams of the 16 bit address and labelling which register holds which part can make this much clearer.

Final Answer:
In the 8085 microprocessor, the high order and low order register pair is the H and L register pair, used together as a 16 bit memory pointer.