8085 and 8080A compatibility: Is 8085 software (restricted to the common instruction set) compatible with the Intel 8080A architecture?

Introduction / Context:
Compatibility between processors affects portability of both source and binary code. The Intel 8085 maintained the 8080A instruction set, adding new instructions and features. This question checks whether you understand that, if you stick to the common subset, the software executes across both CPUs.

Given Data / Assumptions:

• 8085 includes all 8080A instructions and flags.
• 8085 adds extra instructions; these are not available on 8080A.
• We are discussing compatibility at the instruction-set level, not pinouts.

Concept / Approach:
If an 8085 program uses only the instructions available on 8080A, its machine code is indistinguishable from equivalent 8080A code and therefore runs on an 8080A. Problems arise only if the 8085-specific opcodes are used (those have no counterpart on 8080A). This is the standard notion of writing to the “lowest common denominator.”

Step-by-Step Solution:

Verification / Alternative check:
Assemble the same source with an 8080A assembler: if it compiles, the resulting binary will execute equivalently on either CPU (barring timing- or hardware-specific assumptions).

Why Other Options Are Wrong:

• “Incompatible” is too strong and contradicts the shared ISA.
• “Arithmetic only” or “branching excluded” arbitrarily restricts features incorrectly.
• “Requires a hardware adapter” confuses electrical with software compatibility.

Common Pitfalls:
Using 8085-only instructions (e.g., SIM/RIM usage differences) and expecting execution on 8080A; overlooking that device-specific I/O mappings still matter for peripherals even when code is ISA-compatible.

Final Answer:
Compatible: 8085 code using only 8080A instructions runs on 8080A