8080A vs. 8085A software compatibility: Do programs written for the Intel 8080A require modifications to run on the Intel 8085A microprocessor?

Introduction / Context:
The Intel 8085A was designed as an upward-compatible successor to the 8080A. Understanding what “software compatible” means in historical microprocessor families helps developers port legacy code and reuse existing tools. This question tests whether you know that 8080A binaries generally execute on the 8085A as-is.

Given Data / Assumptions:

• Source/binary originally targets 8080A.
• Target CPU is 8085A, which includes a superset of 8080A instructions.
• No reliance on undocumented opcodes or exotic hardware dependencies.

Concept / Approach:
Upward (backward) compatibility means a newer CPU executes code for an older CPU without modification. The 8085A kept the 8080A instruction set and flags while adding new instructions and on-chip features (e.g., serial I/O signals, single 5 V supply). Therefore, standard 8080A programs—both source and binaries—run on the 8085A unchanged.

Step-by-Step Solution:

Verification / Alternative check:
Cross-assemblers can generate identical machine code for both targets when restricted to 8080A mnemonics. Historical documentation and application notes state that 8080A object code will execute on the 8085A.

Why Other Options Are Wrong:

• “Correct statement: changes required” is false for standard code.
• “Only I/O routines must be rewritten”: Not required unless you exploit 8085-specific peripherals.
• “Only arithmetic routines need patching”: The arithmetic/flag model is compatible for 8080A instructions.
• “Compatibility only in monitor ROMs”: Compatibility applies broadly, not just to monitors.

Common Pitfalls:
Confusing hardware pin differences with ISA compatibility; using 8085-only mnemonics and then expecting the same code to run on 8080A (that breaks reverse compatibility, not forward).

Final Answer:
Incorrect statement: 8080A programs run on 8085A without changes