The A5h opcode is reserved in the standard 8051 instruction set. While pure 8051 microcontrollers treat this as an invalid instruction, Oki 8XC154 microcontrollers—commonly found in Honda ECUs—utilize A5h as a proprietary three-byte instruction.

Technical Specification#

In the Oki 8XC154 architecture, the A5h instruction functions as a bit-level move operation:

Instruction Format: A5 XX YY

• A5: Opcode (MOV with two bit-address arguments)
• XX: Bit address of the source
• YY: Bit address of the destination

Important

The Oki implementation of A5h is incompatible with standard 8051 disassemblers, which will fail to parse the instruction correctly.

Compatibility and Disassembly#

To execute Honda ROM code on standard 8051-compatible hardware, the proprietary A5h instruction must be replaced with standard 8051 assembly equivalents.

Instruction Replacement#

The A5 XX YY instruction can be functionally replaced by the following sequence:

MOV C, XX    ; Move source bit to Carry flag
MOV YY, C    ; Move Carry flag to destination bit

Note

It has been confirmed that the state of the Carry flag must be altered during this operation for the ECU logic to function correctly.

Tooling#

To facilitate the analysis of Honda ROMs, the d51 disassembler has been modified to support the Oki-specific A5h opcode.

• Custom Disassembler: Use the Pgmfi-modified d51 to correctly interpret Oki-specific opcodes.
• Verification: This method was experimentally verified by monitoring register states on test hardware to ensure bit-address mapping matches the original Oki implementation.