The A5h opcode is a reserved instruction in the standard 8051 instruction set. While it is not a valid instruction for a standard 8051 microcontroller, it is utilized by the Oki 8XC154 microcontrollers found in various Honda ECUs.

Oki 8XC154 Implementation#

In Oki 8XC154 derivatives, A5h functions as a 3-byte instruction: A5 XX YY.

• A5: Opcode identifier.
• XX: Source bit address.
• YY: Destination bit address.

This instruction performs a bit-level move between two memory addresses. Because this implementation is non-standard, it is incompatible with generic 8051 disassemblers.

Compatibility and Disassembly#

To ensure compatibility with standard 8051 hardware or to facilitate proper disassembly, the A5h instruction must be translated.

Important

Instances of A5h can be replaced with the following sequence to maintain functional parity with standard 8051 architecture:

MOV C, SRCBIT  ; Move source bit to Carry flag
MOV DSTBIT, C  ; Move Carry flag to destination bit

Technical Notes#

• Carry Flag State: It has been confirmed that the state of the Carry flag (C) is modified during this operation. This behavior is required for the ECU to function correctly.
• Disassembly Support: Standard disassemblers will fail to interpret the Oki-specific A5h instruction correctly. Modified versions of the d51 disassembler have been developed to support this specific opcode mapping.

Resources#

For developers working with Honda ROMs, specialized tools are required to handle the Oki-specific instruction set. Refer to the following for modified disassembly utilities:

• d51 Disassembler: A modified version of the d51 disassembler is available to support the Oki A5h implementation.
• Implementation: By replacing the A5h opcode with the two-step MOV sequence, ROMs can be successfully ported or analyzed on standard 8051-compatible hardware.