A checksum is a basic form of error checking used to verify the integrity of data within a ROM. It is calculated by summing the values of the ROM data without carry or overflow checking.

Technical Overview#

Checksums vary in bit-depth (8-bit, 16-bit, 32-bit). Honda OBD0 and OBD1 ECUs utilize 8-bit checksums, ranging from 0 to 255 decimal (00–FF hex).

Note

The ECU performs this calculation to ensure the ROM image has not been corrupted. If the calculated checksum does not match the expected value, the ECU may trigger a diagnostic trouble code or fail to operate.

Manual Checksum Adjustment#

When modifying a ROM, the checksum must be recalculated and updated to maintain compatibility. The following procedure outlines how to manually adjust the checksum using a hex editor.

Prerequisites#

• Hex editing software (e.g., WinHex).
• Windows Calculator (set to Scientific/Hex mode).

Procedure#

1. Calculate Current Hash: Open the ROM file in your hex editor. Navigate to the Tools menu and select Calculate Hash (8-bit). Record the resulting value.
2. Determine Correction Value: Use the Windows Calculator in Hex mode to perform the following calculation:
   • FF - [Current_Checksum] = [Correction_Value]
3. Apply Correction:
   • Locate an unused memory address (typically toward the end of the ROM file) that contains a value of FF.
   • Replace the FF at that location with your calculated [Correction_Value].
   • If no FF value is available at an unused address, use the existing value at that location (e.g., 00) in your calculation (00 - [Current_Checksum]) and replace that value with the result.

Tip

Always maintain a backup of the original ROM file before performing manual edits.

Tools#

• Check8: A utility designed to calculate 8-bit checksums for binary files.
• Hex Editors: Software such as WinHex is recommended for viewing and modifying raw ROM data.