How to localize static data without disassembler?

Sniffing traffic on the I²C bus with logic analyzer, summary byte table, hex search in Ghidra.

What is needed for SREC patching?

Byte changes, checksum recalculation of lines with Python script, preserving file format.

How to suppress artifacts after character replacement?

Digit signature search, zeroing data for unnecessary values with KS recalculation.

Reverse 9S12HY64: patching without disassembler

The article describes reverse engineering of 9S12HY64 firmware without disassembler: I²C sniffing, signature search in Ghidra, SREC patching to change the display. Achieved working result of indicator emulation. The approach is applicable to similar tasks.

By Sudonull Editorial Team • Published 2026-04-14 • Updated 2026-04-14

Patching Static Data in 9S12HY64 Firmware Without Disassembling

Changing how indicators appear on an automotive instrument cluster built around the MC9S12HY64 microcontroller is achieved by analyzing I²C traffic and patching the SREC firmware file. This method avoids disassembling code entirely, instead focusing on identifying signatures in static data using Ghidra. The goal is to replace characters A with P and M with D, while eliminating digit artifacts.

Hardware Analysis and Protocol

The 9S12HY64 microcontroller connects to the display via a 6-pin interface without an external driver. Circuit tracing and reference manuals confirm the I²C protocol. A Saleae logic analyzer captures traffic between the MCU and display across all gear selector modes.

A summary table of the traffic reveals changing bytes that serve as potential signatures:

Mode A1: 5A C9 9A 8D...
Mode M1: 5A C9 9A 8D... (variations in first bytes)

Red-highlighted bytes are targeted for search in the firmware, presumed to represent a 2D array of static message payloads.

Locating Signatures in the Firmware

Ghidra is used for hex searching without HCS12 disassembly. The signature 0x5A, 0xC9, 0x9A, 0x8D pinpoints a memory region. Message format: display byte number + data.

Matching traffic to memory:

First byte identifies the segment (A/M).
Subsequent bytes define character patterns.

From a new cluster’s firmware, messages for P and D were extracted:

P: one-byte replacement.
D: two-byte correction.

Message Comparison:

| Mode | Original Message | Target Message |

|------|------------------|----------------|

| A1 | 5A C9 9A 8D... | 5A XX XX 8D... (P) |

| M1 | 5A C9 9A 8D... | 5A YY ZZ 8D... (D) |

Patching the SREC File

SREC structure: header, address, data, checksum. Byte changes require recalculating the checksum (KS).

Python script for KS calculation:

# Example script (adapted from GitHub)
def calc_srec_checksum(line):
    # Sum of bytes with inversion
    s = sum(bytes.fromhex(line[1:-1]))  # Exclude S and KS
    return format((~s + 1) & 0xFF, '02X')

Changes:

For P: modify the 1st byte in one line.
For D: adjust the 1st and 2nd bytes in two lines.

Recalculate KS; resulting firmware shows partial replacement with digit artifacts.

Eliminating Digit Artifacts

Digits (1–5) are generated as: 00th byte + nibble from the 12th byte (base 0x80 + value). Signature search in firmware for digits, zeroing data for '4' (keeping 1, 2, 3).

Format: byte number – data.
Recalculate KS for modified lines.

Final firmware delivers full P/D emulation without digits.

Key Takeaways

I²C traffic sniffing enables locating static data without code analysis.
Ghidra hex search works effectively even for unknown architectures like HCS12.
SREC patching demands precise checksum handling for validity.
Approach is scalable to other embedded systems using serial protocols.
Method reduces risk by avoiding execution of modified code.

— Editorial Team

Reverse 9S12HY64: patching without disassembler