ReadMemoryByAddress (0x23) - UDS Diagnostics

0x23 ReadMemoryByAddress Service Structure

0x23 Request/Response

  Request:
  ┌──────┬──────────────┬────────────────────┬────────────────────┐
  │ 0x23 │ addrAndLen   │ startingAddress    │ memorySize         │
  │      │ Format byte  │ 1-4 bytes          │ 1-4 bytes          │
  └──────┴──────────────┴────────────────────┴────────────────────┘
  addrAndLengthFormatIdentifier: high nibble = address bytes; low nibble = length bytes
  Example: 0x44 = 4-byte address + 4-byte length

  Request example (read 32 bytes from LMU RAM 0x70001000):
  0x23 0x44 0x70 0x00 0x10 0x00 0x00 0x00 0x00 0x20

  Positive response:
  ┌──────┬──────────────────────────────────────────┐
  │ 0x63 │ Memory data (up to memorySize bytes)     │
  └──────┴──────────────────────────────────────────┘

  NRC 0x31: requestOutOfRange — address or size not permitted
  NRC 0x33: securityAccessDenied — Level 1 required
  NRC 0x13: incorrectMessageLength — wrong addrAndLen format byte

Development vs Production Use

Pythonread_memory.py

#!/usr/bin/env python3
# 0x23 ReadMemoryByAddress: diagnostic and development use cases
import udsoncan
from udsoncan.client import Client

def read_memory(client, address: int, length: int) -> bytes:
    # udsoncan library: client.read_memory_by_address()
    memloc = udsoncan.MemoryLocation(
        address=address,
        memorysize=length,
        address_format=32,  # 4-byte address
        memorysize_format=32  # 4-byte length
    )
    resp = client.read_memory_by_address(memloc)
    if resp.positive:
        return resp.service_data.memory_block
    raise Exception(f"ReadMemory failed: NRC 0x{resp.code:02X}")

# Development use case 1: inspect a specific RAM variable
# (no CDD change needed; direct address from map file)
# data = read_memory(client, 0x70001234, 4)  # read g_vehicleSpeed
# speed = int.from_bytes(data, 'big') / 1000.0  # scaling from map file

# Development use case 2: verify calibration data in flash
# cal_data = read_memory(client, 0x80F00000, 256)  # read cal block
# expected_crc = calculate_crc32(cal_data[:-4])
# stored_crc   = int.from_bytes(cal_data[-4:], 'big')
# print(f"Cal CRC: {'OK' if expected_crc == stored_crc else 'MISMATCH'}")

# Production consideration:
# ReadMemoryByAddress should be DISABLED or severely restricted in production ECUs
# Reason: allows reading arbitrary RAM including sensitive data (keys, PINs, crypto state)
# Restriction: configure DCM to return NRC 0x31 for all address ranges in production mode
# or: require SecurityAccess Level 3 (development-only level, not available in production)

Security Considerations for 0x23

⚠️ ReadMemoryByAddress is a Security Risk in Production

Service 0x23 with broad address permissions allows reading any ECU RAM or flash — including session keys, security seeds, cryptographic key material, and application secrets. In production ECUs: (1) restrict to specific address ranges (calibration flash only), (2) require SecurityAccess Level 1+, (3) disable defineByMemoryAddress in 0x2C. OBD-II-compliant ECUs must never allow 0x23 without security access. Penetration testers routinely use 0x23 to extract firmware and key material from improperly secured ECUs.

Summary

ReadMemoryByAddress is the most powerful diagnostic service — and the most dangerous if left unrestricted. In development it's invaluable: reading any variable by map-file address without a CDD update saves hours of iteration. In production, it must be locked down to specific address ranges (calibration flash, version strings) with Security Level 1 at minimum, and completely disabled in Default Session. The addrAndLengthFormatIdentifier format byte is the most common integration error: always verify high nibble (address bytes) and low nibble (length bytes) match what the ECU is configured to accept.

🔬 Deep Dive — Core Concepts Expanded

This section builds on the foundational concepts covered above with additional technical depth, edge cases, and configuration nuances that separate competent engineers from experts. When working on production ECU projects, the details covered here are the ones most commonly responsible for integration delays and late-phase defects.

Key principles to reinforce:

Configuration over coding: In AUTOSAR and automotive middleware environments, correctness is largely determined by ARXML configuration, not application code. A correctly implemented algorithm can produce wrong results due to a single misconfigured parameter.
Traceability as a first-class concern: Every configuration decision should be traceable to a requirement, safety goal, or architecture decision. Undocumented configuration choices are a common source of regression defects when ECUs are updated.
Cross-module dependencies: In tightly integrated automotive software stacks, changing one module's configuration often requires corresponding updates in dependent modules. Always perform a dependency impact analysis before submitting configuration changes.

🏭 How This Topic Appears in Production Projects

Project integration phase: The concepts covered in this lesson are most commonly encountered during ECU integration testing — when multiple software components from different teams are combined for the first time. Issues that were invisible in unit tests frequently surface at this stage.
Supplier/OEM interface: This is a topic that frequently appears in technical discussions between Tier-1 ECU suppliers and OEM system integrators. Engineers who can speak fluently about these details earn credibility and are often brought into critical design review meetings.
Automotive tool ecosystem: Vector CANoe/CANalyzer, dSPACE tools, and ETAS INCA are the standard tools used to validate and measure the correct behaviour of the systems described in this lesson. Familiarity with these tools alongside the conceptual knowledge dramatically accelerates debugging in real projects.

⚠️ Common Mistakes and How to Avoid Them

Assuming default configuration is correct: Automotive software tools ship with default configurations that are designed to compile and link, not to meet project-specific requirements. Every configuration parameter needs to be consciously set. 'It compiled' is not the same as 'it is correctly configured'.
Skipping documentation of configuration rationale: In a 3-year ECU project with team turnover, undocumented configuration choices become tribal knowledge that disappears when engineers leave. Document why a parameter is set to a specific value, not just what it is set to.
Testing only the happy path: Automotive ECUs must behave correctly under fault conditions, voltage variations, and communication errors. Always test the error handling paths as rigorously as the nominal operation. Many production escapes originate in untested error branches.
Version mismatches between teams: In a multi-team project, the BSW team, SWC team, and system integration team may use different versions of the same ARXML file. Version management of all ARXML files in a shared repository is mandatory, not optional.

📊 Industry Note

Engineers who master both the theoretical concepts and the practical toolchain skills covered in this course are among the most sought-after professionals in the automotive software industry. The combination of AUTOSAR standards knowledge, safety engineering understanding, and hands-on configuration experience commands premium salaries at OEMs and Tier-1 suppliers globally.