General Specification Electrical Function gmw3110 General Motors Local Area Network Enhanced Diagnostic Test Mode Specification

8.6.8 Tester Implications. 
This service can result in responses that are single frame or multiple frame based 
on the PID(s) requested. The tester should not functionally address requests for this service which result in a 
multiple frame response(s) unless it can handle sending the flow control frame(s) to each responding node 
within the allowable network timing parameters. The tester must be careful when requesting multiple PIDs 
functionally. If any ECU only allows requesting a single PID per request message then that ECU would 
generate a negative response even if it supported one of the PIDs in the request message. In addition, 
functionally addressed requests for this service should not contain more than two PIDs as this would generate 
a multi-frame request which would result in the transmission of flow frames from every ECU on the subnet. 
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GMW3110 
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8.7 ReadMemoryByAddress ($23) Service. 
The purpose of this service is to retrieve data from a contiguous 
range of ECU memory addresses. The range of ECU addresses is defined by a starting memory address 
parameter and a length (memory size) parameter included in the request message. This service is intended to 
be used during a device
’s development cycle to allow access to data that may not be available via another 
diagnostic service. The ReadMemoryByAddress service is only available as a one shot request-response 
service. 
8.7.1 Service Description.
The ReadMemoryByAddress service allows a tester to request the ECU to provide 
the data content of a range of ECU memory. The memory data to be read is identified by the parameters 
memoryAddress (MA) and memorySize (MS). The size of the memoryAddress parameter in the request 
message can be 2, 3, or 4 bytes in length depending on the ECU internal addressing scheme. The number of 
bytes that a tester shall use in the memoryAddress portion of the request message for a given ECU shall be 
documented in the appropriate CTS, SSTS, or supplemental diagnostic specification referenced by either of 
the preceding documents. The memorySize parameter in the request message shall always be two bytes. 
The ReadMemoryByAddress positive response message includes the start address (MA), specified in request 
message, and requested memory data. Memory data shall be sent in ascending order beginning with the 
memory location determined by the memoryAddress parameter. 
This diagnostic service shall be used to read RAM, Calibrations and unprotected areas of EEPROM/Flash 
memory. A device may choose to deny access to an address or a range of addresses. This allows production 
ECUs to protect their operating system and other secure data. 
An ECU may restrict the testers ability to read specific addresses or ranges of addresses based on the security 
status of the ECU. If any of the addresses (that fall in the range of the request message) have security 
restrictions then the request shall be rejected unless the tester had previously successfully accessed security. 
Note:
Security access is only required if the ECU is locked based on the status of the MEC and/or the 
vulnerability flag see the SecurityAccess ($27) service for more information.