Debugging self-modifying code in flash

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By default, when downloading an application to flash via J-Link, it is assumed that this application does not change during the debug session. This allows J-Link to do some optimization like caching certain target contents and so speed up debugging (depending on the IDE integration and the behavior of the IDE, reaction time can be 2-3 times faster with caching certain contents). However, there are cases where the application, downloaded at debug session start, may change during debugging it. These case are for example:

  • The application contains self-modifying code
  • There are some constant arrays etc. downloaded as part of the application but these are modified during the execution (e.g. non-volatile configuration data etc.)

When debugging such cases, memory windows etc. in the IDE may show the original (now incorrect) value. In order to debug such cases, J-Link needs to be aware of that certain ranges of the flash memory are considered to be "volatile" during the debug session. This can be achieved by the

ExcludeFlashCacheRange <SAddr>-<EAddr>
// Mark the first 64 KB of the flash as volatile
ExcludeFlashCacheRange 0x08000000-0x0800FFFF

command string. For more information about this command string and how to use command strings in different environments, please refer to UM08001, chapter "Command strings".