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µVISION DEBUGGER: µVISION may perform endless memory read accesses

Information in this knowledgebase article applies to:

  • PK51 up to version 9.56
  • Any 8051 Target Debugger like Monitor-51, ISD51, ULINK2 or third-party debugger connected via AGDI interface


Under certain circumstances, µVision may perform endless memory read accesses with any 8051 target debugger. This can happen under the following conditions:

  • Single-stepping in the C source code, especially when one C source line generates multiple assembly instructions.
  • When you hover the mouse cursor over a variable/expression in the C source window.
  • When you click with your mouse into the disassembly window and then back into a C source window.

In most cases, a user will not notice this problem, but since the bandwidth to the target debugger is usually limited, the debug connection might slow down. It may also happen that the RUN button stays greyed out and the STOP button stays active forever, after a single step has been executed.


There was an issue in the µVision target debugger, which caused these endless read accesses.


You may download the attached file and extract the ZIP file into the folder C:\Keil_v5\C51\BIN\. If you have chosen a different installation folder, the path needs to be adapted accordingly. This updates the S8051.dll file.


  • This update is only intended for PK51 version 9.56! All newer versions of the PK51 tools will include this update.


Request the files attached to this knowledgebase article.

Last Reviewed: Wednesday, January 13, 2021

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