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OllyDbg is a 32-bit assembler level analysing debugger for Microsoft® Windows®. Emphasis on binary code analysis makes it particularly useful in cases where source is unavailable. OllyDbg is a shareware, but you can download and use it for free. Special highlights are:

* Intuitive user interface, no cryptical commands
* Code analysis - traces registers, recognizes procedures, loops, API calls, switches, tables, constants and strings
* Directly loads and debugs DLLs
* Object file scanning - locates routines from object files and libraries
* Allows for user-defined labels, comments and function descriptions
* Understands debugging information in Borland® format
* Saves patches between sessions, writes them back to executable file and updates fixups
* Open architecture - many third-party plugins are available
* No installation - no trash in registry or system directories

* Debugs multithread applications
* Attaches to running programs
* Configurable disassembler, supports both MASM and IDEAL formats
* MMX, 3DNow! and SSE data types and instructions, including Athlon extensions
* Full UNICODE support
* Dynamically recognizes ASCII and UNICODE strings - also in Delphi format!
* Recognizes complex code constructs, like call to jump to procedure
* Decodes calls to more than 1900 standard API and 400 C functions
* Gives context-sensitive help on API functions from external help file
* Sets conditional, logging, memory and hardware breakpoints
* Traces program execution, logs arguments of known functions
* Shows fixups
* Dynamically traces stack frames
* Searches for imprecise commands and masked binary sequences
* Searches whole allocated memory
* Finds references to constant or address range
* Examines and modifies memory, sets breakpoints and pauses program on-the-fly
* Assembles commands into the shortest binary form
* Starts from the floppy disk

and much, much more!