HDUMP - Hexadecimal Dump
A hexadecimal dump (HDUMP) is a formatted display of the raw contents of memory, registers, or a data set, presented in both hexadecimal and character (typically EBCDIC) representations. Its primary purpose in the mainframe environment is to aid in debugging, problem diagnosis, and in-depth analysis of program failures or data anomalies.
Key Characteristics
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- Dual Representation: Each byte of data is displayed as a two-character hexadecimal value (00-FF) and, if printable, its corresponding EBCDIC (or ASCII) character.
- Address-Based: Dumps typically include memory addresses or offsets, allowing analysts to pinpoint specific locations within storage or a data set.
- Source Flexibility: Can be generated automatically by the system during an abend (e.g.,
SYSUDUMP,SYSMDUMP,SVC DUMP), explicitly by utility programs (e.g.,IEBGENERwithDUMPoption), or programmatically within application code. - Diagnostic Tool: Provides a low-level, unfiltered view of data, crucial for understanding the exact state of a program or system component at a specific point in time.
- Requires Interpretation: Effective analysis of an HDUMP demands a strong understanding of z/OS control blocks, program data structures, and EBCDIC character encoding.
- Variable Scope: Dumps can range from small, targeted memory areas to full address space dumps or even complete system dumps.
Use Cases
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- Abend Analysis: Diagnosing program abends (e.g.,
S0C7,S0C4) by examining the contents of registers, program storage, and relevant control blocks at the point of failure. - Data Corruption Investigation: Identifying the cause of corrupted data in files, databases (DB2, IMS), or memory by inspecting the raw hexadecimal values.
- Control Block Examination: Verifying the integrity and contents of z/OS or application control blocks (e.g., TCB, ASCB, DCB, PCB) to understand system or program state.
- Program Logic Verification: Debugging complex COBOL or Assembler logic by tracing the actual values of variables and data structures as they reside in memory.
- Security Incident Response: Analyzing memory or data sets for sensitive information or indicators of compromise following a security event.
- Abend Analysis: Diagnosing program abends (e.g.,
Related Concepts
HDUMPs are intrinsically linked to Abends as they provide the critical evidence for post-mortem analysis. The Interactive Problem Control System (IPCS) is the primary z/OS tool used to format, browse, and analyze system dumps, making raw HDUMPs much more readable by applying DSECTs (Data Section Control Tables) and symbols. Understanding z/OS Control Blocks (e.g., TCB, ASCB, DCB, RB) is paramount for interpreting the data within a dump, as these structures define the system's operational state. Furthermore, familiarity with the EBCDIC character set is essential for correctly interpreting the character portion of a mainframe HDUMP.
- Targeted Dumps: Whenever possible, generate
SVC DUMPsor application-specific dumps for the relevant address space or data area rather than full system dumps to reduce size and improve analysis efficiency. - Utilize IPCS: Always use IPCS to format and analyze dumps. It applies DSECTs, resolves addresses, and provides a structured view, making the raw hexadecimal data much more understandable.
- Understand Data Formats: Be proficient in interpreting various data types (e.g., binary, packed decimal, zoned decimal, floating-point) as they appear in hexadecimal within the dump.
- Define DSECTs: For complex application data structures, define DSECTs in your Assembler or COBOL copybooks; this greatly aids in symbolic dump analysis.
- Secure Handling: Dumps can contain sensitive data, including passwords or proprietary information. Ensure they are handled, stored, and disposed of securely according to organizational data protection policies.