OpenVMS Delta,XDelta Debugger Manual

This manual describes the HP OpenVMS DELTA and XDELTA debuggers, tools primarily used by programmers for debugging system code, device drivers, and other images that execute in privileged processor-access modes or at elevated Interrupt Priority Levels (IPLs) on OpenVMS VAX, Alpha, and I64 systems.

Key Distinctions between DELTA and XDELTA:

• DELTA: Operates as an exception handler within a process context. It is used for debugging programs running at IPL 0 in any processor mode and cannot debug code at elevated IPLs. It typically requires specific privileges (e.g., CMKRNL, CMEXEC) for debugging privileged code. Invocation involves defining a debugger logical name and using RUN/DEBUG, with EXIT to terminate the session.
• XDELTA: Invoked directly from the hardware System Control Block (SCB) in a system context. It is designed for debugging programs at any IPL, including elevated ones, and is not process-specific. XDELTA requires the system to be booted with it loaded into memory and remains active until a reboot without it. No special privileges are required to run XDELTA itself.

Common Features and Debugging Capabilities:

Despite their operational differences, both debuggers share many commands and expressions, offering capabilities such as:

• Memory and Register Inspection: Referencing specific addresses, viewing contents of general-purpose, floating-point, and internal processor registers, and evaluating numeric expressions (primarily in hexadecimal, with left-to-right evaluation).
• Breakpoints: Setting and clearing breakpoints (simple or complex), including via the INI$BRK system routine, with initial breakpoints differing slightly across platforms. It notes warnings about not clearing breakpoint 1 in XDELTA on VAX.
• Program Flow Control: Commands to continue program execution (P), step through single instructions (S), or step over subroutine calls (O).
• Error Reporting: A universal "Eh?" error message for various issues like unrecognized characters, incorrect commands, or inaccessible memory/privileges.
• Advanced Debugging Scenarios: Guidelines for debugging kernel mode code (requiring writable pages and page fault prevention), installed and protected shareable images (requiring specific linking and installation), and working around issues like single-stepping failures or compiler listing mismatches.
• Command Set: Includes commands for setting display modes, navigating memory locations, dumping memory regions, validating queues, forcing system crashes, listing information about loaded executive images, and displaying interrupt stack frames (some commands are platform or debugger specific).

The manual provides detailed descriptions of commands, symbols, and expressions, along with examples and sample debugging sessions for various OpenVMS platforms.