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Unveiling Architectural Complexity: Debugging Methods for Processor Systems

As modern designs incorporate larger on-chip caches and greater complexity, debugging intricate single and multi-processor systems presents unique challenges. This exploration covers traditional and contemporary techniques for exposing internal functionality, including JTAG's Test Access Port (TAP), Probe Mode, and in-circuit emulation (ICE). It highlights automated design tools that reduce errors, simulation utilities that uncover logic flaws, and testing methodologies that ensure robust performance. The discussion contextualizes these methods against the backdrop of multiprocessor design challenges like race conditions and inconsistent behaviors.

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Unveiling Architectural Complexity: Debugging Methods for Processor Systems

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  1. Architectural Complexity: Opening the Black Box Methods for Exposing Internal Functionality of Complex Single and Multiple Processor Systems EECC-756

  2. Modern Design Trends • Larger on-chip caches • Extended levels of cache • System-on-a-chip integration • Overall increasing design complexity All lead to more complex debugging of designs

  3. The Good News • Automated design tools are minimizing design errors • IP reuse minimizes bugs • Simulation tools discover most logic errors before fabrication • Massive test suites allow comprehensive testing • So what happened to Intel with FPU flaw?

  4. Past Methods for Debugging • Signal probing • Bus monitoring • Software debugging

  5. Past Methods for Debugging (cont’d) • Signal probing • More internal logic per pin = less info on pin • Pin inaccessibility due to modern packages (i.e. sockets, BGAs) • Bus monitoring • Caches hide data accesses • Software debugging • Impractical for real-time applications • Little or no hardware support in the past

  6. Solutions • Test Access Port (TAP) • Uses JTAG IEEE1149.1 specification for boundary scan • Probe Mode • Allows step by step analysis of code impact on internal registers • In-circuit Emulation (ICE) • Allows execution tracing • Real-time applicability

  7. Test Access Port (TAP) • Implementation of boundary scan JTAG IEEE1149.1 specification • Allows access to all internal flip-flops in boundary scan chain • Numerous chains serve different functions (i.e. IO flip-flops) • Allows non-destructive snapshot of internal state at any point in time

  8. Test Access Port (cont’d) • Single instruction register • Multiple data registers (scan chains)

  9. Probe Mode • Special processor mode halts program execution • Uses the TAP interface to receive instructions and output internal data • Allows read/write access to any internal registers • Allows memory accesses to test cache functionality

  10. Probe Mode (cont’d)

  11. In-Circuit Emulation (ICE) Support • Special pins provide branching information • Example: Pentium Dual Pipeline • 3 dedicated pins • IU – Asserted when instruction completes in the U instruction pipeline • IV – Asserted when instruction completes in the V instruction pipeline • IBT – (Instruction Branch Taken) Asserted when a branch is taken

  12. In-Circuit Emulation (cont’d) • Branch signal information provides realtime code tracing • Branch trace message buffers provide further information • Branch trace message buffers in conjunction with Probe Mode allow detailed realtime code tracing

  13. Branch Trace Message Buffers • FIFO queue • Can be read through TAP during program execution • Circular mode (trace-back from breakpoint) vs. Jump-to-Probe Mode (maintain instruction stream) • Incident counter expands buffer size • Intel automatically generates a special BTM cycle on local bus to export BTM info

  14. Branch Trace Buffer Logic Implementation

  15. Multiprocessor Issues • Three methods for opening the “black box” on a single processor system • TAP (boundary scan) • Probe Mode • Branch Tracing Methods for ICE • Multiple processor system design also has challenges

  16. Multiprocessor Challenges • Race conditions due to parallel data accesses • Inconsistent and unpredictable network paths • Differing processor behaviors on heterogeneous networks • Communication patterns that restrict performance or scalability

  17. Multiprocessor Solutions : Debugging Code • Create sequential version of code • Execute parallel tasks on a single computer as separate processes • Visualization tools that create space-time diagrams or animations to show 2-dimensional changes of state • Unified Trace Environment (IBM)

  18. Multiprocessor Solutions : Debugging Designs • Ability to monitor communication packets circumvents most visibility problems • Debug messages can be included in packet • Network protocol simulations • Protocol verification programs • (i.e. petri-nets) • Network communication pattern simulators • However ...

  19. Multiprocessor Design Trends • Currently, uniprocessor designs are hitting roadblocks • large dies impractical signal transit time • routing increases exponentially with die size • One possible solution : multiple processors on a single die re-emergence of visibility problems

  20. Conclusion • Several methods available for internal execution tracing of uniprocessors • Test Access Port (JTAG IEEE1149.1) • Probe Mode extension • Branch Tracing • Don’t count out TAP, Probe Mode, and ICE for multiprocessors

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