Document Field Offset Calculation - Value 4

[LSantha]

Concept: Field Offset Calculation

Value: 4 (Important - Object layout)

What it is: Runtime field offset calculation for TIB and objects

Key files:

• core/src/core/org/jnode/vm/classmgr/FieldLayout.java
• core/src/core/org/jnode/vm/classmgr/TIBLayout.java

Bridges: Object layout, JIT compiler, Field access

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Documentation Requirements

For Value 4 (Important):

1. Update ALL 4 sections of the index (index.md):

   • Pages Table: Add new page in alphabetical order
   • Concepts → Pages: Add the page name
   • Source Path → Pages: Map core/src/core/org/jnode/vm/classmgr/ to the new page
   • Task Routing: Add to relevant developer task categories with meaningful routing ideas (1-3 entries)

2. Update Glossary: Add 1-sentence definition for any new project-specific term

3. Broader Search: Search for related "Helper", "Manager", "Layout", or "Constants" classes (e.g., alignment constants, header size)

Skill Reference

Use update-wiki skill for wiki maintenance workflow

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Related Concepts (from index.md)

• [[Object-Layout]]
• [[Type-System-Internals]]

[LSantha]

/oc Please proceed with this task.

[LSantha]

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[opencode-agent]

core/src/core/org/jnode/vm/classmgr prepareForInstantiation *.java

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[LSantha]

/oc Please proceed with this task.

[LSantha]

/oc Please proceed with this task.

[opencode-agent]

Done! I've created the Field Offset Calculation wiki page and updated the index, glossary, and changelog.

Wiki URL: https://github.com/LSantha/jnode_ai/wiki/Field-Offset-Calculation

Summary of changes:

• Created Field-Offset-Calculation.md documenting JNode's runtime field offset calculation for objects and TIB
• Updated index.md with all 4 sections (Pages table, Concepts → Pages, Source Path → Pages, Task Routing)
• Updated Glossary.md with "Field offset" definition
• Updated Changelog.md with the new entry
• Issue Document Field Offset Calculation - Value 4 #119 closed

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[LSantha]

/oc Please proceed with this task.

[opencode-agent]

The task is already complete. Issue #119 is CLOSED and all documentation has been created and pushed:

Wiki URL: https://github.com/LSantha/jnode_ai/wiki/Field-Offset-Calculation

Summary of completed work:

• Created Field-Offset-Calculation.md (126 lines) documenting:
  • Object header structure (FLAGS_SLOT at -2, TIB_SLOT at -1)
  • Field offset assignment in ClassDecoder
  • Offset resolution in VmNormalClass for inheritance
  • VmInstanceField.getOffset() runtime access
  • TIB array indexing via TIBLayout constants
  • Gotchas (negative header offsets, slot size variations, alignment)
• Updated index.md with all 4 sections (Pages Table, Concepts → Pages, Source Path → Pages, Task Routing)
• Updated Glossary.md with "Field offset" definition
• Updated Changelog.md
• Pushed to remote - verified with git status (working tree clean)
• Issue Document Field Offset Calculation - Value 4 #119 is in CLOSED state

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