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WebAssembly · 08 Pages and Byte Offset
Series contents · Technology map
Stage: Memory and Array Boundaries · External example · preview instructions · Storage ID: webassembly-step-08
Goals and learning outcomes
- Learning outcome: explain the role of Pages. Input cases: four distinct integers.
- Deliverable: a table of index·offset·value read. Explain the rationale for handling Pointers, linking it to the code.
Prerequisites
Prepare the experiment results for Trap and Preflight checks from Unit 7, “Traps and Input Validation”.
Key concepts and explanations
Pages
A Wasm memory page is 65536 bytes. Element counts and byte counts use different units.
Pointers
Pointer arguments are interpreted as byte offsets. Passing an Int32 element index directly reads a different location.
Practice setup
First check the execution scope guidance on the example screen. The following describes the environment needed to extend the assignment in a separate project. The iframe preview is an experiment with WebAssembly.Memory and JS reference computations; do not present it as execution of an actual Wasm module. Practice with actual modules in a separate Node 22+ environment or a controlled local web project. Use kernel.wat·kernel.wasm·benchmark.mjs in the existing src/labs/webassembly-02·03 folders, and rebuild with the official wat2wasm tool when modifying WAT. Preserve CSP·isolation, and record measurements yourself.
The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.
Practice tasks
- In day-8.mjs in a separate practice project, print the memory.buffer size and at(index*4). Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: four distinct integers. Baseline: index. Run under the baseline conditions, then record observations and errors in notes/day-8.md.
- Variable: pointer unit; changed value: index×4. Run again through the same input path. Deliverable to save: a table of index·offset·value read. Hypothesis to check: Values are read from the intended integer addresses. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| pointer unit | index | index×4 | Values are read from the intended integer addresses. |
Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.
Completion criteria
- Required deliverable: a table of index·offset·value read. Results are available for both the baseline and changed conditions, allowing comparison of 2 conditions; input values and the execution environment are recorded together.
- The explanation of differences between the two conditions connects both concepts: Pages and Pointers. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Pages affect in this exercise? Input cases: four distinct integers.
- Variable: pointer unit. Hypothesis to check: Values are read from the intended integer addresses. Did the actual results agree with the hypothesis? Explain the evidence in terms of Pointers.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Standalone WAT + Node assertions; iframe CSP blocks Wasm compilation.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Traps and Input Validation · Next: Views and Endianness