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WebAssembly · 16 Aligned and Unaligned Access
Series contents · Technology map
Stage: Deployment and Execution Structure · External example · preview instructions · Storage ID: webassembly-step-16
Goals and learning outcomes
- Learning outcome: explain the role of Alignment. Input cases: the same four bytes.
- Deliverable: creation errors and values read. Explain the rationale for handling DataView, linking it to the code.
Prerequisites
Prepare the experiment results for Overwriting and Repeat conditions from Unit 15, “In-place Transformation”.
Key concepts and explanations
Alignment
The byteOffset of Int32Array must be a multiple of its element size. Distinguish Wasm load constraints from JS view constraints.
DataView
DataView provides byte-based access. It can explicitly read unaligned data.
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-16.mjs in a separate practice project, place data at offset 0 and 1 and compare TypedArray creation with DataView access. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: the same four bytes. Baseline: 0. Run under the baseline conditions, then record observations and errors in notes/day-16.md.
- Variable: offset; changed value: 1. Run again through the same input path. Deliverable to save: creation errors and values read. Hypothesis to check: The difference between TypedArray constraints and byte access is exposed. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| offset | 0 | 1 | The difference between TypedArray constraints and byte access is exposed. |
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: creation errors and values 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: Alignment and DataView. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Alignment affect in this exercise? Input cases: the same four bytes.
- Variable: offset. Hypothesis to check: The difference between TypedArray constraints and byte access is exposed. Did the actual results agree with the hypothesis? Explain the evidence in terms of DataView.
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: In-place Transformation · Next: Laying Out Multiple Arrays