OASIS (SEMI P39) layout files vs GDSII
GDSII and OASIS are both binary stream formats for hierarchical integrated-circuit and photomask layout. OASIS is the newer SEMI standard, engineered to store the same geometry in dramatically smaller files. GDSII, decades older, remains the default interchange format between design tools and mask shops.
What is OASIS?
OASIS — the Open Artwork System Interchange Standard — is a layout stream format published by SEMI as standard P39. It was designed in the early 2000s as a modern successor to GDSII, with the explicit goal of shrinking the very large files that dense, hierarchical mask data produces.
Rather than storing every coordinate as a fixed-width integer the way GDSII does, OASIS leans on compact, variable-length number encoding and a richer set of built-in repetition and shape descriptions. Regular arrays, repeated figures, and common rectangles can be expressed with far fewer bytes, and modal state lets a writer omit values that have not changed from the previous element. The net effect, on real designs, is a file often several times smaller than the equivalent GDSII — the size reduction is the format's whole reason for existing. The full record structure is defined in the paid SEMI P39 specification; this page describes the format at a high level from public sources rather than reproducing it.
GDSII vs OASIS: how they compare
The table below is written in our own words from publicly available descriptions of the two formats.
| Aspect | GDSII | OASIS |
|---|---|---|
| Age / origin | Created by Calma in the late 1970s for its GDS II layout systems | Introduced in the early 2000s as a modern successor |
| Standardization body | De facto industry standard; no formal owner today | Formal SEMI standard (P39) |
| Typical file size | Large; fixed-width integer coordinates | Typically several times smaller for the same design |
| Compression approach | None intrinsic; relies on external gzip if any | Built-in compact encoding, modal state, and rich repetition |
| Adoption | Ubiquitous; the default handoff format everywhere | Growing, especially for very large advanced-node datasets |
| Tool support | Read and written by essentially every layout tool | Supported by many modern tools, though less universal |
| Human-readability | Binary; needs a viewer or parser to inspect | Binary; even more compact, so equally opaque without tools |
When to use GDSII vs OASIS
Choose GDSII when maximum compatibility matters. It is the format almost every design tool, foundry flow, and viewer can read without question, which is why it endures as the interchange default even though newer options exist. For a small or moderate design, its larger size rarely matters.
Choose OASIS when file size is the constraint — typically very large, dense datasets at advanced process nodes, where a GDSII file becomes unwieldy and OASIS can cut it down substantially. The trade-off is that not every tool in a given flow may accept OASIS, so confirm that every stage downstream — including the mask shop — reads it before committing to it as your handoff format. Many teams keep GDSII as the lingua franca and reach for OASIS only where the size win is worth the compatibility check.
Does Glyph support OASIS?
Yes. Glyph reads and writes OASIS (.oas/.oasis) as well as GDSII — open a .oas file the same way you'd open a .gds file, and export one from or by picking a .oas filename in Save as…. Both directions run entirely in your browser, so your files never leave your machine.
Need a quick one-off conversion? Convert OASIS to GDSII or GDSII to OASIS in the free browser-based converter — no upload, no installation — or start from the general OASIS converter if your target isn't GDSII.
OASIS export is compressed by default and is not a save-in-place format: it squares round path caps — the one documented geometry loss — and drops bitmaps and construction shapes. Text size, font and justification do survive, carried as OASIS standard (S_GDS_PROPERTY) element properties.
For the full GDSII record catalogue and structure, see the GDSII file format reference. To start editing a .gds file now, open the Glyph editor. Once your handoff format is settled, see maskless lithography for how that file gets exposed without a physical photomask.
Further reading
Primary and reference sources — described and linked, never reproduced here:
- SEMI P39, Specification for OASIS — Open Artwork System Interchange Standard — the official standard: SEMI store product page. The authoritative (paid) definition of the format.
- Wikipedia, "Open Artwork System Interchange Standard" — en.wikipedia.org/wiki/Open_Artwork_System_Interchange_Standard. Background on OASIS and its goals.
- KLayout — klayout.de. A widely used open-source layout viewer/editor that supports both GDSII and OASIS.
- Wikipedia, "GDSII" — en.wikipedia.org/wiki/GDSII. History and background on the older format.
OASIS is a SEMI standard and "OASIS" is a trademark of SEMI; Glyph is not affiliated with, endorsed by, or sponsored by SEMI. The names are used here descriptively to compare file formats. KLayout, Oasis montaj and other product names mentioned on this page are trademarks of their respective owners, used for identification only; no affiliation or endorsement by Glyph is implied.
This comparison is maintained by the authors of Glyph's from-scratch GDSII and OASIS readers and writers — the same team behind NANYTE's maskless lithography tooling.
Frequently asked questions
Is OASIS smaller than GDSII?
Yes — OASIS files are typically several times smaller than the equivalent GDSII design. OASIS achieves this with compact variable-length number encoding, modal state that omits unchanged values, and a richer set of built-in repetition and shape descriptions, instead of GDSII's fixed-width integer coordinates.
Does Glyph support OASIS as well as GDSII?
Yes. Glyph reads and writes both GDSII and OASIS (.oas/.oasis) entirely in your browser — opening or exporting either format runs locally, and your design never leaves your machine.
Can I view an OASIS file online?
Yes. Glyph opens OASIS (.oas/.oasis) layout files — the SEMI P39 IC and photomask format, not the Oasis montaj geoscience software — in your browser with nothing to install, and the file is read locally, never uploaded. You can view and edit the layout and export it back to OASIS or to GDSII.
Should I hand off a design as GDSII or OASIS?
Use GDSII when maximum compatibility matters — it is read by essentially every layout tool and mask shop without question. Use OASIS when file size is the real constraint, typically for very large, dense datasets at advanced process nodes, and only after confirming every downstream tool in your flow, including the mask shop, accepts it.
Does converting between GDSII and OASIS lose anything?
In Glyph, OASIS export squares round path caps, because OASIS has no round-cap concept, and does not carry over bitmaps or construction-only shapes — the same exclusions GDSII export applies. Text size, font and justification do survive, carried as OASIS standard (S_GDS_PROPERTY) element properties. GDSII element properties (PROPATTR/PROPVALUE pairs) read from a .gds file are kept for GDSII re-export but are not written to OASIS.
Is my file uploaded anywhere when I convert between GDSII and OASIS?
No. Both directions run entirely inside your browser tab, using the same reader and writer as the Glyph editor — there is no server involved and your file never leaves your machine.