GDS File

The .gds file is the established exchange format for integrated circuit (IC) and MEMS mask layout data, formally known as GDSII (Graphic Database System II) or the Calma Stream Format. Developed by the Calma Company in 1978, it became the de facto tape-out format that chip designers deliver to semiconductor foundries for photomask fabrication.

A .gds file is a binary stream composed of length-prefixed records, stored in big-endian byte order. Each record begins with a 4-byte header encoding the record length, type, and data type, followed by the data payload. The file always opens with a HEADER record (hex signature 00 06 00 02) and closes with ENDLIB. Between those markers the format organizes IC geometry in a strict hierarchy: library → structures (cells) → elements. Supported element types include:

• BOUNDARY - filled polygons (the primary shape)
• PATH - wire segments with a defined width
• SREF / AREF - single and array cell references for compact hierarchy
• TEXT - string labels with position, orientation, and magnification
• BOX and NODE - additional annotation elements

All geometry resides on numbered layers, with integer coordinates scaled by a UNITS record that maps database units to meters.

A .gds file stores only geometry and hierarchy - it is not a schematic or netlist. File sizes range from kilobytes for a single cell to many gigabytes for a full-reticle mask set. Large files are commonly distributed as .gds.gz using external gzip compression. For modern high-density designs, the OASIS format offers better compression and faster parsing and is increasingly preferred over .gds, though .gds remains the dominant interchange standard across the EDA industry and is accepted by virtually every commercial foundry.