338 lines
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338 lines
18 KiB
HTML
<HTML>
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<HEAD><TITLE>Computer Aids for VLSI Design</TITLE></HEAD>
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<BODY BGCOLOR="#FFFFFF">
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<HR BREAK>
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<CENTER><FONT SIZE=6><B>Computer Aids for VLSI Design</B></FONT></CENTER>
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<CENTER><FONT SIZE=5><B><I>Steven M. Rubin</I></B></FONT></CENTER>
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<CENTER><FONT SIZE=4><B>Copyright © 1994</B></FONT></CENTER>
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<HR>
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<H1><CENTER>Appendix C: <A NAME="GDSFormat">GDS II Format</A></CENTER></H1>
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<CENTER><TABLE BORDER=0><TR><TD><A HREF="chapb.html"><IMG SRC="../images/iconbackarrow.gif" ALT="Prev" BORDER=0></A></TD>
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<TD><B><CENTER><A HREF="../index.html">Appendix 3 of 7</A></CENTER></B></TD>
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<TD><A HREF="chapd.html"><IMG SRC="../images/iconforearrow.gif" ALT="Next" BORDER=0></A></TD></TR></TABLE></CENTER>
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<HR>
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<P>
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In the design of integrated circuits, the most popular format for
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interchange is the Calma <B>GDS II</B> stream format (GDS II is a trademark
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of Calma Company, a wholly owned subsidiary of General Electric Company, U.S.A.).
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For many years, this format was the only one of its kind and many other
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vendors accepted it in their systems.
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Although Calma has updated the format as their CAD systems have developed,
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they have maintained backward compatibility so that no GDS II files
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become obsolete.
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This is important because
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GDS II is a binary format that makes assumptions about integer and
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floating-point representations.
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<P>
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A GDS II circuit description is a collection of cells that may contain
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geometry or other cell references.
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These cells, called <B><A NAME="GDSStructure">structures</A></B> in GDS II parlance, have alphanumeric
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names up to 32 characters long.
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A library of these structures is contained in a file that consists of
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a library header, a sequence of structures, and a library tail.
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Each structure in the sequence consists of a structure header, a sequence
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of <B><A NAME="GDSElement">elements</A></B>, and a structure tail.
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There are seven kinds of elements: <B>boundary</B> defines a filled polygon,
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<B>path</B> defines a wire, <B>structure reference</B> invokes a subcell,
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<B>array reference</B> invokes an array of subcells, <B>text</B> is for
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documentation, <B>node</B> defines an electrical path, and <B>box</B>
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places rectangular geometry.
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<P><HR><BR><H2>C.1 Record Format</H2><P>
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In order to understand the precise format of the above GDS II components,
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it is first necessary to describe the general record format.
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Each GDS II record has a 4-byte header that specifies the record size and
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function.
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The first 2 bytes form a 16-bit integer that contains the record length
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in bytes.
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This length includes the 4-byte header and must always be an even
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number.
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The end of a record can contain a single null byte if the record contents
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is an odd number of bytes long.
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The third byte of the header contains the type of the record and the
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fourth byte contains the type of the data.
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Since the data type is constant for each record type, this 2-byte field
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defines the possible records as shown in Figs. C.1 and C.2.
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<P>
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<CENTER><TABLE BORDER=2><TR><TD><TABLE>
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<TR><TD><B>File Header Records:</B></TD><TD><B>Bytes 3 and 4</B> </TD><TD><B>Parameter Type</B></TD></TR>
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<TR><TD COLSPAN=3><HR></TD></TR>
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<TR><TD>HEADER</TD><TD><CENTER>0002</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>BGNLIB</TD><TD><CENTER>0102</CENTER></TD><TD>12 2-byte integers</TD></TR>
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<TR><TD>LIBNAME</TD><TD><CENTER>0206</CENTER></TD><TD>ASCII string</TD></TR>
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<TR><TD>REFLIBS</TD><TD><CENTER>1F06</CENTER></TD><TD>2 45-character ASCII strings</TD></TR>
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<TR><TD>FONTS</TD><TD><CENTER>2006</CENTER></TD><TD>4 44-character ASCII strings</TD></TR>
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<TR><TD>ATTRTABLE</TD><TD><CENTER>2306</CENTER></TD><TD>44-character ASCII string</TD></TR>
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<TR><TD>GENERATIONS</TD><TD><CENTER>2202</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>FORMAT</TD><TD><CENTER>3602</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>MASK</TD><TD><CENTER>3706</CENTER></TD><TD>ASCII string</TD></TR>
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<TR><TD>ENDMASKS</TD><TD><CENTER>3800</CENTER></TD><TD>No data</TD></TR>
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<TR><TD>UNITS</TD><TD><CENTER>0305</CENTER></TD><TD>2 8-byte floats</TD></TR>
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<TR><TD COLSPAN=3> </TD></TR>
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<TR><TD><B>File Tail Records:</B></TD><TD><B>Bytes 3 and 4</B></TD><TD><B>Parameter Type</B></TD></TR>
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<TR><TD COLSPAN=3><HR></TD></TR>
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<TR><TD>ENDLIB</TD><TD><CENTER>0400</CENTER></TD><TD>No data</TD></TR>
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<TR><TD COLSPAN=3> </TD></TR>
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<TR><TD><B>Structure Header Records: </B></TD><TD><B>Bytes 3 and 4</B></TD><TD><B>Parameter Type</B></TD></TR>
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<TR><TD COLSPAN=3><HR></TD></TR>
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<TR><TD>BGNSTR</TD><TD><CENTER>0502</CENTER></TD><TD>12 2-byte integers</TD></TR>
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<TR><TD>STRNAME</TD><TD><CENTER>0606</CENTER></TD><TD>Up to 32-characters ASCII string</TD></TR>
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<TR><TD COLSPAN=3> </TD></TR>
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<TR><TD><B>Structure Tail Records:</B></TD><TD><B>Bytes 3 and 4</B></TD><TD><B>Parameter Type</B></TD></TR>
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<TR><TD COLSPAN=3><HR></TD></TR>
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<TR><TD>ENDSTR</TD><TD><CENTER>0700</CENTER></TD><TD>No data</TD></TR>
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</TABLE></TD></TR><TR><TD><B>FIGURE C.1 </B>GDS II header record types.</TD></TR></TABLE></CENTER>
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<P>
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Magnetic tapes containing GDS II files will have 2048 byte blocks that
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contain these records.
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The block size is standardized but has no bearing on record length or
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position.
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There is also a capability for circuits that require multiple
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reels of tape.
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<P>
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<CENTER><TABLE BORDER=2><TR><TD><TABLE>
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<TR><TD><B>Element Header Records:</B></TD><TD><B>Bytes 3 and 4 </B></TD><TD><B>Parameter Type</B></TD></TR>
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<TR><TD COLSPAN=3><HR></TD></TR>
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<TR><TD>BOUNDARY</TD><TD><CENTER>0800</CENTER></TD><TD>No data</TD></TR>
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<TR><TD>PATH</TD><TD><CENTER>0900</CENTER></TD><TD>No data</TD></TR>
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<TR><TD>SREF</TD><TD><CENTER>0A00</CENTER></TD><TD>No data</TD></TR>
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<TR><TD>AREF</TD><TD><CENTER>0B00</CENTER></TD><TD>No data</TD></TR>
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<TR><TD>TEXT</TD><TD><CENTER>0C00</CENTER></TD><TD>No data</TD></TR>
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<TR><TD>NODE</TD><TD><CENTER>1500</CENTER></TD><TD>No data</TD></TR>
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<TR><TD>BOX</TD><TD><CENTER>2D00</CENTER></TD><TD>No data</TD></TR>
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<TR><TD COLSPAN=3> </TD></TR>
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<TR><TD><B>Element Contents Records: </B></TD><TD><B>Bytes 3 and 4</B></TD><TD><B>Parameter Type</B></TD></TR>
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<TR><TD COLSPAN=3><HR></TD></TR>
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<TR><TD>ELFLAGS</TD><TD><CENTER>2601</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>PLEX</TD><TD><CENTER>2F03</CENTER></TD><TD>4-byte integer</TD></TR>
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<TR><TD>LAYER</TD><TD><CENTER>0D02</CENTER></TD><TD>2-byte integers</TD></TR>
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<TR><TD>DATATYPE</TD><TD><CENTER>0E02</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>XY</TD><TD><CENTER>1003</CENTER></TD><TD>Up to 200 4-byte integer pairs</TD></TR>
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<TR><TD>PATHTYPE</TD><TD><CENTER>2102</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>WIDTH</TD><TD><CENTER>0F03</CENTER></TD><TD>4-byte integer</TD></TR>
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<TR><TD>SNAME</TD><TD><CENTER>1206</CENTER></TD><TD>Up to 32-character ASCII string</TD></TR>
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<TR><TD>STRANS</TD><TD><CENTER>1A01</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>MAG</TD><TD><CENTER>1B05</CENTER></TD><TD>8-byte float</TD></TR>
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<TR><TD>ANGLE</TD><TD><CENTER>1C05</CENTER></TD><TD>8-byte float</TD></TR>
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<TR><TD>COLROW</TD><TD><CENTER>1302</CENTER></TD><TD>2 2-byte integers</TD></TR>
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<TR><TD>TEXTTYPE</TD><TD><CENTER>1602</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>PRESENTATION</TD><TD><CENTER>1701</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>ASCII STRING</TD><TD><CENTER>1906</CENTER></TD><TD>Up to 512-character string</TD></TR>
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<TR><TD>NODETYPE</TD><TD><CENTER>2A02</CENTER></TD><TD>2-byte integer</TD></TR>
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<TR><TD>BOXTYPE</TD><TD><CENTER>2E02</CENTER></TD><TD>2-byte integer</TD></TR>
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</TABLE></TD></TR><TR><TD><B>FIGURE C.2 </B>GDS II element record types.</TD></TR></TABLE></CENTER>
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<P><HR><BR><H2>C.2 <A NAME="GDSLibrary">Library</A> Head and Tail</H2><P>
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A GDS II file header always begins with a HEADER record the parameter of which
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contains the GDS II version number used to write the file.
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For example, the bytes 0, 6, 0, 2, 0, 1 at the start of the file constitute
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the header record for a version-1 file.
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Following the HEADER comes a BGNLIB record that contains the date of the
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last modification and the date of the last access to the file.
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Dates take six 2-byte integers to store the year, month, day, hour, minute, and
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second.
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The third record of a file is the LIBNAME, which identifies the name of
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this library file.
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For example, the bytes 0, 8, 2, 6, "C", "H", "I", "P" define a
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library named "CHIP."
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Following the LIBNAME record there may be any of the optional header records:
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REFLIBS to name up to two reference libraries, FONTS to name up to four
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character fonts, ATTRTABLE to name an attribute file, GENERATIONS to
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indicate the number of old file copies to keep, and FORMAT to indicate the
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nature of this file.
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The strings in the REFLIBS, FONTS, and ATTRTABLE records must be the
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specified length, padded with zero bytes.
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<P>
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The parameter to FORMAT has the value 0 for an archived file and the
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value 1 for a filtered file.
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Filtered files contain only a subset of the mask layers and that subset
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is described with one or more MASK records followed by an ENDMASK
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record.
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The string parameter in a MASK record names layers and sequences of layers;
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for example, "1 3 5-7."
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<P>
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The final record of a file header is the UNITS record and it is not optional.
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The parameters to this record contain the number of user units per
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database unit (typically less than 1 to allow granularity of user
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specification) and the number of meters per database unit (typically much
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less than 1 for IC specifications).
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<P>
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Eight-byte floating-point numbers have a sign at the top of the first
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byte, a 7-bit exponent in the rest of that byte, and 7 more bytes that
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compose a mantissa (all to the right of an implied decimal point).
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The exponent is a factor of 16 in excess-64 notation (that is, the mantissa
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is multiplied by 16 raised to the true value of the exponent, where the
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true value is its integer representation minus 64).
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<P>
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Following the file header records come the structure records.
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After the last structure has been defined, the file terminates with
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a simple ENDLIB record.
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Note that there is no provision for the specification of a root
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structure to define a circuit; this must be tracked
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by the designer.
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<P><HR><BR><H2>C.3 Structure Head and Tail</H2><P>
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Each structure has two header records and one tail record that sandwich
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an arbitrary list of elements.
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The first structure header is the BGNSTR record, which contains the creation
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date and the last modification date.
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Following that is the STRNAME record, which names the structure using
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any alphabetic or numeric characters, the dollar sign, or the underscore.
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The structure is then open and any of the seven elements can be listed.
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<P>
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The last record of a structure is the ENDSTR.
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Following it must be another BGNSTR or the end of the library, ENDLIB.
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<P><HR><BR><H2>C.4 <A NAME="GDSBoundary">Boundary Element</A></H2><P>
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The boundary element defines a filled polygon.
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It begins with a BOUNDARY record, has an optional ELFLAGS and PLEX record,
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and then has required LAYER, DATATYPE, and XY records.
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<P>
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The ELFLAGS record, which appears optionally in every element, has two
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flags in its parameter to indicate template data (if bit 16 is set)
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or external data (if bit 15 is set).
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This record should be ignored on input and excluded from output.
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Note that the GDS II integer has bit 1 in the leftmost or most significant
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position so these two flags are in the least significant bits.
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<P>
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The PLEX record is also optional to every element and defines element
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structuring by aggregating those that have common plex numbers.
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Although a 4-byte integer is available for plex numbering, the
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high byte (first byte) is a flag that indicates the head of the plex if its least
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significant bit (bit 8) is set.
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<P>
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The LAYER record is required to define which layer (numbered 0 to 63) is to
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be used for this boundary.
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The meaning of these layers is not defined rigorously and must be
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determined for each design environment and library.
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<P>
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The DATATYPE record contains unimportant information and its argument should
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be zero.
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<P>
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The XY record contains anywhere from four to 200 coordinate pairs
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that define the outline of the polygon.
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The number of points in this record is defined by the record length.
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Note that boundaries must be closed explicitly, so the first and last
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coordinate values must be the same.
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<P><HR><BR><H2>C.5 <A NAME="GDSPath">Path Element</A></H2><P>
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A path is an open figure with a nonzero width that is typically used
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to place wires.
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This element is initiated with a PATH record followed by the optional ELFLAGS
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and PLEX records.
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The LAYER record must follow to identify the desired path material.
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Also, a DATATYPE record must appear and an XY record to define the coordinates
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of the path.
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From two to 200 points may be given in a path.
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<P>
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Prior to the XY record of a path specification there may be two
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optional records called PATHTYPE and WIDTH.
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The PATHTYPE record describes the nature of the path segment
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ends, according to its parameter value.
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If the value is 0, the segments will have square ends that terminate
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at the path vertices.
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The value 1 indicates rounded ends and the value 2 indicates
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square ends that overlap their vertices by one-half of their width.
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The width of the path is defined by the optional WIDTH record.
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If the width value is negative, then it will be independent of any
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structure scaling (from MAG records, see next section).
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<P><HR><BR><H2>C.6 <A NAME="GDSStructureRef">Structure Reference Element</A></H2><P>
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Hierarchy is achieved by allowing structure references (instances)
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to appear in other structures.
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The SREF record indicates a structure reference and is followed
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by the optional ELFLAGS and PLEX records.
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The SNAME record then names the desired structure and an XY record
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contains a single coordinate to place this instance.
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It is legal to make reference to structures that have not yet been
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defined with STRNAME.
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<P>
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Prior to the XY record there may be optional transformation records.
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The STRANS record must appear first if structure transformations are desired.
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Its parameter has bit flags that indicate mirroring in <I>x</I> before rotation
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(if bit 1 is set), the use of absolute magnification (if bit 14 is set),
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and the use of absolute rotation (if bit 15 is set).
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The magnification and rotation amounts may then be specified in the
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optional MAG and ANGLE records.
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The rotation angle is in counterclockwise degrees.
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<P><HR><BR><H2>C.7 <A NAME="GDSArray">Array of Structures Element</A></H2><P>
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For convenience, an array of structure instances can be specified with
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the AREF record.
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Following the optional ELFLAGS and PLEX records comes the SNAME to identify
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the structure being arrayed.
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Next, the optional transformation records STRANS, MAG, and ANGLE give
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the orientation of the instances.
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A COLROW record must follow to specify the number of columns and
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the number of rows in the array.
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The final record is an XY with three points: the coordinate of the
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corner instance, the coordinate of the last instance in the columnar
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direction, and the coordinate of the last instance in the row direction.
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From this information, the amount of instance overlap or separation
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can be determined.
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Note that flipping arrays (in which alternating rows or columns are mirrored
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to abut along the same side) can be implemented with multiple arrays that
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are interlaced and spaced apart to describe alternating rows or columns.
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<P><HR><BR><H2>C.8 <A NAME="GDSText">Text Element</A></H2><P>
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Messages can be included in a circuit with the TEXT record.
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The optional ELFLAGS and PLEX follow with the mandatory LAYER record
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after that.
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A TEXTTYPE record with a zero argument must then appear.
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An optional PRESENTATION record specifies the font in bits 11 and 12,
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the vertical presentation in bits 13 and 14 (0 for top, 1 for
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middle, 2 for bottom), and the horizontal presentation in bits 15 and 16
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(0 for left, 1 for center, 2 for right).
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Optional PATHTYPE, WIDTH, STRANS, MAG, and ANGLE records may appear to
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affect the text.
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The last two records are required: an XY with a single coordinate to
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locate the text and a STRING record to specify the actual text.
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<P><HR><BR><H2>C.9 <A NAME="GDSNode">Node Element</A></H2><P>
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Electrical nets may be specified with the NODE record.
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The optional ELFLAGS and PLEX records follow and the required LAYER
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record is next.
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A NODETYPE record must appear with a zero argument, followed by an XY record
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with one to 50 points that identify coordinates on the electrical net.
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The information in this element is not graphical and does not affect the
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manufactured circuit.
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Rather, it is for other CAD systems that use topological information.
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<P><HR><BR><H2>C.10 <A NAME="GDSBox">Box Element</A></H2><P>
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The last element of a GDS II file is the box.
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Following the BOX record are the optional ELFLAGS and PLEX records,
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a mandatory LAYER record, a BOXTYPE record with a zero argument,
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and an XY record.
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The XY must contain five points that describe a closed, four-sided box.
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Unlike the boundary, this is not a filled figure.
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Therefore it cannot be used for IC geometry.
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<P>
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<HR>
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<TABLE COLSPEC="L40 L40 L40 L40 L40 L40 L40 L40 L40 L40" BORDER=0><TR>
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<TD> </TD>
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<TD><ADDRESS><A HREF="mailto:strubin@rulabinsky.com"><CENTER>Steven M. Rubin</CENTER></A></ADDRESS></TD>
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