CGX File Format - WRcad

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The Computer Graphics eXchange (CGX) format is a simple binary data format somewhat similar to GDSII, but designed to be more compact and efficient. This format, developed by Whiteley Research, is hereby placed in the public domain without restriction.

Like GDSII, files consist of a sequential list of variable-length records. It has simplified record structure, but extensions in data flexibility. If is more compact than GDSII and is more efficient to read and write. The advantages of CGX are smaller files and potentially faster read/write than GDSII.

CGX Format Identifier

The first three bytes of a CGX file are 'c', 'g', and 'x'. The fourth byte is an integer format level. A parser designed to handle a certain level will accept that level and any value lower. Presently, the only existing level is 0, thus this byte should be set to 0.

CGX Data Types

CGX uses the same long (4-byte) and short (2-byte) integer formats as GDSII, and the same 8-byte floating point format. These are the only numerical data types defined.

A date is stored as 8 bytes, as shown in the following table. These are the same numerical fields as used in GDSII, though the format is different (bytes are used where possible, rather than shorts). The third column gives the value in terms of the members of the tm structure from the C library.

short	year	tm_year + 1900
byte	month	tm_mon + 1
byte	day	tm_mday
byte	hour	tm_hour
byte	minute	tm_min
byte	second	tm_sec
byte	0

As in GDSII, string data are terminated with a null byte, with an additional null byte added if necessary to make the total length even.

CGX Data Records

The four-byte file header is followed by any number of data records, the last of which signals the end of data. There are 11 defined record types. Each record begins with a 4-byte header:

short	recsize
byte	rectype
byte	flags

The recsize field is a short integer giving the total record size, including the header. Thus, as in GDSII, records are limited to 64K bytes in length. The record size will always be an even number. The rectype byte is set to a small integer to define the type of record. The flags byte is used in some of the record types, otherwise it is ignored.

The defined record types are given in the table below.

rectype	name
0	LIBRARY
1	STRUCT
2	CPRPTY
3	PROPERTY
4	LAYER
5	BOX
6	POLY
7	WIRE
8	TEXT
9	SREF
10	ENDLIB

It is allowable to define additional record types for local or proprietary purposes. If a parser encounters an unknown record type, it may skip over the record, ignoring it.

LIBRARY record

The LIBRARY record should be the first data record in the file, and can appear once only.

The flags byte of the record header can be used for a version number, which identifies in some way the remaining data in the file.

The LIBRARY record contains the following fields:

bytes	field name	purpose
8	munit	machine units
8	uunit	user units
8	cdate	library creation date
8	mdate	library modification date
?	libname	library name string

The first two fields are double-precision numbers that define the scale factors for the data in the file. These are interpreted in the same way as the similar fields in the header of a GDSII file.

The second two fields represent creation and modification dates for the file content.

A name string for the library follows. Strings are null-byte terminated, and an additional null byte is added if necessary so that the total length is even.

STRUCT record

The STRUCT record opens a cell structure. Records that follow will be assigned to that cell, until another STRUCT record is seen.