extended display identification data (DDC) (EDID)
The Video Electronics Standards Association(VESA) has standardized the interface and communication between the graphics card and the displays or monitors with the Display Data Channel ( DDC). Corresponding contacts are provided in the VGA interface and in the DVI connector for this communication. The transmitted data and their formats are defined in the Extended Display Identification Data (EDID) and are transmitted via a connector contact.
The EDID data is queried via a functionally extended interrupt 10h. With this interrupt, the graphics cards can query the status of the display. The EDID data blocks are 128 bytes or 256 bytes long depending on the device and consist of an 8 byte header, which marks the beginning of the data block, followed by data fields for the manufacturer code, which is assigned by Microsoft, the product identification, a numeric serial number, the date of manufacture, the EDID version, the monitor's mode of operation( analog/ digital), screen size, gamma factor, monitor characteristics, color characteristics (10 bytes), screen resolution, timing (18 bytes), refresh rate, pixel clock and line frequency, product identification and checksum.
Using this EDID data, the graphics card can adjust to the monitor characteristics.
Since EDID was already used for analog displays and CRT monitors, VESA has developed an improved standard for the identification of digital displays with Display Identification( DisplayID).