Background
The channel for transmitting the EDID from the display to the graphics card is usually the I²C bus, defined in DDC2B (DDC1 used a different serial format which never gained popularity).
Before DDC and EDID were defined, there was no standard way for a graphics card to know what kind of display device it was connected to. Some VGA connectors in personal computers provided a basic form of identification by connecting one, two or three pins to ground, but this coding was not standardized.
The EDID is often stored in the monitor in a memory device called a serial PROM (programmable read-only memory) or EEPROM (electrically erasable PROM) and is accessible via the I²C bus at address 0x50. The EDID PROM can often be read by the host PC even if the display itself is turned off.
Many software packages can read and display the EDID information, such as read-edid and PowerStrip for Microsoft Windows and XFree86 (which will output the EDID to the log if verbose logging is on (startx -- -logverbose 6)) for Linux and BSD unix. Mac OS X natively reads EDID information (see /var/log/system.log or hold down Cmd-V on startup) and programs such as SwitchResX or DisplayConfigX can display the information as well as use it to define custom resolutions.
Read more about this topic: Extended Display Identification Data
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