EDID

Extended Display IDentification is data about the monitor, which the monitor provides to the video card. EDID is the programmatical way to check if a video mode is supported. However, this procedure is rather complex. (See VBE.)

Reading EDID

The first step is, naturally, to actually get the data. This is done through the BIOS - INT 0x10, AX=4F15h, BL=01h, CX = Controller Unit, DX = EDID block, ES:DI = 128-Byte Buffer.

mov ax, 0x4f15
mov bl, 0x01
xor cx, cx
xor dx, dx
int 0x10
;AL = 0x4F if function supported
;AH = status (0 is success, 1 is fail)
;ES:DI contains the EDID

Note that this code will only run in Real Mode or Virtual 8086 Mode.

EDID Data

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EDID record:

Offset (Bytes)	Length	Description
00h	8 Bytes	Padding
08h	1 Word	Manufacture ID (Big-Endian)
0Ah	1 Word	EDID ID code
0Ch	1 DWord	Serial Number
10h	1 Byte	Manufacture Week
11h	1 Byte	Manufacture Year
12h	1 Byte	EDID Version
13h	1 Byte	EDID Revision
14h	1 Byte	Video Input Type
15h	1 Byte	Max Horizontal Size (cm)
16h	1 Byte	Max Vertical Size (cm)
17h	1 Byte	Gama Factor
18h	1 Byte	DPMS Flags
19h	10 Bytes	Chroma Information
23h	1 Byte	Established Timings 1
24h	1 Byte	Established Timings 2
25h	1 Byte	Manufacture's Reserved Timings
26h	8 Words	Standard Timing Identification
36h	18 Bytes	Detailed Timing Description 1
48h	18 Bytes	Detailed Timing Description 2
5Ah	18 Bytes	Detailed Timing Description 3
6Ch	18 Bytes	Detailed Timing Description 4
7Eh	1 Byte	Unused
7Fh	1 Byte	Checksum (Low Byte of 16-bit sum of 00-7Eh)

Video Input Type:

Bit(s)	Description
0	Separate Sync
1	Composite Sync
2	Sync on green
3-4	Unused?
5-6	Voltage Level
7	Digital Signal

DPMS Flags:

Bit(s)	Description
0-2	Unused?
3	Display Type (1=RGB)
4	Unused?
5	Active Off Supported
6	Suspend Supported
7	Standby Supported

Chroma Information:

Byte	Description
0	Green X'/Y' and Red X'/Y'
1	White X'/Y' and Blue X'/Y'
2	Red Y
3	Red X
4	Green Y
5	Green X
6	Blue Y
7	Blue X
8	White Y
9	White X

Detailed Timing Description:

Byte	Description
0	Horizontal Frequency (kHz)
1	Vertical Frequency (Hz)
2	Horizontal Active Time
3	Horizontal Blanking Time
4	Horizontal Active Time / Horizontal Blanking Time
5	Vertical Active time
6	Vertical Blanking Time
7	Vertical Active Time / Vertical Blanking Time
8	Horizontal Sync Offset
9	Horizontal Sync Pulse-width
10	Vertical Sync Offset / Vertical Sync Pulse-width
11	Vertical/Horizontal Sync Offset / Pulse-width
12	Horizontal Image Size (mm)
13	Vertical Image Size (mm)
14	Horizontal Image Size / Vertical Image Size
15	Horizontal Border
16	Vertical Border
17	Type Of Display

Type Of Display:

Bit(s)	Description
7	Interlaced
5-6	Stereo Mode (00 - No Stereo, 01 - Right Stereo Sync High, 10 - Left Stereo Sync High)
4-3	Sync Type (00 - Analog Composite, 01 - Bipolar Analog Composite, 10 - Digital Composite, 11 - Digital Separate)
2	Serrate (OR Vertical Sync Polarity IF 'Sync Type' == 11)
1	Sync Location (OR Horizontal Sync Polarity IF 'Sync Type' == 11)
0	Unused?

Get preferred resolution

void get_preferred_resolution(uint8_t *edid, int *x, int *y) {
    *x = edid[0x38] | ((int) (edid[0x3A] & 0xF0) << 4);
    *y = edid[0x3B] | ((int) (edid[0x3D] & 0xF0) << 4);
}

See Also

External Links

• Online parser for EDID information
• VBE/DDC 1.1 Specification
• VESA EDID 1.4 Specification