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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 = DX = 0, 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.


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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?
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