Talk:VGA Hardware: Difference between revisions
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I assume this title was meant to be "Memory Layout in 4-colour (2-bit) modes". In this case AFAIK it's very similar to 4-bit/16-colour modes, except that only 2 planes are used (instead of 4 planes). The same applies to monochrome (1-bit, 2-colour) modes, where only one plane is used.
== Detection ==
There's at least 3 different "hints" that could be used to help VGA detection.
The first hint is the class code in the device's PCI configuration space. If the class code is 0x03 (display device), the sub-class code is 0x00 and the interface code is 0x00, then (according to PCI standards) the device is: ''"VGA-compatible controller. Memory addresses 0A0000h through 0BFFFFh. I/O addresses 3B0h to 3BBh and 3C0h to 3DFh and all aliases of these addresses."''.
Other class codes define other types of display devices. Example, 0x030000 is VGA, 0x030001 is "8514-compatible controller", 0x030100 is "XGA controller", 0x030200 is "3D controller" and 0x038000 is "Other display controller". Also, class code 0x00???? means that "Device was built before Class Code definitions were finalized".
The second hint is VBE. For "Function 00h - Return VBE Controller Information", Bit 1 in the Capabilities field will be either set (Controller is not VGA compatible) or clear (Controller is VGA compatible).
The third hint is BIOS function Int 0x10, ax = 0x1A00 ("GET DISPLAY COMBINATION CODE (PS,VGA/MCGA)"). I'll let Ralph Brown describe it (see http://www.ctyme.com/intr/rb-0219.htm). I'm assuming that if this function is supported and returns 0x07 or 0x08 then it's VGA compatible (however, if this function is supported and returns 0xFF then the controller may or may not be VGA compatible).
Of course I call these "hints" because I don't think any of them are necessarily definitive. A better idea might be to get the PCI vendor and device IDs and implement a "compatible enough to work" white list and a "not compatible enough" black list (and then rely on the hints above for video cards that aren't in either list).
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