DeviceID & DeviceKey

[lmstearn]

Browsing through some old AHK code using EnumDisplayDevices there was this snippet:

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if (SubStr(DeviceKey,1,18)="\Registry\Machine\")
        DeviceKey := SubStr(DeviceKey,19)

EnumDisplayDevicesA (or its Unicode equivalent) is pretty much the same as it was from Win2k days, assuming the "Not used" tag for DeviceId, and "Reserved" tags for DeviceKey operated then.
As a matter of interest, trawled through the local registry for all occurrences of the DeviceKey value, wherewith some, or all, are likely used by the AMD driver for an old card in the rig, and came up with these:

Spoiler

{$8-$4-$4-$4-$8} := some possible private GUID ref for the monitor ($N => replaced the hex numerics)

The deviceKey value for the first monitor here, is, for example,
"\Registry\Machine\System\CurrentControlSet\Control\Video\{$8-$4-$4-$4-$8}\0000"

to be found in \Device\Video0 key of Computer\HKEY_LOCAL_MACHINE\HARDWARE\DEVICEMAP\VIDEO

Other references to the value in brackets were found in these:
Computer\HKEY_CURRENT_USER\SOFTWARE\ATI\ACE\Settings\ADL\MaxCofuncPaths
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\DirectX\{$8-$4-$4-$4-$8}
Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\UnitedVideo\CONTROL\VIDEO\{$8-$4-$4-$4-$8}
Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Video\{$8-$4-$4-$4-$8}
Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Enum\PCI\VEN_1002&DEV_683D&SUBSYS_25561458&REV_00\4&b77c4c1&0&0008\Device Parameters
Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\amdkmdag\Video
Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\BasicDisplay\Video
Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\UnitedVideo\CONTROL\VIDEO\{$8-$4-$4-$4-$8}
Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Video\{$8-$4-$4-$4-$8}
Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\PCI\VEN_1002&DEV_683D&SUBSYS_25561458&REV_00\4&b77c4c1&0&0008\Device Parameters
Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\amdkmdag\Video
Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\BasicDisplay\Video
Computer\HKEY_USERS\.DEFAULT\Software\ATI\ACE\Settings\ADL\MaxCofuncPaths
Computer\HKEY_USERS\S-1-5-18\Software\ATI\ACE\Settings\ADL\MaxCofuncPaths
Computer\HKEY_USERS\S-1-5-21-3723142744-2120397182-728992050-1000\SOFTWARE\ATI\ACE\Settings\ADL\MaxCofuncPaths

If we were to ever use the deviceKey itself, it surely is just the GUID hex numeric with the curly braces, so extraction goes something like:

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startPos := Instr(DeviceKey, "{")
DeviceKey := SubStr(DeviceKey,startPos, Instr(DeviceKey, "}") - startPos + 1))

From the list, it looks like they are for driver software, and their alteration or removal from the reg will break things quickly. One might infer from the "Reserved" attribute explained here the API has a "private" interface, as it handles quasi "security" keys.
Google doesn't reveal a lot of stuff about it- a lot of interesting threads dealing with low level APIs found in places like OSROnline used to have a high search relevance, not so much these days.