OSX kernel (here) takes /efi/platform/system-id, makes following byte stream:
0x2A, 0x06, 0x19, 0x90, 0xD3, 0x8D, 0x44, 0x40, 0xA1, 0x39, 0xC4, 0x97, 0x70, 0x37, 0x65, 0xAC, <16 bytes from /efi/platform/system-id>
and then does SHA1 hash of that, and then use 16 bytes from that as IOPlatformUUID (Hardware UUID).