in believe that all readers of this forum have upgraded to 10.10.5 and 10.11.6 neither of which are supported by apple, and i am using a R9 280x not flashed which is also not supported by apple.
i believe there is a solution, but which radeon card is appropriate i have yet to discover.
“Most” after-market graphics cards (targeted at the PC market) work fine after the OS boots.
What “most” cards lack is a EFI section in the on-board firmware. Without this EFI section, the Mac cannot properly initialise the card during the EFI firmware boot environment. This results on no display output aka “no boot screen output”. However, once Mac OS takes over, it can load the kernel extensions (kext, also known as a driver) for the graphics card chip set. If Apple has shipped any graphics card with that family chipset, then Mac OS will have a driver (kext). The kext will match multiple cards from multiple vendors.
The exception is typically any new card that utilises a chipset that was first released after Mac OS El Capitan 10.11.6. Since there are no more El Capitan software updates (apart from Security Updates), El Capitan will not get a new driver/kext from Apple. (Oops, this forum is for Yosemite, the same “expectation/rules” apply)
Nvidia have now taken over the role of distributing drivers for their cards directly. There’s a chance that drivers from Nvidia might support newer cards/chipsets on El Capitan (read the Nvidia release notes).
“Some” after-market cards have firmware storage that is large enough to contain both a BIOS and an EFI section. Apple-branded cards typically ship with both BIOS and EFI sections in their firmware. The community has taken advantage of this fact, and there are numerous cases of the EFI part being extracted from an Apple card, and then grafted onto the firmware for an after-market PC brand card. Sometimes there’s a bit of “fix-up” required to map the video-path to the correct physical connector (e.g. DVI-D port 1 on a two-or-more port card). Generally though, most “grafts” simply do the job.
I have an XFX-brand ATI/AMD Radeon 5770 that I flashed to have an EFI section (as well as the original BIOS section). It works well for both EFI boot screen output and acceleration in Mac OS.
I also have an EVGA-brand Nvidia GTX 680. Here we fall foul of the move from 32-bit-EFI code to 64-bit-EFI code. For a while, Nvidia firmware had 32-bit code, then Nvidia started to use EFI Byte Code also known as EBC. EBC is an intermediate code that is interpreted. Motherboard EFI is required to have an EBC interpreter, therefore a graphics card that has an EFI section encoded as EBC is compatible with both 32 and 64 bit motherboard EFI Firmware. Great! Unfortunately, around the 680 generation Nvidia/Apple started to ship cards with just BIOS and 64-bit-EFI. Our original MacPros cannot initialise those cards because of the 64-bit EFI code in the ROM. They still work once Mac OS has booted, you just cannot see the EFI boot screens.
Having said all of that, “most” cards revert to the lowest-common-denominator which is typically analog VGA output if they are not otherwise initialised. If your card has VGA capability, either having a dedicated VGA connector, or if you have DVI-I connector(s), then you can use a VGA monitor-cable-adaptor combination to see boot screens over VGA.
The Netkas forums are where I went when working on my XFX. I learned a lot from the posts over there...
