enable the registers on the Macbook Pro 5,3

Startup.nsh.md

Put the EFI Shell as EFI_System_Partition/EFI/BOOT/Bootx64.efi and the following script as EFI_System_Partition/EFI/BOOT/Startup.nsh

REM Enabling VGA on the PCI Bridge that the NVidia card is connected to
REM Setting Bridge Control Register (0x3e) to 8
mm 0010003E 8 -PCI

REM Enabling NVidia Card "I/O Space Access" and setting it as Bus Master
REM Setting Command Register (0x04) to 7
mm 03000004 7 -PCI

REM Disabling the integrated NVidia Card.
REM Having only one VGA card is less confusing for Windows. It's on the gMux.
mm 750 2 -IO

REM Switch to the EFI System Partition
fs1:
REM Run Microsoft's Bootloader
EFI\Microsoft\Boot\Bootmgfw.efi

Explanation:

1. start efi shell

2. find the vga controller: (the -b flag reduces output to one page) Code: pci -b here you can see all devices, find the display (vga) controller, in my case: Code: 00 01 00 00

3. check the device config: Code: pci -i 00 01 00 -b

a) on page 1 (in my case) i can see the values for the Command Register (4), the bits (00) I/O space enabled and (02) bus master are set to 1, so nothing to change here.

b) on page 3 (in my case) i can see the values for the Bridge Control Register (3E), the bit (03) VGA Enable is set to 0, this should be 1

4. set the bit mentioned in 3b): Code: mm 0001003E -PCI 8 explanation: a) device is on 000100 (see 2)) b) the Bridge Control Register is 3E (see 3b)) c) the VGA Enable bit is (03), so if you imagine an 8 bit register (00001000 = Bit (07) to Bit (00)) and you want to set bit (03) to 1, you have to feed the register with 00001000 binary, which is 8 decimal).

5. check pci device config again (same code as for 3)) and you should see that the VGA Enable bit (03) in register (3E) is now set to 1.