I did new tests on ASUS:
- I installed WinXP x64 in legacy mode on a 100MB FAT32 + 5GB NTFS MBR disk
- I installed iGPU Intel driver
- I made an image of NTFS partition in Acronis BR
- I cleaned the disk and created GPT partitions:
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select disk 0
clean
convert gpt
create partition efi size=100
format quick fs=fat32 label="System"
create partition msr size=16
create partition primary size=5120
format quick fs=ntfs label="Windows"
create partition primary
format quick fs=ntfs label="Data"
- using Acronis BR, I restored the system from the image to the NTFS partition (Windows)
- I have copied efi files to the FAT32 partition and winload.efi to NTFS
- in the BCD file I turned on the debugger and connected a second computer with WinDbg
If I boot the system with CSM enabled and Secure Boot disabled then I have error IRQL_NOT_LESS_OR_EQUAL (0a) caused by VIDEOPRT.SYS:
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FAULTING_IP:
nt!MmMapIoSpace+46e
fffff800`018e2cae 668b4e1a mov cx,word ptr [rsi+1Ah]
If I boot the system with CSM disabled and Secure Boot enabled then I have error PAGE_FAULT_IN_NONPAGED_AREA (50) caused by igxpmp64.sys:
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FAULTING_IP:
hal!x86BiosExecuteInterrupt+76d
fffff800`01ca664d 0fb608 movzx ecx,byte ptr [rax]
This means that WinXP can also be booted from a GPT partition in pure UEFI with the Secure Boot option enabled
WinDbg log CSM on, Secure Boot off: https://pastebin.com/RkxNLrQR
WinDbg log CSM off, Secure Boot on: https://pastebin.com/KPCduyAF