rEFInd - possible boot 32-bit OS from 64-bit UEFI?

[Gelip]

Can I boot 32-bit Windows on UEFI 64-bit if I compile rEFInd as EBC (EFI Byte Code) ?

[Roderick W. Smith]

I don't think so. There are at least two issues here:

• Although rEFInd might compile as an EBC application, it includes some platform-specific code that will fall back to completely untested generic defaults. If you're very lucky, those might work OK, but I'd place the odds of a failure as quite high. (I'd quite like to get an EBC binary working, but the last I looked into it, it required some exotic development tools that I don't have. IIRC, they were pricey commercial tools.)
• More importantly, rEFInd doesn't launch OS kernels directly (except for Linux kernels with EFI stub support, which fit into a special category all their own). Instead, rEFInd launches follow-on boot loaders. This is how Windows is launched -- rEFInd launches Microsoft's own EFI boot loader. That loader is, AFAIK, platform-specific, and AFAIK it will launch only a like-architecture kernel, so you'd need some alternative way of launching the Windows kernel. I don't know of any way around this limitation. To be sure, it could theoretically be done; it's been done for macOS (for launching 64-bit macOS on 32-bit EFIs) and Linux (GRUB can do it), so getting it to work for Windows is probably possible. I just don't know of any tool to do it. If there were a way around this issue, you could just use the stock 64-bit rEFInd binary.

All that said, there may be another way to get the effect you want: You can use rEFInd's ability to launch a BIOS-mode boot loader to boot Windows in BIOS/CSM/legacy mode. This would require an EFI with a CSM (which most 64-bit AMD64/x86-64/X64 EFIs provide), and you'd probably need Windows to be at least partially installed on an MBR disk rather than a GPT disk. (A hybrid MBR might work, too, but those are ugly and dangerous. There may also be hacks available to get Windows to boot in BIOS mode from GPT disks, but I've not looked into the matter in quite a while.) If it's a desktop computer with multiple disks, these limitations are probably not too onerous, even if you have big disks, since you could keep Windows on an MBR disk but still use GPT disks for data storage.

Depending on your needs, another option is to run 32-bit Windows in a virtualized environment (VirtualBox, KVM/QEMU, VMware, etc.) inside another OS. This is likely to be easier to set up than a dual-boot involving a BIOS/CSM/legacy-mode reboot and the hoops that entails, but it comes with its own set of limitations -- but also its own advantages.