Commit 37b4a909 authored by Simon Glass's avatar Simon Glass Committed by Bin Meng

x86: Mention running U-Boot in 64-bit mode in the README

This feature is not supported. Document this, and add some details on how it
might be implemented.
Signed-off-by: default avatarSimon Glass <>
Reviewed-by: default avatarBin Meng <>
parent 007adbc2
......@@ -1049,11 +1049,29 @@ provides the same EFI run-time services) is not currently supported on x86.
See README.efi for details of EFI support in U-Boot.
64-bit Support
U-Boot supports booting a 64-bit kernel directly and is able to change to
64-bit mode to do so. It also supports (with CONFIG_EFI_STUB) booting from
both 32-bit and 64-bit UEFI. However, U-Boot itself is currently always built
in 32-bit mode. Some access to the full memory range is provided with
The development work to make U-Boot itself run in 64-bit mode has not yet
been attempted. The best approach would likely be to build a 32-bit SPL
image for U-Boot, with CONFIG_SPL_BUILD. This could then handle the early CPU
init in 16-bit and 32-bit mode, running the FSP and any other binaries that
are needed. Then it could change to 64-bit model and jump to U-Boot proper.
Given U-Boot's extensive 64-bit support this has not been a high priority,
but it would be a nice addition.
- Audio
- Chrome OS verified boot
- Building U-Boot to run in 64-bit mode
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