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UEFI specification 2.7A, EFI_BLOCK_IO_PROTOCOL, page 566. The ioalign property does define the alignment of data buffer. If the alignment is required and our buffer is not aligned, or if the data buffer is not multiple of Blocksize, we need to use bounce buffer to perform the block IO. This is much like with BIOS version, except there the INT13 needs buffer to be located in low memory. Additionally, we need to handle disk writes properly.