I recently received an email form Bela Lubkin, who pointed out some mistakes I made in my previous hack:

In grub-0.97_macrpro_esb2_ahci_stage1.patch, I happened to randomly notice a bug. (Ran across it while googling information to get my Dell notebook w/Ubuntu 8.10 to use ahci rather than ata_piix driver...)

The bug: you've moved the setup of the stack segment register (%ss) after the setup of the stack pointer (%sp). I don't have full context (didn't bother to find the stage1.S full file you're patching), so I don't know if it's OK that you are pushing %edx onto [%old-ss:$STAGE1_STACKSEG]. But probably not. But even worse is the "sti /* we're safe again */". Ancient 8086 mistake. You can't enable interrupts until the stack is setup correctly. Move the %ss setup code back to where it was.

I assume you moved it because you wanted to preserve the fact that %ax == 0 on exit of this bit of code. Well, I did find the grub 0.97 source to make sure: both %al and %ah are subsequently overwritten before being used. You don't have to preserve it.

You can save the whole push/pop %dx: find the comment "%dl may have been clobbered ...", move your code immediately before its `popw %dx; pushw %dx'. This does mean your hack isn't effective if grub is being booted from a floppy, but ... not a problem.

You can also save a few more code bytes. I assume this is being compiled as 16-bit (8086) code, e.g. with ".code16" GNU `as` directive. Thus, the instructions `push %edx' and `pop %edx' need a code32 prefix; replace with `push %dx; pop %dx'. Replace `mov $0xcfc,%dx' with `mov $0xfc,%dl'. Replace `xorl %eax,%eax' with `xor %ax,%ax'.

And he was even kind enough to send me a fix for these, so many thanks to him.

Here are links for the new patch he sent me, and an updated stage1 binary.