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[romprefix] Do not clobber stack segment when returning to BIOS
Commit c429bf0 ("[romprefix] Store boot bus:dev.fn address as autoboot device location") introduced a regression by using register %cx to temporarily hold the PCI bus:dev.fn address, despite the fact that %cx was already being used to hold the stored BIOS stack segment. Consequently, when returning to the BIOS after a failed or cancelled boot attempt, iPXE would end up calling INT 18 with the stack segment set equal to the PCI bus:dev.fn address. Writing to essentially random areas of memory tends to upset even the more robust BIOSes. Fix by using register %ax to temporarily hold the PCI bus:dev.fn address. Reported-by: Anton D. Kachalov <mouse@yandex-team.ru> Tested-by: Anton D. Kachalov <mouse@yandex-team.ru> Signed-off-by: Michael Brown <mcb30@ipxe.org>
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