[SeaBIOS] A20: ahci_process_op is called with A20 off

Xu, Anthony anthony.xu at intel.com
Fri May 12 01:22:02 CEST 2017

> SeaBIOS has a couple of different methods to accomplish this mode
> switching - it can directly switch modes (C16_BIG switch) or it can
> use a helper in SMM mode to perform the switch (C16_SMM).  The
> preferred method is C16_SMM as C16_BIG isn't available in VM86 mode
> and it also clobbers the 16bit segment registers.

Hi Kevin,

Why does SeaBios use VM86 mode?
vtx supports real mode long time ago.

C16_SMM is slow because it causes several extra switches between VM and QEMU.
I would prefer C16_BIG for better performance.
Does SeaBios know if it runs in real mode or VM86 mode?
Can SeaBios choose C16_SMM/C16_BIG based on VCPU mode?


More information about the SeaBIOS mailing list