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Re: gate A20 and i386-qemu port

From: Vladimir 'phcoder' Serbinenko
Subject: Re: gate A20 and i386-qemu port
Date: Sat, 14 Nov 2009 13:48:00 +0100
User-agent: Mozilla-Thunderbird (X11/20091109)

Robert Millan wrote:
> I'm wondering how come the i386-qemu port works if none of its
> initialization code takes into account gate A20.  Does this
> mean it's supposed to be already disabled in initial CPU state,
> and only enabled by BIOS?
> Or maybe the code only works by chance and would break unexpectedly
> in specific situations?
Without disabling A20 even and odd mibibytes will be squashed together.
Following code can check A20:
*((volatile grub_uint8_t *) 0x200000) = 0;
*((volatile grub_uint8_t *) 0x300000) = 1;
if (*((volatile grub_uint8_t *) 0x200000))
  grub_printf ("Expect bugs\n");
  grub_printf ("You're safe\n");

Vladimir 'phcoder' Serbinenko

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