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From:   Brian Gerst <brgerst@gmail.com>
Date:   Thu, 27 Sep 2018 14:22:55 -0400
Message-ID: <CAMzpN2iC7-CaxAsgrq-jwujgMYRqPf_5DLihKUHF5F6a1nxabQ@mail.gmail.com>
Subject: Re: [PATCH] Reduce boot header size with 1 byte
To:     xyproto@archlinux.org
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Sep 27, 2018 at 9:42 AM <xyproto@archlinux.org> wrote:
>
> From: Alexander F. R=C3=B8dseth <xyproto@archlinux.org>
>
> Only ah needs to be set to 0 before calling interrupt 0x16 for waiting
> for a keypress.
>
> This patch changes the line that uses xor so that it only zeroes "ah" ins=
tead of "ax".
> This saves a byte.
>
> Signed-off-by: Alexander F. R=C3=B8dseth <xyproto@archlinux.org>
> ---
>  arch/x86/boot/header.S | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 850b8762e889..905cb96f43d4 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -71,14 +71,15 @@ msg_loop:
>         jmp     msg_loop
>
>  bs_die:
> -       # Allow the user to press a key, then reboot
> -       xorw    %ax, %ax
> +       # Allow the user to press a key
> +       xorb    %ah, %ah

Because this is code running in 16-bit real mode, xorw does not need a
16-bit prefix and only uses 2 bytes.  You save nothing by using xorb.

--
Brian Gerst