[x86 setup] Make sure AH=00h when setting a video mode

Passing a u8 into a register doesn't mean gcc will zero-extend it.
Also, don't depend on thhe register not to change.

Per bug report from Saul Tamari.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/arch/i386/boot/video-vga.c b/arch/i386/boot/video-vga.c
index d660e60..aef02f9 100644 (file)
--- a/arch/i386/boot/video-vga.c
+++ b/arch/i386/boot/video-vga.c
@@ -73,9 +73,10 @@ static u8 vga_set_basic_mode(void)
                mode = 3;
 
        /* Set the mode */
+       ax = mode;
        asm volatile(INT10
-                    : : "a" (mode)
-                    : "ebx", "ecx", "edx", "esi", "edi");
+                    : "+a" (ax)
+                    : : "ebx", "ecx", "edx", "esi", "edi");
        do_restore = 1;
        return mode;
 }