[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>

1 files changed, 3 insertions, 2 deletions

diff --git a/arch/i386/boot/video-vga.c b/arch/i386/boot/video-vga.c
index d660e608cd58..aef02f9ec0c1 100644
--- 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;
 }