Re: [PATCH] Reduce boot header size with 1 byte
On Thu, Sep 27, 2018 at 9:42 AM wrote: > > From: Alexander F. Rødseth > > 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" instead > of "ax". > This saves a byte. > > Signed-off-by: Alexander F. Rødseth > --- > 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
Re: [PATCH] Reduce boot header size with 1 byte
On 9/27/18 6:41 AM, xypr...@archlinux.org wrote: > From: Alexander F. Rødseth > > 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" instead > of "ax". > This saves a byte. > > Signed-off-by: Alexander F. Rødseth Hi, You should Cc: the X86 maintainers on this patch. Otherwise it will just be lost in the ether. thanks. > --- > 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 > int $0x16 > + > + # Boostrap by reading sector 1, head 0, track 0 from drive > int $0x19 > > - # int 0x19 should never return. In case it does anyway, > - # invoke the BIOS reset code... > - ljmp$0xf000,$0xfff0 > + # If boostrap somehow did not work, invoke the BIOS reset code > + ljmp$0xf000, $0xfff0 > > #ifdef CONFIG_EFI_STUB > .org0x3c > -- ~Randy
[PATCH] Reduce boot header size with 1 byte
From: Alexander F. Rødseth 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" instead of "ax". This saves a byte. Signed-off-by: Alexander F. Rødseth --- 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 int $0x16 + + # Boostrap by reading sector 1, head 0, track 0 from drive int $0x19 - # int 0x19 should never return. In case it does anyway, - # invoke the BIOS reset code... - ljmp$0xf000,$0xfff0 + # If boostrap somehow did not work, invoke the BIOS reset code + ljmp$0xf000, $0xfff0 #ifdef CONFIG_EFI_STUB .org0x3c -- 2.19.0
