[PATCH] Trim a few bytes from boot.img

[PATCH] Trim a few bytes from boot.img

[Colin Watson]

A while back, I was looking for eight extra bytes in boot.img so that I
could put a keyboard modifier check in there, allowing me to avoid
printing "GRUB loading" by default, which my boss^3 has been asking me
for, without crippling debugging in the process.  I knew I could find a
few by shortening error messages (e.g. "Hard Disk" -> "HD"), but I asked
Colin King, one of our kernel hackers, if he could find a nicer
approach.  He came up with a patch which I massaged a bit into this.
What do people think?

It seems as if it might be generally useful to have a bit of spare space
in here, although I would certainly appreciate it if people didn't use
it all at once. ;-)

2010-02-10  Colin King  <colin.king@ubuntu.com>
2010-02-10  Colin Watson  <cjwatson@ubuntu.com>

	Shrink the pre-partition-table part of boot.img by eight bytes.

	* boot/i386/pc/boot.S (ERR): New macro.
	(chs_mode): Use ERR.
	(geometry_error): Likewise.
	(hd_probe_error): Remove.  This is only used once, so we wrwite
	it inline instead.
	(read_error): Instead of printing read_error_string, just set up
	%si and fall through to ...
	(error_message): ... this new function, also used by ERR.

=== modified file 'boot/i386/pc/boot.S'
--- boot/i386/pc/boot.S	2010-01-03 22:05:07 +0000
+++ boot/i386/pc/boot.S	2010-02-10 11:32:08 +0000
@@ -27,6 +27,7 @@
 
 	/* Print message string */
 #define MSG(x)	movw $x, %si; call LOCAL(message)
+#define ERR(x)	movw $x, %si; jmp LOCAL(error_message)
 
 	.file	"boot.S"
 
@@ -233,7 +234,7 @@ LOCAL(chs_mode):
 	jz	LOCAL(floppy_probe)
 
 	/* Nope, we definitely have a hard disk, and we're screwed. */
-	jmp	LOCAL(hd_probe_error)
+	ERR(hd_probe_error_string)
 
 LOCAL(final_init):
 	/* set the mode to zero */
@@ -360,22 +361,15 @@ LOCAL(copy_buffer):
  * BIOS Geometry translation error (past the end of the disk geometry!).
  */
 LOCAL(geometry_error):
-	MSG(geometry_error_string)
-	jmp	LOCAL(general_error)
-
-/*
- * Disk probe failure.
- */
-LOCAL(hd_probe_error):
-	MSG(hd_probe_error_string)
-	jmp	LOCAL(general_error)
+	ERR(geometry_error_string)
 
 /*
  * Read error on the disk.
  */
 LOCAL(read_error):
-	MSG(read_error_string)
-
+	movw	$read_error_string, %si
+LOCAL(error_message):
+	call	LOCAL(message)
 LOCAL(general_error):
 	MSG(general_error_string)
 

Thanks,

-- 
Colin Watson                                       [cjwatson@ubuntu.com]

Re: [PATCH] Trim a few bytes from boot.img

[Vladimir 'φ-coder/phcoder' Serbinenko]

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Colin Watson wrote:
> A while back, I was looking for eight extra bytes in boot.img so that I
> could put a keyboard modifier check in there, allowing me to avoid
> printing "GRUB loading" by default, which my boss^3 has been asking me
> for, without crippling debugging in the process.  I knew I could find a
> few by shortening error messages (e.g. "Hard Disk" -> "HD"), but I asked
> Colin King, one of our kernel hackers, if he could find a nicer
> approach.  He came up with a patch which I massaged a bit into this.
> What do people think?
>
> It seems as if it might be generally useful to have a bit of spare space
> in here, although I would certainly appreciate it if people didn't use
> it all at once. ;-)
>   
Looks goo. Can you merge it into experimental?
> 2010-02-10  Colin King  <colin.king@ubuntu.com>
> 2010-02-10  Colin Watson  <cjwatson@ubuntu.com>
>
> 	Shrink the pre-partition-table part of boot.img by eight bytes.
>
> 	* boot/i386/pc/boot.S (ERR): New macro.
> 	(chs_mode): Use ERR.
> 	(geometry_error): Likewise.
> 	(hd_probe_error): Remove.  This is only used once, so we wrwite
> 	it inline instead.
> 	(read_error): Instead of printing read_error_string, just set up
> 	%si and fall through to ...
> 	(error_message): ... this new function, also used by ERR.
>
> === modified file 'boot/i386/pc/boot.S'
> --- boot/i386/pc/boot.S	2010-01-03 22:05:07 +0000
> +++ boot/i386/pc/boot.S	2010-02-10 11:32:08 +0000
> @@ -27,6 +27,7 @@
>  
>  	/* Print message string */
>  #define MSG(x)	movw $x, %si; call LOCAL(message)
> +#define ERR(x)	movw $x, %si; jmp LOCAL(error_message)
>  
>  	.file	"boot.S"
>  
> @@ -233,7 +234,7 @@ LOCAL(chs_mode):
>  	jz	LOCAL(floppy_probe)
>  
>  	/* Nope, we definitely have a hard disk, and we're screwed. */
> -	jmp	LOCAL(hd_probe_error)
> +	ERR(hd_probe_error_string)
>  
>  LOCAL(final_init):
>  	/* set the mode to zero */
> @@ -360,22 +361,15 @@ LOCAL(copy_buffer):
>   * BIOS Geometry translation error (past the end of the disk geometry!).
>   */
>  LOCAL(geometry_error):
> -	MSG(geometry_error_string)
> -	jmp	LOCAL(general_error)
> -
> -/*
> - * Disk probe failure.
> - */
> -LOCAL(hd_probe_error):
> -	MSG(hd_probe_error_string)
> -	jmp	LOCAL(general_error)
> +	ERR(geometry_error_string)
>  
>  /*
>   * Read error on the disk.
>   */
>  LOCAL(read_error):
> -	MSG(read_error_string)
> -
> +	movw	$read_error_string, %si
> +LOCAL(error_message):
> +	call	LOCAL(message)
>  LOCAL(general_error):
>  	MSG(general_error_string)
>  
>
> Thanks,
>
>   


-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko



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Re: [PATCH] Trim a few bytes from boot.img

[Colin Watson]

On Tue, Feb 16, 2010 at 12:53:51PM +0100, Vladimir 'φ-coder/phcoder' Serbinenko wrote:
> Colin Watson wrote:
> > A while back, I was looking for eight extra bytes in boot.img so that I
> > could put a keyboard modifier check in there, allowing me to avoid
> > printing "GRUB loading" by default, which my boss^3 has been asking me
> > for, without crippling debugging in the process.  I knew I could find a
> > few by shortening error messages (e.g. "Hard Disk" -> "HD"), but I asked
> > Colin King, one of our kernel hackers, if he could find a nicer
> > approach.  He came up with a patch which I massaged a bit into this.
> > What do people think?
> >
> > It seems as if it might be generally useful to have a bit of spare space
> > in here, although I would certainly appreciate it if people didn't use
> > it all at once. ;-)
> 
> Looks goo. Can you merge it into experimental?

Done.  It's /people/cjwatson/trim-boot-img as well.

Thanks,

-- 
Colin Watson                                       [cjwatson@ubuntu.com]