Syslinux - Mar 2017 - where to swap

It is good to see this

----- Forwarded message from syslinux-bot for Martin Str|mberg <ams at
ludd.ltu.se> -----

Date: Sat, 25 Mar 2017 05:54:09 -0700
From: syslinux-bot for Martin Str|mberg <ams at ludd.ltu.se>
To: Syslinux commits mailing list <syslinux-commits at zytor.com>
Cc: ams at ludd.ltu.se
Subject: [syslinux:master] mbr/isohdpfx.S: correct pointer for heads/sectors
X-Mailer: Syslinux-git-log-daemon

Commit-ID:  48e94f4fa7b3c32cbd43b6e57c64bc933f76d059
Gitweb:    
http://www.syslinux.org/commit/48e94f4fa7b3c32cbd43b6e57c64bc933f76d059
Author:     Martin Str|mberg <ams at ludd.ltu.se>
AuthorDate: Sat, 25 Mar 2017 08:15:04 -0400
Committer:  Gene Cumm <gene.cumm at gmail.com>
CommitDate: Sat, 25 Mar 2017 08:15:10 -0400

mbr/isohdpfx.S: correct pointer for heads/sectors

sectors is pushed first then heads but pointers were in reverse order.

Signed-off-by: Gene Cumm <gene.cumm at gmail.com>

---
 mbr/isohdpfx.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mbr/isohdpfx.S b/mbr/isohdpfx.S
index 17e1efe..a693ccb 100644
--- a/mbr/isohdpfx.S
+++ b/mbr/isohdpfx.S
@@ -48,8 +48,8 @@ isolinux_start_hybrid		= 0x7c00+64+4
 stack		= 0x7c00
 partoffset	= (stack-8)
 driveno		= (stack-14)
-heads		= (stack-16)
-sectors		= (stack-18)
+sectors		= (stack-16)
+heads		= (stack-18)
 ebios_flag	= (stack-20)
 secpercyl	= (stack-24)
 
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----- End forwarded message -----


And please see also the patch in
http://www.syslinux.org/archives/2017-March/025706.html

In that patch is the push to stack order swapped.


So my question is:  Where to swap?




Groeten
Geert Stappers
-- 
Leven en laten leven

On Sat, Mar 25, 2017 at 06:51:50PM +0100, Geert Stappers via Syslinux
wrote:
> It is good to see this
> ----- Forwarded message from syslinux-bot for Martin Str|mberg <ams at
ludd.ltu.se> -----
> Date: Sat, 25 Mar 2017 05:54:09 -0700
> From: syslinux-bot for Martin Str|mberg <ams at ludd.ltu.se>
> Subject: [syslinux:master] mbr/isohdpfx.S: correct pointer for
heads/sectors
> Commit-ID:  48e94f4fa7b3c32cbd43b6e57c64bc933f76d059
> Gitweb:    
http://www.syslinux.org/commit/48e94f4fa7b3c32cbd43b6e57c64bc933f76d059
> Author:     Martin Str|mberg <ams at ludd.ltu.se>
...
> diff --git a/mbr/isohdpfx.S b/mbr/isohdpfx.S
> index 17e1efe..a693ccb 100644
> --- a/mbr/isohdpfx.S
> +++ b/mbr/isohdpfx.S
> @@ -48,8 +48,8 @@ isolinux_start_hybrid		= 0x7c00+64+4
>  stack		= 0x7c00
>  partoffset	= (stack-8)
>  driveno		= (stack-14)
> -heads		= (stack-16)
> -sectors		= (stack-18)
> +sectors		= (stack-16)
> +heads		= (stack-18)
>  ebios_flag	= (stack-20)
>  secpercyl	= (stack-24)
Yes. It is. However by now we know that that correction is wrong. You
need to revert that and use the one in
<http://www.syslinux.org/archives/2017-March/025706.html>. Here pasted
(so might not apply cleanly):

diff --git a/mbr/isohdpfx.S b/mbr/isohdpfx.S
index 17e1efe..14eca14 100644
--- a/mbr/isohdpfx.S
+++ b/mbr/isohdpfx.S
@@ -151,7 +151,7 @@ next:

   /* Check to see if we have EBIOS */
      pushw %dx	      /* drive number */
-    movb      $0x41, %ah      /* %al == 0 already */
+    movb      $0x41, %ah
     movw      $0x55aa, %bx
     xorw     %cx, %cx
     xorb %dh, %dh
@@ -167,20 +167,22 @@ next:
       read_sector_cbios: movb $0x42, %ah ;  jmp read_common */
     movl	 $0xeb42b4+((read_common-read_sector_cbios-4) << 24),
(read_sector_cbios)
-	jmp	1f
+	jmp	2f
1:
+	xor	%cx, %cx	/* Clear EBIOS flag. */
+2:
	popw	%dx
 	pushw	%cx		/* EBIOS flag */

	/* Get (C)HS geometry */
     movb	     $0x08, %ah
     int    $0x13
-	    andw	$0x3f, %cx	/* Sector count */
     popw	%bx		/* EBIOS flag */
-	pushw		%cx	      /* -16: Save sectors on the stack */
     movzbw	%dh, %ax     /* dh = max head */
     incw %ax     	/* From 0-based max to count */
-	 pushw	%ax		/* -18: Save heads on the stack */
+   pushw %ax		/* -16: Save heads on the stack */
+   andw  $0x3f, %cx	/* Sector count */
+   pushw %cx    	/* -18: Save sectors on the stack */
    mulw	%cx		/* Heads*sectors -> sectors per cylinder */

    pushw   %bx 		/* -20: EBIOS flag */


Given David's last report the part:
-       jmp     1f
+       jmp     2f
1:
+       xor     %cx, %cx        /* Clear EBIOS flag. */
+2:

can be ignored. His BIOS isn't corrputing CX (which I
thought). However that part doesn't hurt if it remains.


-- 
MartinS

Hi,

Martin Str|mberg wrote:
> Given David's last report the part:
> -       jmp     1f
> +       jmp     2f
> 1:
> +       xor     %cx, %cx        /* Clear EBIOS flag. */
> +2:
> 
> can be ignored. His BIOS isn't corrputing CX (which I
> thought). However that part doesn't hurt if it remains.
I think it still has good reason to exist. The test

	cmpw	$0xaa55, %bx
	jne	1f

could cause a jump which could push a non-zero CX to the stack.
(Whatever reason there might be not to get the 0xaa55 signature
 but to get non-zero CX.)

I have now updated the Debian bug report:
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=857597#55


Have a nice day :)

Thomas