[syslinux] "isolinux.bin missing or corrupt" when booting USB flash drive in old PC

[Martin Str|mberg]

On Wed, Mar 22, 2017 at 12:06:12PM +0100, Thomas Schmitt via Syslinux wrote:
> There is a suspicious discrepancy in the code:
> 
>   heads		= (stack-16)
>   sectors	= (stack-18)
> 
>   ...
> 	/* here we computer CHS values or just do some dummy computation for EBIOS */
> 	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 */
> 	mulw	%cx		/* Heads*sectors -> sectors per cylinder */
> 
> Does "-16: Save sectors on the stack" contradict "sectors = (stack-18)" ?

Yes.

Congratulations Thomas, you've found the bug! Very well done. Thank you.

Changing the code to
sectors     = (stack-16)
heads       = (stack-18)

lets my always fail EBIOS detection version to boot the iso.

Please David if you see this, try this.


Patch:
diff --git a/mbr/isohdpfx.S b/mbr/isohdpfx.S
index 6d8ab36..5a8b267 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)
 

-- 
MartinS