[syslinux] isohybrid and ISO images whose size is not a multiple of 2048 bytes vs. VirtualBox

Ady ady-sf at hotmail.com
Tue Feb 17 14:52:46 PST 2015

> An alternative would be to request a cylinder size which
> is divisible by 2048. E.g.
>   isohybrid -h 252 -s 63
> The cylinder sizes 64x32 and 255x63 emerged as best
> candidates when we compared our collections of questionable
> experience and rumors. More substance is not to see behind
> them.
> (Actually Cylinder/Head/Sector addressing is so brain
>  damaged that only ill firmware can insist in it having
>  a particular form. All sane firmwares will simply use
>  LBA numbers for addressing.)
Let's not forget that the goal usually is to select CHS values so that 
the resulting (optical and/or other) media would boot correctly in as 
many systems as possible while maintaining a "sane" file system. This 
is the reason for the recommended '-h' and '-s' values.

I'd rather not comment (at least, not at this opportunity) on the 
(minimal and preferred) conditions to be (hard-code) included in the 
source code.

Now, if the proposed (temporal?) workaround is to use a different pair 
of '-h' and '-s' values, I would suggest choosing them according to 
commonly-used conventions or standards for CHS translations (e.g. 
h={15,16,32,64,128,240,255}; s={16,32,63}) instead of completely 
arbitrary values. That is, in addition to making the total size a 
multiple of 2048 bytes, avoiding partial cylinders and maintaining the 
resulting amount of cylinders to equal or less than 1024.


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