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

[Thomas Schmitt]

Hi,

my proposal is based on a mathematical proof (see mail
before) and yours is correct too. They cannot but match.

Let's inspect your examples:

Ady wrote:
>  ISO size: 1'085'736'960 bytes ( > 1GiB)

This are exactly 132 cylinders of 255x63.
132 is divisible by 4. So there is no padding needed.

> Cylinders (1st attempt)=132 
> [...]
>  2120580 / 4 = 530145, so the answer is "yes".

My calculation gets align_factor = 4.
So isohybrid would pad up to quadruple cylinders.
Well, 132 is already such a quadruple.


> When comparing your code with my suggested calculation, I see that your 
> conditionals do not account for the ISO image size, but only for 
> "Heads" and "Sectors_per_track".

The existing code in isohybrid already handles the size
for padding up to a target granularity. Currently the
target is always a single cylinder.
My new code only decides about the granularity of 1, 2,
or 4 cylinders.


>  ISO size: 1'085'736'960 bytes+2048 bytes
> [...]
> 136 * 255 * 63 = 2184840
> 2184840 / 4 = 546210 so the answer is "yes".

My proposal:
Granularity is still 4 cylinders.
Image size is 132.0002489884843 cylinders.
So the target size is the next multiple of 4: 136 cylinders.

Your algorithm is more complicated than mine, because it
tries to do the part of the job which is already
implemented in isohybrid:

    cylsize = head * sector * 512;
    frac = isostat.st_size % cylsize;
    padding = (frac > 0) ? cylsize - frac : 0;

isostat.st_size is the ISO image size in bytes.
cylsize is the current granularity of 1 cylinder.
My proposal has 1, 2, or 4 cylinders instead.

    cylsize = head * sector * 512;
    align_cylsize = cylsize * align_factor;
    frac = isostat.st_size % align_cylsize;
    padding = (frac > 0) ? align_cylsize - frac : 0;

Numbers inserted:

    cylsize = 255 * 63 * 512 = 8225280
    align_cylsize = 8225280 * 4 = 32901120 
    frac = (1085736960 + 2048) % 32901120 = 2048
    padding = 32901120 - 2048 = 32899072

This means that the image will be padded up by nearly 32 MB
to 1085736960 + 2048 + 32899072 = 1118638080 bytes.
This are  1118638080 / 255 / 63 / 512 = 136 cylinders.

Same result as yours.


Have a nice day :)

Thomas