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

Ady ady-sf at hotmail.com
Thu Feb 19 16:21:04 PST 2015

> Hi,
> > Hopefully I am explaining it better this time.
> Our ideas of implementation are supposed to yield the
> same result.
Hmm, I am not sure. Or perhaps I am misunderstanding your code.

Let me try with an example (or two). For this example, let's assume:
 -h 255
 -s 63
 ISO size: 1'085'736'960 bytes ( > 1GiB)
This size happens to be a multiple of:
 63, 255 and 2048

So, using these values in my suggested loop.

1_ Calculate a potential Cylinders (amount) value;

  trunc(roundup( ISO_size / (Heads * Sectors_per_track * 512))

Cylinders (1st attempt)=

Cylinders (1st attempt)=

Cylinders (1st attempt)=132 

2_ Calculate 'Cylinders * Heads * Sectors_per_track';

 132 * 255 * 63 = 2120580

3._ Is the result of the calculation (as explained in the previous step 
#2) a multiple of '4'?

 2120580 / 4 = 530145, so the answer is "yes".

3.a_ If it is, continue (to step #4).

Since the answer was "yes", we end the loop and we have found the 
minimal Cylinders amount that achieves our conditions.

Now, if I take your code:
  if (Heads * Sectors_per_track) is divisible by 4
     # Pad up to next full cylinder
     align_factor = 1
  else if (Heads * Sectors_per_track) is divisible by 2
     # Pad up to next full double cylinder 
     align_factor = 2
     # Pad up to next full quatruple cylinder (nearly 32 MB)
     align_factor = 4

For the example I am presenting here, "Heads*Sectors_per_track" is 
255*63, so it is not a multiple of 4 nor of 2. Would you add anyway 
nearly 32MiB? I assume you wouldn't. Instead, I guess you would first 
evaluate whether it is already a full "valid" cylinder and that no 
additional zero-padding would be required.

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".

So let's assume a new example. Let's add a few bytes to the size of the 
ISO image:
 -h 255
 -s 63
 ISO size: 1'085'736'960 bytes+2048 bytes
 ISO size: 1'085'739'008 bytes  ( > 1GiB)

Now let's perform the calculations for this new example:

Cylinders (1st attempt)=

Cylinders (1st attempt)=

Cylinders (1st attempt)=133

 133 * 255 * 63 = 2136645

Is it multiple of '4'?

 2136645 / 4 = 534161.25, so the answer is "no".

3.b_ If it is not, then add '+1' to the attempted Cylinders (amount) 
value and loop.

 134 * 255 * 63 = 2152710

Is it multiple of '4'?

 2152710 / 4 = 538177.5 so the answer is "no".

 135 * 255 * 63 = 2168775

Is it multiple of '4'?

 2168775 / 4 = 542193.75 so the answer is "no".

 136 * 255 * 63 = 2184840

 2184840 / 4 = 546210 so the answer is "yes".

Since the answer was "yes", we end the loop and we have found the 
minimal Cylinders amount that achieves our conditions.

In this second example, we have started with a first attempt of 133 for 
the amount of complete Cylinders, and we would had to add 3 more, up to 

Other values of "ISO_size", "Heads" and "Sectors_per_track" might lead 
my suggested loop to add either 0, 1, 2, or 3 complete Cylinders. The 
zero-padding could be between 0 bytes and "almost 4 complete Cylinders" 
(4 complete Cylinders minus 2048 bytes).

Now, considering our mutual slight misunderstandings throughout this 
email thread, I have to wonder whether the wording you (Thomas) used 
for your 3 variants of zero-padding, together with the 3 conditionals, 
are equivalent to the 4 possible loops I am suggesting.

Although the size of the input image doesn't seem to be considered in 
the conditionals you (Thomas) presented, I could assume that this is 
just "implied" in the pseudo-code, similarly to what happens in my 
suggested loop (the size of the image is used in the calculations 
before the loop, not part of it). If it is not, then we have a first 
difference in our approach.

The second difference could be the 3 conditionals vs the 4 loops. As 
seen in my second example above, depending on the size of the input 
image the required padding varies, even with a "fixed" pair of "Heads" 
and "Sectors_per_track" values. Although I have not tried to find 
specific examples for having to add between 1 and 2 complete Cylinders 
(of zero-padding bytes), the second example above should show that the 
possibility indeed exists.

So, unless I am misinterpreting the wording in your code:
 "# Pad up to next full ... cylinder" ,
my second example suggests that having 4 ranges (coded as 4 loops or 
using conditionals or however it could be done) might be able to 
achieve better optimal values (less zero-padding) at least for some 

I must apologize for the long emails. Certainly if I would know how to 
code it myself it would be shorter to explain.

Thank you for your attention.


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