Software to put on IDE

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By [D-Tail]

Ascended (8249)

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08-05-2004, 20:01

When you say 'create new partition' (dunno the shortcut for that right now), and you input '32M' (for creating a 32MB-sized partition), it won't range from #0000 to #FFFF (which is exactly 32MB in 65536 clusters), but it will make its range in #0000 to #FF79. This way, the cluster size remains limited (yeah, those additional 32 clusters will double your clustersize! It's sad indeed Wink).

By Latok

msx guru (3836)

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08-05-2004, 21:43

I don't really understand, yet. 32mb in 65536 clusters would mean 1 cluster is 2kb, right? But above, it states it's 8 kb or even 16kb?

By Latok

msx guru (3836)

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08-05-2004, 22:02

Waaaa, wait. 32mb=32768kb. 65536 clusters would mean 1 cluster is 0,5kb. What's wrong here?

By Latok

msx guru (3836)

Latok's picture

08-05-2004, 22:06

And 0,5kb = 512 bytes. Which is.......1 sector. But we were talking clusters....

By [D-Tail]

Ascended (8249)

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08-05-2004, 22:40

... Hehe, confusing, those calculations? Tongue

Hokay. We have 32MB.
That's 32768kB.

...erm...

WHOOOW!! My bad Smile It's 65536 sectors Smile yes... Sorry for the inconvenience Tongue

Clusters stand aside from it, that's merely how FAT12 has been designed. Anyway, when you have a partition of EXACTLY 32MB (that is, 65536 sectors Tongue), so 32768kB. So, an easy calculation (1st grade math class Smile) proves us that a sector has a size of 1/2kByte, that is 512 bytes. For some reason, the standard cluster size of a 32MB partition is 16kB. When you decrease the sector size of that partition from #10000 to #FF7A (that's #86, so, 134 sectors, not 32, another miscalculation by me :S), the clustersize becomes 8kB. Dunno exactly why, but that's how it seems.

By the way, the sector size of a partition being #10000 means the range is #0000 - #FFFF. The range of a #FF7A sectors sized partition is #0000 - #FF79. In normal kBytes, that's 65402/2=32701kByte, so that's about 99,7% of 32MB. So actually you exchange 0,3% of 32MB for a HUGE lot of extra space for smaller programs. At least the overall level of slack space will be decreased Smile

By Latok

msx guru (3836)

Latok's picture

08-05-2004, 22:51

Interesting stuff, idd. And I believe indeed fdisk 'knows' this feature and uses #ff7a sectors when creating a 32mb fat12 drive, right? So the clustersize is 8kb.

By [D-Tail]

Ascended (8249)

[D-Tail]'s picture

08-05-2004, 23:00

Ow OFCOURSE!!! (Damn, I'm finally getting somewhere Smile)

So WHY is it called FAT12? Because it can define an absolute position on disk in 12 bits, so ranging from 0 to 4095. The definition of a cluster is as follows: it's the smallest size of a disk which can be used to put files on. So, we have 32768kBs on our partition, which are 65536 sectors.

So there are 65536 sectors on a 32MB partition, divided by 4096 clusters. That makes our math easy again: 65536/4096=16, so the cluster size of a 32768kB partition is 16/2=8kB. So now I'm still wondering why certain FDISKs make the cluster size of an exactly 32MB filled up partition 16kB... Because we could do the same trick with only 8kB...

[edit]YAY! By means of this post, I've got another title Big smile[/edit]

By Latok

msx guru (3836)

Latok's picture

08-05-2004, 23:24

Erhm, indeed. 65536 sectors and 4096 positions. That makes 16 sectors per cluster. That's 8kbyte per cluster, because one sector is 512 bytes, so 16 sectors is indeed 8 kbyte. I agree with D-Tail Tongue Why do certain FDISKs make the clustersize 16kb? Do they, actually? Wink

By [D-Tail]

Ascended (8249)

[D-Tail]'s picture

08-05-2004, 23:30

Well... That was at least the argument, I believe. I recall it was stated in some manual, why a 32MB partition should be a little smaller than a real 32MB partition, thus decreasing the cluster size. Right now I will examine the FDISK310.TXT file Tongue

By [D-Tail]

Ascended (8249)

[D-Tail]'s picture

08-05-2004, 23:43

Well... The manual just states the following:

[(c) Jon de Schrijder]

|size in sectors|FATtype|MediaID|rootDIRentries|sectors/cluster|
|---------------+-------+-------+--------------+---------------+
|  32733-65404  | FAT12 |  F0   |     256      |      16       |
| 65405-262610  | FAT16 |  F0   |     512      |       4       |

I've got a slight idea why FAT12 isn't defined beyond 65404 sectors. Well... Assuming we deal only with FAT12.
I think it has something to do with the sectors used by the BOOT, FAT, ROOT, etcetera. These things could consume 132 sectors altogether, thus the actual partition space combined with the 'administrative things (ROOT, FAT, etc)' would be 65536 sectors exactly. So when you're about to make an exactly 32MB sized partition, it will mean you use 32MB for data. Just a few kBs will be added because of the administrative part of the partition.

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