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Index > OS Construction > The 18 Mb FAT32 (USB boot and run) limit

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Mike Gonta



Joined: 26 Dec 2010
Posts: 238
Location: the-ideom
Mike Gonta
Not all PC's that support USB booting support the int 13h extensions for LBA addressing. The BIOS emulation
applies only to the legacy int 13h functions which use CHS addressing.
Code:
DRIVE     | Extensions  | LBA support  | C.   H.  S.         | LBA
__________|_____________|______________|_____________________|________________
          |             |
0         | No          | No           | 1024*  2*18 = 18Mb  | N/A
80h       | No          | No           | 1024*255*63 =  8Gb  | N/A
0 or 80h  | Yes         | No           | 1024*255*63 =  8Gb  | N/A
0 or 80h  | Yes         | Yes          | 1024*255*63 =  8Gb  | Size of drive
    

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Mike Gonta
the-ideom - now you know how to compile

https://mikegonta.com


Last edited by Mike Gonta on 22 Apr 2012, 09:25; edited 3 times in total
Post 20 Apr 2012, 06:17
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smiddy



Joined: 31 Oct 2004
Posts: 559
smiddy
Interesting!

It has been a while so forgive my ignorance, wouldn't you have a bigger File Allocation Table with FAT32 (in sectors need per FAT) versus FAT12? Not that size may matter since we're using FASM to write our OS.

Does this also apply to USB CD/DVD/BD drives?
Post 20 Apr 2012, 10:13
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