The 18 Mb FAT32 (USB boot and run) limit

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Mike Gonta

Joined: 26 Dec 2010
Posts: 243

Mike Gonta 20 Apr 2012, 06:17

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
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https://mikegonta.com

Last edited by Mike Gonta on 22 Apr 2012, 09:25; edited 3 times in total

20 Apr 2012, 06:17

smiddy

Joined: 31 Oct 2004
Posts: 557

smiddy 20 Apr 2012, 10:13

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?

20 Apr 2012, 10:13

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