DOS HDD Size Limits - the history

[pearce_jj]

Looking for some input on the time line of the 16MB hard disk partition limit.

It seems DOS 2.1 will work with 32MB partitions because it can use 8K clusters, but I had thought that the maximum cluster size was 4K at that point.

Detail:

• Virtual machine running IBM PC-DOS 2.1 with a 40MB hard drive.
• fdisk 2.00 (c) 1983 sees this disk as 647 cylinders (63 sectors per track, 2 heads).
• Creating a 511 cylinder partition this can then be formatted fine and is just under 32MB.
• DOS has created the parition using 8K clusters, which I didn't think it would.

Cheers!

 

[per]

Are refering to MS-DOS 2.11, or PC-DOS 2.1?

I thought PC-DOS 2.1 only could partion disks up to 10MB, but I may be wrong since I have never actually tried to partion any drive using that version.

 

[pearce_jj]

Hi, thanks, it's IBM PC-DOS 2.1.

 

[modem7]

Looking for some input on the time line of the 16MB hard disk partition limit.

From http://e-articles.info/e/a/title/DOS-Versions/

"PC DOS 2.0 was introduced on March 8, 1983 (along with the IBM PC XT), and was virtually a complete rewrite over the previous versions. DOS 2.0 added many new features and functions (mostly derived from UNIX), including a tree-structured (hierarchical) file system, support for hard disk drives up to 16.76MB (15.98 MiB) using FAT12 ..."

 

[pearce_jj]

This is the confusing thing. Every reference I can find states that DOS 3 added support for 32MB paritions via FAT-16, yet I've shown that DOS 2.1 could also support 32MB partitions with FAT-12 using 8K clusters.

 

[modem7]

Having a better look on the Internet, I see much about the FAT12 limit being quoted as 32MB.

The Wikipedia article on 'File Allocation Table' includes, "MS-DOS 3.0 could still access MS-DOS 2.0 style 8 KB-cluster partitions".

The "16MB maximum" figure that's quoted in places may have come about because in the days of DOS 2, hard drive sizes were small and accordingly, FDISK in DOS 2 always chose to use 4K sized clusters.

 

[pearce_jj]

Interesting, thanks. Seems to be much confusion on this in the Wikipedia FAT article and on the MS KB - from the latter,

MS-DOS Versions 2.x
MS-DOS 2.x supports one type 01 partition of up to 15 megabytes (MB) in size, which uses a 12-bit file allocation table (FAT). Fdisk creates only one MS-DOS partition per drive.

I have read that DOS 2.0 was hard-coded for the 10MB HDD in the PC/XT, DOS 2.1 adding support for disks generally. Although I don't have 2.0 media to test that.

But since 2.1 supported 32MB partition, 'support for larger disks' in DOS 3.0 was via extended partitions only then, FAT-16 being only an efficiency thing.

Cheers!