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More than two hard disks in DOS

Investigating the rather odd behavior of the Microsoft OS/2 1.21 disk driver led me to Compaq and their EXTDISK.SYS driver. While experimenting with various setups, I realized that DOS versions older than 5.0 do not support more than two hard disks exposed by the system’s BIOS, and will in fact quite likely hang early during boot-up if there are “too many” hard disks.

This seems to have been one of the many things that “everyone knew” back in the day, similar to the fact that DOS versions older than 3.3 may hang while booting from disks with significantly more than 17 sectors per track. The code near LOG2LOOP label attempts to calculate the cluster shift for the FAT file system, but gets stuck in an endless loop because the BPB for a drive was never initialized and contains zeros. I believe clone BIOSes with support for more than two IDE hard disks generally started appearing only since 1994 or so, and assumed (not unreasonably) that the user would be installing DOS 5.0 or later.

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