by tzs | 2025-04-06 15:41:12+00:00
I don't know if this had much affect on anything, but another thing that hindered using FreeBSD for some users was that Linux worked better as a dual boot system with DOS/Windows on a typical home PC.

There were two problems.

The first was that FreeBSD really wanted to own the whole disk. If you wanted to dual boot with DOS/Windows you were supposed to put FreeBSD on a separate disk. Linux was OK with just having a partition on the same disk you had DOS/Windows on. For those of us whose PCs only had one hard disk, buying a copy of Partition Magic was cheaper than buying a second hard disk.

The reason for this was that the FreeBSD developers felt that multiple operating system on the same disk was not safe due to the lack of standards for how to emulate a cylinder/head/sector (CHS) addressing scheme on disks that used logical block addressing (LBA). They were technically correct, but greatly overestimated the practical risks.

In the early days PC hard disks used CHS addressing, and the system software such as the PC BIOS worked in those terms. Software using the BIOS such as DOS applications and DOS itself worked with CHS addresses and the number of cylinders, heads, and sectors per track (called the "drive geometry") they saw matched the actual physical geometry of the drive.

The INT 13h BIOS interface for low level disk access allowed for a maximum of 1024 cylinders, 256 heads, and 63 sectors per track (giving a maximum possible drive size of 8 GB if the sectors were 512 bytes).

At some point as disks got bigger drives with more than 63 sectors per track became available. If you had a drive with for example 400 cylinders, 16 heads, and 256 sectors per track you would only be able to access about 1/4 of the drive using CHS addressing that uses the actual drive geometry.

It wasn't really practical to change the INT 13h interface to give the sectors per track more bits, and so we entered the era of made up drive geometries. The BIOS would see that the disk geometry is 400/16/256 and make up a geometry with the same capacity that fit within the limits, such as 400/256/16.

Another place with made up geometry was SCSI disks. SCSI used LBA addressing. If you had a SCSI disk on your PC whatever implemented INT 13h handling for that (typically the BIOS ROM on your SCSI host adaptor) would make up a geometry. Different host adaptor makers might use different algorithms for making up that geometry. Non-SCSI disk interfaces for PCs also moved to LBA addressing, and so the need to make up a geometry for INT 13h arose with those too, and different disk controller vendors might use a different made up geometry.

So suppose you had a DOS/Windows PC, you repartitioned your one disk to make room for FreeBSD, and went to install FreeBSD. FreeBSD does not use the INT 13h BIOS interface. It uses its own drivers to talk to the low level disk controller hardware and those drivers use LBA addressing.

It can read the partition map and find the entry for the partition you want to install on. But the entries in the partition map use CHS addressing. FreeBSD would need to translate the CHS addresses from the partition map into LBA addresses, and to do that it would need to know the disk geometry that whatever created the partition map was using. If it didn't get that right and assumed a made up geometry that didn't match the partitioner's made up geometry the actual space for DOS/Windows and the actual space for FreeBSD could end up overlapping.

In practice you can almost always figure out from looking at the partition map what geometry the partitioner used with enough accuracy to avoid stomping on someone else's partition. Partitions started at track boundaries, and typically the next partition started as close as possible to the end of the previous partition and that sufficiently narrows down where the partition is supposed to be in LBA address space.

That was the approach taken by most SCSI vendors and it worked fine. I think eventually FreeBSD did start doing this too but by then Linux had become dominant in the "Dual boot DOS/Windows and a Unix-like OS on my one disk PC" market.

The other problem was CD-ROM support. FreeBSD was slow to support IDE CD-ROM drives. Even people who had SCSI on their home PC and used SCSI hard disks were much more likely to have an IDE CD-ROM than a SCSI CD-ROM. SCSI CD-ROM drives were several times more expensive and it wasn't the interface that was the bottleneck so SCSI CD-ROM just didn't make much sense on a home PC.

For many then it came down to with Linux they could install they didn't need a two disk system and they could install from a convenient CD-ROM, but for FreeBSD they would need a dedicated disk for it and would have to deal with a stack of floppies.