2021-03-20 - Adventures in Partitions

No, this post isn't about Poland

I've always been fascinated by the history of programming and to that end I recently bought myself an old computer. I installed Windows 98 SE, Windows NT 4.0, Windows 3.1 and OS/2 2.1 on it and installed a variety of programming packages. Previously I was using virtual machines to host the OS but I found the virtual screen difficult to read and the virtualization program has compatibility versions with windows updates. Having dedicated hardware means things can run full screen and I shouldn’t have to deal with updates.

It's been an interesting experience setting up all these OSs. For one thing I learned things I didn't expect to learn and for another things I expected to be problems weren't. My primary concern with setting up this system was drivers. I envisioned days spent trying to get things to work and googling obscure error messages but that hasn't really been the case. For the most part things just worked and I was able to find drivers for the things I wanted. Dell had drivers downloads for both Windows 98 and Windows NT 4.0 and I even found USB mass storage drivers for both of those as well. I also found a tool that patches the SVGA driver for Windows 3.1 so that you can run it at a resolution above 640x480. I am missing some drivers, like Windows 98 SE can't read NTFS partitions but Windows NT 4.0 can read FAT32 and both can connect to the network and read USB sticks so that’s not a huge issue. When I get around to working with OS/2 I want to try and figure out how to get it to read the CD Drive and display at a higher resolution but those issues don’t stop it from working.

What I did have a problem with is hard drive partitions.

Firstly do you know how computers boot from a hard drive? well it turns out that it's a three step process. First you have the Master Boot Record (MBR) which sits at the start of the hard drive. The computer executes this section first and it loads partition information and passes execution off to some other bit of code. The actual operations performed depends on the MBR installed. A basic one will just find an active partition and execute the Volume Boot Record (VBR) while a more advanced one will switch over to a boot manager program. The VBR works the same as the MBR but for a partition and that is more OS specific. The VBR locates, loads and starts the actual OS.

The other thing I learned about was how partitions are defined and how the computer requests data from them. It turns out that the MBR has space for four partition slots which are stored after the start up code. These partition slots contain information about where the partition is on the disk, how big it is, and what kind of partition it is. This limits the maximum number of primary partitions on a disk to four. You can have extended partitions which are basically partitions containing other partitions but those caused me issues so I never used them. Newer hard drive setups replace the MBR with something more expandable but that's not really relevant to this old computer.

Now on to accessing data. Originally data was accessed on a hard drive using Cylinder-head-sector addressing. Hard drives are made up of a stack of platters. CHS forms a kind of 3D coordinate system for locating data on these platters. The Head value is a vertical coordinate and selects which platter and which side of the platter to get data from. Head is the term for the component that reads the data from the platter so by selecting which head to use you select which platter to read from. The Cylinder or Track value is a radial value which indicates a ring on the platter to get data from. The Sector value is an angular value which indicates which section of the ring the data is in. This system was used because early hard drives were rather simple and so the computer had to tell them exactly where to find the data they wanted. As hard drives got more advanced, and specifically as they got more built in controller logic, this scheme was less necessary. CHS was eventually replaced by Logical block Addressing which accesses data on a hard drive using a single numerical index and leaves it up to the hard drive itself to figure out where that block of data actually is.

The reason this is important is because the format you have for encoding these addresses determines how large of a hard drive you can access. The original IBM BIOS implantation of CHS had 10 bits for cylinder, 8 bits for head, and 6 bits for sector. With a 512 byte sectors this gives 8064 MiB (63 sectors x 1024 cylinders x 256 heads x 512 bytes) of addressable space. There's only 63 sectors in a track because numbering starts at 1. This was replaced by 28-bit LBA which allows for 268,435,456 sectors or 128 GiB and later 48-bit LBA which supports up to 128 PiB. One more wrinkle though because the MBR only has 4 bytes to store the size of a partition. If we are using 28-bit LBA that's fine but with 48-bit LBA we lose 16 bits which limits the maximum number of sectors in a partition to 4,294,967,296 or 2 TiB.

The hard drive installed in the computer is 232 GiB (250 GB) but the BIOS and the partition manager I was using only sees it as 128 GiBs likely because they are using 28-bit LBA. FDISK for Windows 98 reported the drive as only being 65,535 MiB but that’s likely because it’s using a 16 bit value somewhere. Windows NT 4.0 reported the drive as being 8064 MiB likely because it was using CHS. The other problem with the Windows NT 4.0 setup program is that it can only create 4 GiB NTFS partitions because it first creates them as super sized FAT 16 partitions for some reason. The OS itself can create larger partitions but those have to be created after you have it installed. There’s also apparently a bug where the main NT OS files have to be within the first 8064 MiB of the drive or the loader can’t find it. DOS and Windows 3.1 were surprisingly easy to setup. The FAT 16 implementation used by them can only be 2 GiB so I created a partition of that size and they happily installed into it. I tried the same for OS/2 but it saw the partition as only being 32 MiB for some reason and got really confused about the other partitions. I ended up having to let it create its own 32 MiB partition and then expanded it to 2 GiB afterwards. It seems to be okay with that.

But now I can programing in C, C++, QuickBasic, Visual Basic, ASP, Pascal and Assembly so that’s nice.