Comment by soneil | Hacker News Reader

Comment by soneil | Hacker News Reader

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It goes back way further than that. The first IBM harddrive was the IBM 350 for the IBM 305 RAMDAC. It was 5 million characters. Not bytes, bytes weren't "a thing" yet. 5,000,000 characters. The very first harddrive was base-10.

Here's my theory. In the beginning, everything was base10. Because humans.

Binary addressing made sense for RAM. Especially since it makes decoding address lines into chip selects (or slabs of core, or whatever) a piece of cake, having chips be a round number in binary made life easier for everyone.

Then early DOS systems (CP/M comes to mind particularly) mapped disk sectors to RAM regions, so to enable this shortcut, disk sectors became RAM-shaped. The 512-byte sector was born. File sizes can be written in bytes, but what actually matters is how many sectors they take up. So file sizing inherited this shortcut.

But these shortcuts never affected "real computers", only the hamstrung crap people were running at home.

So today we have multiple ecosystems. Some born out of real computers, some with a heavy DOS inheritance. Some of us were taught DOS's limitations as truth, and some of us weren't.

RAMAC, not RAMDAC: https://en.wikipedia.org/wiki/History_of_IBM_magnetic_disk_d...

However it doesn't seem to be divided into sectors at all, more like each track is like a loop of magnetic tape. In that context it makes a bit more sense to use decimal units, measuring in bits per second like for serial comms.

Or maybe there were some extra characters used for ECC? 5 million / 100 / 100 = 500 characters per track, leaves 72 bits over for that purpose if the actual size was 512.

First floppy disks - also from IBM - had 128-byte sectors. IIRC, it was chosen because it was the smallest power of two that could store an 80-column line of text (made standard by IBM punched cards).

Disk controllers need to know how many bytes to read for each sector, and the easiest way to do this is by detecting overflow of an n-bit counter. Comparing with 80 or 100 would take more circuitry.

Almost all computers have used power-of-2 sized sectors. The alternative would involve wasted bits (e.g. you can't store as much information in 256 1000-byte units as 256 1024-byte units, so you lose address space) or have to write multiplies and divides and modulos in filesystem code running on machines that don't have opcodes for any of those.

You can get away with those on machines with 64 bit address spaces and TFLOPs of math capacity. You can't on anything older or smaller.

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