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I wanted to learn more about systems and so I was reading about x86 global descriptor tables for memory segments as one does, and I came across this table from here:

GDT Table Entry Format

I think I understand why all the information is needed, but I don't understand why values are broken up in this way.

In particular, why did the designers prefer the above layout over something like:

first 32-bits: base next 20-bits: limit next 12-bits: access-byte and flags

?

I've come across lots of places that seem to describe the table entry layout, but it seems to be harder to find information about why things turned out this way.

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Because the x86 architecture was originally 16-bit, it then evolved addendums as it had to address more and more memory, and work with larger and larger registers.

The layout wasn't changed for various reasons:

  • Backward compatibility
  • Maximising compatability with other vendors in order to steal their customers.
  • Maximising compatability with operating systems in order to steal customers.
  • Lazyness. Why change what already works, we already have working circuits.

In short this is what happens when a system undergoes organic growth. It works (perhaps even very well) but doesn't quite make clean sense.

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