How are segment types distinguished from one another?

Question

How are different types of segment identified as different? Wikipedia states:

Segments can be defined to be either code, data, or system segments

It also states:

Descriptors with bit 12 clear are "system descriptors" and are used for specialized purposes.

So, I gather that this "system bit" is used to identify a system segment, but what about code and data segments? Additionally, since these identifiers are stored separately instead of as a single 2-bit value, can you get combination-types, like system code and system data segments?

https://wiki.osdev.org/Global_Descriptor_Table says the Ex bit makes a descriptor a code-segment descriptor instead of data. https://wiki.osdev.org/GDT_Tutorial#What_should_i_put_in_my_GDT.3F also has more details on what kinds of GDT entries there can be. If you want the full details, see Intel's manuals (vol.3 especially). Links in the x86 tag wiki: https://stackoverflow.com/tags/x86/info — Peter Cordes, Sep 04 '18 at 21:59

score 3 · Accepted Answer · answered Sep 05 '18 at 08:19

The Type field of a segment descriptor is not one bit but four.

The S field defines if the descriptor is a system descriptor (bit is clear) or not.
The interpretation of the Type field depends on the value of S.

Non system descriptors

System descriptors

To better understand how protected/long mode segmentation works I advice you dropping Wikipedia in favour of the Intel manuals.

How are segment types distinguished from one another?

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