[Erwin]

I thought it would be the memory segmentation system introduced by the 286 processor: https://en.wikipedia.org/wiki/Segment_descriptor & https://en.wikipedia.org/wiki/Global_Descriptor_Table

There was a lot of unusual things back then; now I think memory is flat so your stack address 1234 is the same as your code and data address 1234 (the exception being thread-local variables).