Microkernels are better

Posted Feb 26, 2013 10:46 UTC (Tue) by khim (subscriber, #9252) [Link]

Didn't Intel use that to get round a small address bus? And wasn't it absolutely useless for security?

They are designed to be used for security and, e.g. OS/2 1.x used them for security. Sadly they broken the backward-compatibility on the way: you could use segments solely to extend memory or for security on 80286 — but not simultaneously. 80386 solved that problem but it introduced UNIX-like paging model and everyone forgot about segments.

iirc, assuming a 4K segment, addressing 4K+1 in segment 1 would get you the first byte of segment 2. On Primes, different segment meant different memory.

You can do that on 80386, too: it really depends on how your GDT/IDT/LDT are arganized. You can even change sizes of segments on the fly. Actually this architecture was pretty sophisticated and flexible, but it was pushed to the slow-path (and eventually eradicated in x86-64) when AMD and Intel found out that nobody uses it.

Posted Feb 26, 2013 10:51 UTC (Tue) by mpr22 (subscriber, #60784) [Link]

• real mode: 16-bit segment register; left-shift segment by four bits and add the offset to get a 20-bit physical address.
• 16-bit protected mode (80286 and later): 16-bit segment register; when loading a segment register, use the top 14 bits to do a table lookup to get the base address of the segment; add the offset to the base address to get a 24-bit physical address.
• 32-bit protected mode (80386 and later): 16-bit segment register; when loading a segment register use the top 14 bits to do a table lookup to get the virtual base address and size of the segment; add the offset to the base address to get a 32-bit virtual address, which is then resolved to a physical address either on a 1:1 basis (paging disabled) or via the paging unit.