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Rev

To be fair, the question title says "8086" not "x86". Upvoted anyway for the modern details, but CS base on boot was a lower address on older CPUs like 8086 where physical addresses were only 20 bits, presumably corresponding to the initial CS value since unreal mode was new with 286 or 386. The OP also mentions wanting to write an emulator for a "custom CPU", so their best bet would just be to have part of the address space act as ROM, that's by far the most simple thing.
– Peter Cordes
Commented Feb 5 at 7:01

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