Where system stores 16 segment addresses while segment registers are 4?

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In real mode memory addressing there are 16 segments made with the size of 64KB, and there are 4 segment registers, so where system store those 16 addresses?

 0. 00000
 1. 10000
 2. 20000
 3. 30000
      :
      :
14. E0000
15. F0000

For example, if my system respectively uses the first 4 segment starting addresses in CS, DS, ES, and SS. Where will the remaining 12 addresses be stored?

x86-16 memory-segmentation cpu-registers assembly cpu-architecture
1 Answers

There is no need to store them anywhere. You load your segment address (which doesn't have to be on a 64K boundary, by the way - only at a multiple of 16) into the segment register of your choice/need when you use it.

For example, let's say I want to load some value at physical address 0x12345 into al. Then I'd do mov ds, 0x1234 followed by mov al, [0x5] (note that this instruction implicitly uses ds:, but you could also specify it manually) - or mov ds, 0x1000; mov al, [0x2345], or any other combination that adds up to the value I want (physical address = segment * 16 + offset). I'd usually organize my memory in such a way that the things I often need together are reachable with the same segment, i.e. are within 64K, so I wouldn't have to switch segments all the time.

user_18710330
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