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 Z80 case
Suppose we need to create a segment occupying virtual addresses 8000-C000. Since we are free to place our segment anywhere in the linear address space, we choose the linear addresses at our convenience. Let's say we choose a linear address 0x20000:
        start = 0x20000
        end   = start + 0x4000 = 0x24000
The segment base must be selected so that the virtual address in our segment will be 0x8000. Let's find it using the following equation:
        VirtualAddress = LinearAddress - (SegmentBase << 4);
        0x8000         = 0x20000 - (base << 4);
        base << 4      = 0x20000 - 0x8000
        base << 4      = 0x18000
        base           = 0x1800
After solving this equation, we see that the segment base is equal to 0x1800.

Now we can create a segment entering:

        segment start address:  0x20000
        segment end address:    0x24000
        segment base:           0x1800

Please note that the end address never belongs to the segment in IDA.

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