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that overflows 20 bits is that the address "wraps around" to the zero end of the address range, i.e. it is taken modulo 2^20 (2^20 = 1048576 = 0x100000). However, the 80286 has 24 address bits and computes effective addresses to 24 bits even in real mode. Therefore, for the segment 0xFFFF and offset greater than 0x000F, the 80286 would actually make an access into the beginning of the second megabyte of memory, whereas the 80186 and earlier would access an address equal to [offset]-0x10,

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