Editing Real mode (section)

==Addressing capacity==
The 8086, 8088, and 80186 have a 20-bit address bus, but the unusual segmented addressing scheme Intel chose for these processors actually produces effective addresses which can have 21 significant bits.  This scheme shifts a 16-bit segment number left four bits (making a 20-bit number with four least-significant zeros) before adding to it a 16-bit address offset; the maximum sum occurs when both the segment and offset are 0xFFFF, yielding 0xFFFF0 + 0xFFFF = 0x10FFEF. On the 8086, 8088, and 80186, the result of an effective address 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, which is at the beginning of the first megabyte.  (Note that on the 80186 and earlier, the first [[kilobyte]] of the address space, starting at address 0, is the permanent, immovable location of the interrupt vector table.)  So, the actual amount of memory addressable by the 80286 and later x86 CPUs in real mode is 1 [[megabyte|MB]] + 64 [[kilobyte|KB]] – 16 [[byte|B]] = 1,114,096 B.