Q5Computer Architecture

Real mode addressing is the addressing scheme used by x86 processors when operating in a mode fully compatible with the original 8086 processor, in which the 20-bit physical memory address is computed by taking a 16-bit segment register value, shifting it left by 4 bits (equivalent to multiplying by 16), and adding it to a separate 16-bit offset value: Physical Address = (Segment × 16) + Offset. This scheme limits the total directly addressable physical memory to 2^20 = 1 MB, and does not provide any hardware memory protection between different programs — all later x86 processors still start up in real mode for backward compatibility before switching to protected mode (which supports much larger address spaces and hardware memory protection) during operating system boot.