Mastering Assembly Programming

Real mode

Segment registers are a rather interesting topic, as they are the ones that tell the processor which memory areas may be accessed and how exactly they may be accessed. In real mode, segment registers used to contain a 16-bit segment address. The difference between a normal address and segment address is that the latter is shifted 4 bits to the right when stored in the segment register. For example, if a certain segment register was loaded with the 0x1234 value, it, in fact, was pointing to the address 0x12340; therefore, pointers in real mode were rather offsets into segments pointed to by segment registers. As an example, let's take the DI register (as we are talking about a 16-bit real mode now), which is used with the DS (data ...