Intro

Memory access in real mode is done using something called segmentation.

The original 8086 processor used mostly 16-bit registers. But a 16-bit register can only represent $2^{16}=65536\text{ bytes}=64\text{ KB}$. So theoretically, the CPU should only have been only be able to access 64 KB of memory instead of 1 MB.

As a solution to this, segmentation was introduced. Instead of using a single address, memory locations were accessed using segment:offset

8086 Segment Registers

The 8086 introduced several special segment registers:

1. CS (Code Segment)
2. SS (Stack Segment)
3. DS (Data Segment)
4. ES (Extra Segment)

Calculating Physical Addresses

In real mode, the CPU converts a segment:offset pair into an actual physical memory address using this formula: $\text{𝚙𝚑𝚢𝚜𝚒𝚌𝚊𝚕 𝚊𝚍𝚍𝚛𝚎𝚜𝚜}=(\text{𝚜𝚎𝚐𝚖𝚎𝚗𝚝}\times 16)+\text{𝚘𝚏𝚏𝚜𝚎𝚝}$

For example, suppose:

Code Segment = 0x7c0
Offset       = 0x0

Then the physical address becomes: $\text{𝟶𝚡𝟽𝚌𝟶}\times \text{𝟶𝚡𝟷𝟶}+\text{𝟶𝚡𝟶}=\text{𝟶𝚡𝟽𝚌𝟶𝟶}$