The Real mode
Introduction
Real mode as its name suggest is real, that means everything is real in it.
The memory which is addressed is real i.e. it really exists at the addressed
location. The real mode was put into the processors after 80286 so that they run
programs of their predecessor and also for loading the operating system
and switching into the Protected mode.
In real addressing mode the memory is addressed by 16bits of addresses.
That is you can address at most (2^16=) 1 MB of physical memory.
Memory Structure
| Linear Address Range |
Memory Type |
Use |
| 0 - 3FF |
Ram |
Real-mode Interrupt Vector Table (IVT) |
| 400 - 4FF |
Ram |
Bios Data Ares (BDA) |
| 500 - 9FBFF |
Ram |
Free Memory (Below 1 MB) 630K |
| 9FC00 - 9FFFF |
Ram |
Extended Bios Data Area (EBDA) |
| A0000 - BFFFF |
Video Ram |
VGA Frame buffer |
| C0000 - C7FFF |
Rom |
Video Bios 32K |
| C8000 - EFFFF |
Nothing |
|
| F0000 - FFFFF |
Rom |
Motherboard Bios 64K |
|
|
| Ok |
|
0% |
| Useful |
|
25% |
| Very Useful |
|
25% |
| Excellent |
|
50% |
|
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Created on: 24 June 2005
Updated on: 27 Jan 2006
