1. AKT211 – CAO 05 – x86 Architecture: Intel 8088 (2)
Ghifar
Parahyangan Catholic University
Sept 26, 2011

2. Outline
Operation Modes
Real-Mode
Protected-Mode

3. Operation Modes Real mode operation Protected mode operation
only in 8086/8088
allows the microprocessor to address only the first 1MB of memory space
Protected mode operation
80286 and above
allows the microprocessor to address above the first 1MB of memory as well as within the first 1MB of memory

4. Real Mode uses a segment and an offset register
segment defines the beginning address of any 64KB memory segment
offset selects any location within 64KB memory segment
E.g. :
segment : 1000H
offset : F000
append 1000H with 0H = 10000H
so that, the location : 1F000H

5. Real Mode
Default {segment:offset} in real mode ( )

6. Protected Mode
Segment address is no longer present, substituted with selector and descriptor
But, offset is still present !
Selector : selects a descriptor from a descriptor table
Descriptor: describes the memory segment’s location, length, and access rights
The instructions are identical with real mode
Most programs written to function in real mode will function without change in the protected mode
2 descriptor tables with the segment registers
Global Descriptors: contain segment definition that apply to all programs
Local Descriptors: contain segment definition that unique to an application
Each descriptor tables contains 8,192 descriptors

7. Protected Mode SELECTOR OFFSET Descriptor Table Base Address
Segment Descriptor
Linear Address

8. Descriptor Format

9. Segment Register format during protected-mode

10. Example of the accessing using protected-mode

11. Any Question ?

12. THANK YOU