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

Outline Operation Modes Real-Mode Protected-Mode

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

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

Real Mode Default {segment:offset} in real mode (8086 - 80286)

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

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

Descriptor Format

Segment Register format during protected-mode

Example of the accessing using protected-mode

Any Question ?

THANK YOU