ITFN 2601Introduction to Operating

Systems

Lecture 16

Segmentation

Segmentation vs Paging?

Paging is incidental

Page-data is heterogeneous

Statically Sized

Segments are intentional

Segment data is homogenous

Dynamically Sized

Pure Segmentation

Segment size varies and changesExternal Fragmentation

Also called “Checkerboarding”Swapping large segments for smallResolution: Compaction

MULTICS Implementation

Similar to Multi-Level PagingAddress contains Segment # for tableSegment table holds Segment DescriptorsDescriptor contains page table locationStandard Page Table

Has 218 segments 216 Pages With 36-bit entries234 Total Pages

MULTICS Segment Descriptor

Page Table addressSegment SizeScale and Paged bits

Multics Lookup

Pentium Implementation

Local/Global Descriptor TableLocal contains program informationGlobal is OS information3-level Paging (Segment +2-level Virtual)

213 Global + 213 Local (per Program) Segments214 Total Segments per program232 Pages with 32-bit entries246 Total Pages

Pentium Code Descriptor

In-memorySystem or Application segmentType & Protection (Code/Data)Segment Scale (16-bits or all 32?)

Pentium 2-level Paging

Pentium Protection

2-bit Protection EntryAccess granted to all

higher levels00 (0) Can access all11 (3) Can access 3

Easy to decide access rights

Must make paths for system calls