ITFN 2601 Introduction to Operating Systems

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ITFN 2601Introduction to Operating Systems

• Lecture 16
• Segmentation

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Segmentation vs Paging?

• Paging is incidental
• Page-data is heterogeneous
• Statically Sized

• Segments are intentional
• Segment data is homogenous
• Dynamically Sized

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Pure Segmentation

• Segment size varies and changes
• External Fragmentation
• Also called Checkerboarding
• Swapping large segments for small
• Resolution Compaction

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MULTICS Implementation

• Similar to Multi-Level Paging
• Address contains Segment for table
• Segment table holds Segment Descriptors
• Descriptor contains page table location
• Standard Page Table
• Has 218 segments
• 216 Pages With 36-bit entries
• 234 Total Pages

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MULTICS Segment Descriptor

• Page Table address
• Segment Size
• Scale and Paged bits

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Multics Lookup
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Pentium Implementation

• Local/Global Descriptor Table
• Local contains program information
• Global is OS information
• 3-level Paging (Segment 2-level Virtual)
• 213 Global 213 Local (per Program) Segments
• 214 Total Segments per program
• 232 Pages with 32-bit entries
• 246 Total Pages

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Pentium Code Descriptor

• In-memory
• System or Application segment
• Type Protection (Code/Data)
• Segment Scale (16-bits or all 32?)

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Pentium 2-level Paging
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Pentium Protection

• 2-bit Protection Entry
• Access granted to all higher levels
• 00 (0) Can access all
• 11 (3) Can access 3
• Easy to decide access rights
• Must make paths for system calls