Combining segmentation and paging

Introduction

Segmentation, taken alone, can have some drawbacks:

• It may result in external fragmentation.

• A growing segment can impose a change in its location.

Merging segmentation and paging can solve these issues.

• First, the logical address space of a process is partitioned into segments.

• Then, each segment is divided into pages.

However, this introduces a growing complexity.

Memory management unit (MMU)

Logical address to physical address translation

The MMU must contain 3 base registers and 1 limit register.

• 1 base register for the segmentation table.

• 1 limit register for the segmentation table.

• 1 base register for the page table.

• 1 base register for the memory frame.

Access to memory unfolds into 3 steps:

• Access to the segmentation table.

• Access to the page table.

• Access to the physical address.

Entry of the segmentation table.

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Entry of the page table.

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The perm field is now associated with the segment.