Lecture Notes on Segmentation
Overview of Segmentation
Segmentation is a memory management scheme that divides a process into several segments of different sizes before placing them into main memory, offering an alternative to paging which utilizes fixed-sized pages.
Key Points
- Difference between Paging and Segmentation:
- Paging divides a process into fixed-sized pages without considering the content, potentially splitting related data across multiple pages.
- Segmentation divides a process based on the logical division of the program, such as functions or modules, allowing related data to stay together.
Issues with Paging
- Example Problem: If a function spans two pages, during execution, the CPU may execute the first part but not have immediate access to the second part, causing execution issues or page faults.
Advantages of Segmentation
- User-Centric: Segmentation divides memory based on the structure of the program as defined by the user.
- Non-Uniform Size: Segments can have variable sizes which allow them to match the actual size requirements of the program components.
Memory Addressing in Segmentation
- Logical and Physical Addresses:
- The CPU generates a logical address consisting of a segment number and an offset.
- The system must translate this logical address into a physical address using a segment table.
How Segmentation Works
- Segment Table: Stores the base address and limit (size) of each segment.
- Address Translation:
- Segment number determines which segment to refer to.
- Offset (given by
d bits) specifies the position within the segment.
Example of Address Translation
- Suppose a logical address is divided into a segment number (s) and an offset (d):
- Find the base address of the segment from the segment table.
- Validate that the offset does not exceed the segment’s size (Segment Limit).
- Calculate the physical address by adding the offset to the segment’s base address.
Error Handling
- If an offset exceeds the segment size, a trap is created signaling an error, preventing data from being wrongly accessed.
Implications and Summary
- Segmentation aligns more closely with how programmers structure programs, offering flexibility and more effective memory utilization than paging.
- Each segment can be individually protected and have different sizes, which improves memory management and protection mechanisms compared to fixed-size paging.
Final Thoughts
Segmentation offers a more practical approach to memory management from a program logical structure and user’s standpoint, making it a potent alternative to paging for complex software applications.