Understanding memory management through segmentation mechanisms
Segmentation is a memory management technique that divides the memory into different segments based on the logical divisions of a program, such as code, data, and stack. Each segment has its own base address and limit, which helps in accessing and managing memory efficiently.
Unlike paging, which divides memory into fixed-size blocks, segmentation deals with variable-sized segments that correspond to logical units of a program.
Segments can grow or shrink based on program needs
Aligns with program structure (code, data, stack)
Logical addresses mapped to physical addresses
Definition: A data structure that maintains information about all segments of a process.
Function: Each entry in the segment table holds the base address and limit of a segment.
Structure: Contains entries with fields for the segment base address, segment limit, and status bits (e.g., valid/invalid bit, access permissions).
Definition: A special register that holds the base address of the segment table.
Function: Points to the segment table in memory, enabling the CPU to access segment information.
Definition: The hardware component responsible for translating logical addresses to physical addresses using the segment table.
Function: Facilitates address translation by accessing the segment table and combining the segment base address with the offset.
The CPU generates a logical address that consists of a segment number and an offset within that segment.
The MMU uses the segment number to index into the segment table and retrieve the base address and limit of the segment.
The MMU checks if the offset is within the segment limit. If valid, the logical address is translated into a physical address by adding the offset to the segment base address.
The physical address is used to access the desired memory location.
The logical address is composed of a segment number (SN) and an offset (O).
Example: If the logical address is SN:O = 3:0x456 and the segment size is 4KB, SN is 3 and the offset is 0x456.
The MMU uses the segment number to index into the segment table.
Retrieves the base address and limit for the segment.
The MMU checks if the offset is within the segment limit.
If the offset is valid, the physical address is calculated by adding the segment base address to the offset.
Example: If the segment base address is 0x2000 and the offset is 0x456, the physical address is 0x2000 + 0x456 = 0x2456.
The physical address is used to access the desired data in RAM.
Here is a simplified block diagram of segmentation hardware:
Generates logical address (Segment Number + Offset)
Points to the segment table in memory
Contains base address and limit for each segment
Translates logical address to physical address
Accessed using physical address
Maps segment numbers to base addresses and limits
Holds the base address of the segment table
Translates logical addresses to physical addresses using segment table entries
Aligns with the logical divisions of a program, making it easier to manage code, data, and stack separately
Each segment can have its own access permissions, enhancing security
Segments can grow or shrink as needed, reducing fragmentation
| Benefit | Description |
|---|---|
| Logical Structure | Memory is divided according to the logical structure of the program, making it more intuitive for programmers |
| Segment Protection | Each segment can have different access rights (read, write, execute), providing fine-grained security |
| Variable Size | Segments can be of variable sizes, matching the actual needs of different program components |
| Dynamic Growth |