Memory Management Hardware

Memory management hardware plays a crucial role in managing the memory resources of a computer system efficiently.

Two important concepts related to memory management hardware are memory segmentation and logical addressing.

Here’s an explanation of each:

Memory Segmentation:

Memory segmentation is a memory management technique where the logical address space is divided into variable-sized segments. Each segment represents a portion of a program or data with a specific purpose or attribute. Segments can vary in size and can represent code, data, stack, or other parts of a program.

Logical Address:

A logical address is an address generated by the CPU during program execution. It represents a location in the logical address space of a process.

• Logical addresses are translated into physical addresses by the memory management unit (MMU) using segmentation and paging techniques.

→ Segmentation and Paging: Logical addresses are typically composed of a segment selector and an offset within the segment. The segment selector is used to index into the segment descriptor table to obtain the base address of the segment. The offset is then added to the base address to generate the logical address.

→ Address Translation: Once the logical address is generated, the MMU translates it into a physical address. This translation involves mapping the logical address to its corresponding physical address in main memory. The MMU performs this translation based on the segment descriptor information and other memory management structures.

→ Virtual Memory: Logical addressing is crucial for implementing virtual memory systems, where the logical address space of a process can exceed the physical memory capacity. Virtual memory systems use techniques like paging to transparently swap data between main memory and secondary storage, allowing processes to access more memory than physically available.

Note:

1. Segment Descriptor: Memory segmentation relies on segment descriptors, which are data structures stored in a table maintained by the operating system. Each segment descriptor contains information about a segment, such as its base address, length, access rights (read-only, read-write), and other attributes.
2. Segmentation Registers: The CPU is equipped with segmentation registers that hold segment descriptors. These registers include a segment selector and a segment base address. When a program references memory using a logical address, the CPU uses the segment selector to locate the corresponding segment descriptor in the descriptor table. It then uses the segment base address to calculate the physical address of the memory location.
3. Protection and Privilege Levels: Memory segmentation allows for memory protection and privilege levels. Access rights specified in segment descriptors control which segments can be accessed by which processes or users. This helps prevent unauthorized access to memory regions and ensures data integrity and security.

Segmented – Page Mapping:

Segmented-page mapping, also known as segmentation with paging, is a memory management technique used in operating systems to organize and manage memory efficiently.

• It combines the advantages of both segmentation and paging to overcome the limitations of each approach individually.

Let’s see how segmented-page mapping works:

1. Segmentation:
   • Segmentation divides the logical address space into variable-sized segments, each representing a different type of data or a logical unit, such as code, stack, or heap.
   • Each segment has its own base address and length, allowing for flexibility in managing memory allocation for different parts of a program.
   • Segmentation helps in organizing memory logically, simplifying memory management and supporting the modular structure of programs.
2. Paging:
   • Paging divides physical memory into fixed-sized blocks called pages.
   • The logical address space is divided into fixed-sized blocks called pages as well.
   • Paging allows for efficient use of physical memory by allocating memory in fixed-sized chunks and enables the use of virtual memory, where parts of the program that are not immediately needed can be swapped out to secondary storage.
3. Segmented-Page Mapping:
   • In segmented-page mapping, the logical address space is first divided into segments, similar to segmentation.
   • Each segment is further divided into fixed-sized pages, similar to paging.
   • Thus, each segment consists of one or more pages.
   • When a program accesses memory, the logical address generated by the CPU contains both a segment number and a page number.
   • The segment number is used to locate the base address of the segment descriptor table, which contains information about the segment, including its base address in physical memory.
   • The page number is then used to index into the page table associated with the segment to find the actual physical address of the data.
   • This combination of segmentation and paging allows for efficient memory management by providing both flexibility in memory allocation (segmentation) and efficient use of physical memory (paging).

Numerical Example:

A numerical example may clarify the operation of the memory management unit. Consider the 20- bit logic address specified in fig. 13.29 (a).

Memory Protection:

Memory protection is a crucial aspect of computer systems that ensures the integrity and security of memory resources.

• It involves mechanisms to control access to memory regions, preventing unauthorized processes or users from accessing or modifying memory locations that they are not supposed to.

• These mechanisms control access to memory regions and dictate what operations (such as read, write, and execute) are permitted for different processes or users.

Here’s how various memory protection schemes are typically implemented:

1. Full Read and Write Privileges:
   • This scheme allows a process or user to read from and write to a memory region without any restrictions.
   • It’s commonly used for areas of memory that contain data or instructions that need to be modified frequently by user programs or the operating system itself.
   • However, providing full read and write privileges to all memory can pose security risks, as it allows any process to manipulate critical system data.
2. Read-Only (Write Protection):
   • In this scheme, a memory region is marked as read-only, preventing any write operations to that region.
   • Read-only memory regions are often used for storing code segments or constant data that should not be modified during program execution.
   • Write protection helps prevent accidental or malicious changes to critical system data or program instructions.
3. Execute-Only (Program Protection):
   • Execute-only memory regions allow the execution of code but prevent both read and write operations.
   • This protection scheme is commonly used to prevent code injection attacks, where an attacker attempts to execute malicious code injected into a writable memory region.
   • By restricting access to execute-only, the system can ensure that only trusted code is executed, enhancing system security.
4. System-Only (Operating System Protection):
   • Memory regions marked for system-only access are restricted to privileged processes or the operating system kernel.
   • Ordinary user processes are prevented from accessing or modifying these regions, enhancing system stability and security.
   • System-only protection helps prevent unauthorized access to critical system data structures or kernel code, reducing the risk of system crashes or security breaches.