Operating Systems Lecture 8: Mechanism of address translation

Understanding Virtual to Physical Address Translation

Introduction to Address Translation

• The lecture focuses on the process of translating virtual addresses to physical addresses, using a simple C program as an example.

• The compiled assembly code consists of instructions that manipulate memory and registers, illustrating how high-level code translates into lower-level operations.

Memory Image and Virtual Address Space

• The memory image includes various components such as executable instructions, heap allocations, and local variables stored in the stack.

• This virtual address space is created by the operating system during process setup, which involves loading code from disk and allocating necessary memory areas.

Physical Memory Allocation

• In practice, the entire memory image may not be contiguous; it can be split into smaller chunks (paging) and distributed across RAM.

• When a CPU requests data at a virtual address (e.g., 128), it requires translation to the corresponding physical address (e.g., 32896).

Role of Memory Management Unit (MMU)

• The MMU performs address translations by adding a base value to virtual addresses while checking against bounds to prevent out-of-bounds access.

• If a requested address exceeds the allocated space for a process, the MMU will signal an error instead of fetching invalid data.

Operating System's Role in Address Translation

• The OS provides base and bound values to the MMU but does not participate in every translation; this task is handled by hardware.

• The MMU operates independently after receiving initial parameters from the OS, ensuring efficient memory access without constant OS intervention.

Privileged Mode of Execution

• Hardware must support privileged execution modes so that only the OS can set critical translation information like base and bound values.

• This separation ensures security by preventing user processes from altering their own memory management settings.

Summary of Key Components

• Essential instructions for setting base and bound values are restricted to privileged mode for safety reasons.

Understanding Memory Management in Operating Systems

Role of Hardware in Address Translation

• The hardware automatically translates memory addresses on every access, generating faults when a user accesses an out-of-bounds address. This triggers a trap to the operating system, halting the offending process.

Operating System's Responsibilities

• The operating system tracks all memory allocations, knowing where each process's code and data reside within physical memory. It allocates memory upon process requests and maintains records of allocated spaces.

• Information about allocated memory is stored as part of each process's Process Control Block (PCB), which includes details like base and bound values for address translation provided to the hardware.

Context Switching and Memory Updates

• During context switches, the operating system must update the hardware with new base and bound values for the currently running process to ensure accurate address translation. Any illegal memory access also requires handling by the operating system.

• The operating system functions as a central coordinator, managing where processes are located in memory while allowing continuous CPU operations without direct involvement from the OS during load/store instructions.

Memory Allocation Schemes: Base and Bound vs Segmentation

• The base and bound scheme allocates an entire chunk of contiguous memory for a process; however, segmentation offers a more flexible approach by dividing processes into smaller segments (code, stack) that can be placed separately in RAM.

• Segmentation allows better management of sparse address spaces by only allocating necessary chunks rather than large contiguous blocks, optimizing memory usage for processes with limited active data or code.

Challenges with Segmentation