Lecture 5 Address Space - Physical Memory Management

Outline

1. Storage Hierarchy of Computer

2. Address and Address Space
3. The Function of Virtual Memory

Physical address and logical address

• Physical Address (PA): Used for addressing units at the level of memory chips, corresponding to the address bus connected to the processor and CPU
• Logical address (LA): Used to specify the address of an operand or an instruction when the CPU executes machine instructions. It is also the address used in user programming
• Linear Address (or Virtual Address): Similar to a logical address, it is also an unreal address
  • Logical Address refers to the address before CPU converts it to segmented memory management
  • Linear Address refers to the address before CPU converts it to paged memory management

Relationship between logical address and physical address

• Conversion from Logical Address to Physical Address

  • Logical Address -> Linear Address (Virtual Address) -> Physical Address

• The logical address is the same as the virtual address when there is no segment memory management

• The logical address, virtual address, and physical address are all the same when there is no page memory management

Storage hierarchy of computer

Multi-layer storage structure of computer

Abstraction of memory resource by an OS

Memory management

• Memory management method in the OS
  • Relocation
  • Segmentation
  • Paging
  • Virtual memory/storage
• The memory management of OS highly depends on the hardware
  • Tightly coupled with computer's storage architecture
  • MMU (Memory Management Unit): the hardware that handles storage access requests of the CPU

Outline

1. Storage Hierarchy of Computers

2. Address and Address Space

3. The Function of Virtual Memory

Definition of address space

• Physical address space: the address space of physical memory
  • It starts from address $0$ and goes up to $MAX_{phy}$
• Virtual address space: the address space of virtual memory
  • It starts from address $0$ and goes up to $MAX_{virt}$
• Logical address space: the address space used by a program during execution
  • It starts from address $0$ and goes up to $MAX_{prog}$

The perspectives of these three address spaces are different

Generation of logical address

Timing of address generation

• At the compiling time
  • Assuming the starting address is known
  • If the starting address changes, the program must be recompiled
• At the loading time
  • If the starting address is unknown in compilation, the compiler should generate relocatable code
  • At the loading time, the position may not be fixed, and absolute (virtual) addresses are generated
• At the execution time
  • The code cannot be modified during execution
  • Requires address translation (mapping) hardware support

Address generation process

• CPU
  • ALU: needs the content in logical address
  • MMU: translates logical address to physical address
  • CPU control logic: sends Physical Address request to the bus
• Memory
  • Send the content of Physical Address to the CPU
  • Receives CPU data to the physical address
• OS
  • Build the mapping between logical address and physical address

Address checking

Outline

1. Storage Hierarchy of Computers
2. Address and Address Space

3. The Function of Virtual Memory

Cache of external storage

Virtual memory can be used as a cache for external storage

• Frequently used data is stored in physical memory
• Infrequently used data is stored in external storage
• Running programs use virtual memory address, no need to care about whether they are stored in physical memory or external storage

Simplify the compilation and loading of App

Each running program has its own independent address space, regardless of where the code and data are actually stored in physical memory. This simplifies:

• The process of compiling and linking executable programs
• The process of loading executable programs by the OS
• Sharing: dynamic link libraries, shared memory
• Memory allocation: physical memory is not contiguous, but appears contiguous in virtual memory
  

Protect data

Virtual memory can protect data

• The independent address space makes it easy to distinguish the memory of different processes
• The address translation mechanism can perform check for read/write/execute permissions
• The address translation mechanism can perform privilege level check