15. Address Translation Flashcards
What is address translation?
Transforming each virtual address to a physical address the information is actually located.
What is the illusion of virtualising memory?
The illusion is that all programs have their own private memory which contains the code and data.
What happens when a line of code is executed?
Fetch instruction and execute instruction.
Hence, there is a total of two memory accesses per line
What is the base and bounds technique?
We have two registers: base and bounds.
Every virtual address will be added with base and checked against bounds to ensure it is authorised.
What is another name for the base and bounds technique?
Dynamic relocation
Where are the base and bounds registers located at?
They are hardware components stored in the CPU
What is the memory management unit (MMU)?
The part of the processor that does address translation
What are the two types of values that bounds registers typically hold? Which is the convention we use?
- Size of address space
- Physical address of the end of the address space.
The one we use is the first method.
Note: address space is virtual
What happens when the virtual address is out of bounds?
An exception will be raised
How do we know if the privilege is user mode or kernel mode, from the perspective of memory?
We store a single big to indicate them
What is free list?
A data structure that manages address space and allows you to find free address space easily
What are the hardware requirements for dynamic relocation?
- Privilege mode
- Base or bounds registers
- Ability to translate virtual addresses and check if within bounds
- Privileged instructions to update base or bounds
- Privileged instructions to register exception handlers
- Ability to raise exceptions
What are the OS requirements for dynamic relocation?
- Memory management
- Base or bounds management
- Exception handling
