Virtual Memory

1
Virtual Memory

• Operating Systems
• Lecture 8

2
Multi-tasking OS
0x9000
OS
0x8000
MS Word
0x7000
Excel
0x4000
Outlook
0x0000
3
Virtual Memory

• Protection
• A process should be able to reference its own
  address space
• Abstraction
• Every process thinks it owns the entire RAM e.g.
  0x0000 to 0x9000
• Makes it easier for the compiler to generate
  object (assembly) code

4
Virtual Memory

• Virtual Memory manager translates the virtual (or
  fake) addresses

Compiler
Physical address
Address Translation (MMU)
Translate Virtual -gt Actual
RAM
virtual address
Assembly language Load Store
CPU
5
Memory Manager
Compiler
Physical address
Address Translation (MMU)
RAM
legal addr? Illegal?
Assembly language Load Store
virtual address
Illegal Access Exception
CPU
Data
6
Virtual Memory

• Virtual memory makes the compiler job much
  simpler
• Sequential addresses
• MMU maps and remaps the virtual addresses to
  actual memory
• If some part of the memory is not used
• Memory may be multiplexed between processes
• Just as the process abstraction permits the CPU
  to be multiplexed between processes

7
Memory Translation

• Hardware provides two registers and an adder
• Keep a base register to remap addresses
• Add the current address to the base register
  address
• Protection Check against an upper bound

8
Memory Translation
Process A
0x3000
0x9000
MMU
process A
Base 0x4000 Bound0x9000
read 0x2000
read 0x3000
0x4000
0x0000
9
Virtual Memory
Physical Memory
Virtual memory
0
MMU
0
0
OS
0
0