Virtual Memory

Intro. to Virtual Memory

Motivation

• Adding disks to memory hierarchy

  • Need to devise a mechanism to “connect” memory and disk in the memory hierarchy

• Simplifying memory for applications

  • Applications should see the straightforward memory layout we saw earlier ->
  • User-space applications should think they own all of memory
  • So we give them a virtual view of memory

• Protection between processes

  • With a bare system, addresses issued with loads/stores are real physical addresses
  • This means any program can issue any address, therefore can access any part of memory, even areas which it doesn't own

• Memory fragmentation

  • As programs come and go, the storage is "fragmented".
  • Therefore, at some stage programs have to be moved around to compact the storage.

Address Translation

We want to translate virtual addresses to physical addresses.

Page Table

VPN	PPN	Status	Dirty	Protection
0xFFFF F004	PPN/DPN	-	0/1	-
...	...	...	...	...

Status Bits

On each memory access, first check if page table entry is “valid”.

• Valid/on → In main memory, read/write data as directed by process.

• Not Valid/off → On disk

  • Trigger page fault exception, OS intervenes to allocate the page into DRAM (trap handler);
  • If out of memory, select a page to replace in DRAM
  • Store outgoing page to disk, and page table entry that maps that VPN->PPN is marked as invalid/DPN
  • Read requested page from disk into DRAM and update with a valid PPN
  • Finally, read/write data as directed by process.

Dirty Bit

When a page gets replaced: - Dirty bit on: Write outgoing page back to disk. - Dirty bit off: No disk write.

Protection

Multi-Level Page Tables

TLB

Address	Value
Virtual Address	\(A_{\text{virt}}\)
Physical Address	\(A_{\text{phys}}\)
Page Size	\(P = 2^{o}\)
Associative	\(A_{\text{assoc}}\)

Address	Width
VPN	\(A_{\text{virt}} - o\)
PPN	\(A_{\text{phys}} - o\)
Offset	\(o\)

Parameters	Value
# Virtual Pages	\(2^{A_{\text{virt}} - o}\)
# Physical Pages	\(2^{A_{\text{phys}} - o}\)
# TLB Entries	\(2^{A_{\text{virt}} - o} / A_{\text{assoc}}\)
# Page Table Entries	\(2^{A_{\text{virt}} - o}\)

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