9.4 Protection and Virtual memory

Question 1

What does memory protection refer to (what are programs and the kernal allowed to do)

Answer 1

• Ensuring programs do not access other parts of memory
• Kernel can access anything

Question 2

Describe how memory is protected (ab as)

Answer 2

• Only allowed access to memory in range ab, ab + as
• ink flag for kernel

Question 3

What instructions does the kernel use for managing program memory space protection

Answer 3

• SETAB SETAS

Question 4

How does memory protection change existing data access instructions (safeguarding)

Answer 4

• Conditional check for ink, otherwise check if in range

Works same for branch instructions

Question 4

What happens if this code executes on MacOS

Answer 4

• Returns valid pointer (doesn’t work on linux)
• Would cause errors down the line as computer does not have 10TB

Question 4

What is virtual memory

Answer 4

• Abstraction such that program thinks there is more memory than available
• Managed by kernel

Question 5

What is a memory page

Answer 5

• Similar to cache blocks, data is copied from disk to main memory (and vice versa) in pages (set of memory)
• Doesn’t make sense to just copy accross single word, when often sequential access is needed.

Question 6

How do virtual addresses work

Answer 6

• Page table stores actual address (maps virtual to actual incl memory type and address etc.)

Question 7

What are some properties of the page table (use, no entries, entry size etc.)

Answer 7

Question 8

What is a page fault

Answer 8

When memory is on disk not main memory and needs to be copied (ie cache miss)

Question 9

What is page replacement (eviction)

Answer 9

Page is evicted from main memory back to disk

Question 10

What is the point of a Translation Lookaside Buffer (TLB)

Answer 10

Caches addreses of data (LRU), so no need to consult page table

Question 12

How does protection apply to virtual memory

Answer 12

Read, write, execute permission bits