13. Virtual Addresses

Question 1

Where does the stack start and which direction does it grow?

Answer 1

The stack starts at the top of the address space and grows down

Question 2

Where does the heap start and which direction does it grow?

Answer 2

The heap starts towards the bottom of the address space and grows up

Question 3

Will the stack and the heap ever meet (grow into each other)?

Answer 3

Probably not. That would require the stack, heap, or both to be huge.

Question 4

What 3 things are required to implement the address space abstraction?

Answer 4

1. Address translation, meaning 0x100 for Process 1 is not the same location as 0x100 for Process 2
2. Protection (privacy), meaning processes can only access their own address space
3. Memory management, meaning the ability to have a greater sum of address spaces than the amount of physical memory on the machine

Question 5

How does the address space contain a level of indirection?

Answer 5

The address space abstraction brings with it the breaking of the direct connection between a memory address (as perceived by the process) and a physical memory location

This means that there is no guarantee that 0x100 for the process is 0x100 on the physical memory

Question 6

Forcing a process to translate a reference (mem address) is an example of a level of indirection. What features does this level of indirection provide the kernel with?

Answer 6

A process address has to be translated into a corresponding physical memory address. This level of indirection allows the kernel greater control over the physical memory.

References to physical memory addresses can be revoked (removed from page table?), shared (added to page table), moved, and altered (mapped to another location)

Question 7

What are the two components of the memory interface?

Answer 7

Load and store.

Load(address) loads data from the given address, usually into a register or another memory location

Store(address, value) stores value to the given address.

Question 8

What does a physical address point to?

Answer 8

Physical memory

Question 9

What does a virtual address point to?

Answer 9

Something that acts like memory

Question 10

What do we call addresses accessed via the memory interface (load and store)?

Answer 10

Virtual addresses.

These virtual addresses point to something that acts like memory, but is really an address that needs to be translated into a corresponding physical address

Question 11

What are the 4 locations that data referred to by a virtual address might be?

Answer 11

In memory, on disk, in memory on another machine, or at a port on a hardware device

Question 12

What expectations does a process have about the permanence of a store at a particular virtual address?

Answer 12

It depends on where the virtual address points to.

If it points to physical memory, it expects the data to be stored transiently (temporarily).

If it points to the disk, it expects the data to be stored permanently.

If it points to a device port, then it will depend on how the device.

Question 13

What permissions and protections come with virtual addresses?

Answer 13

Some virtual addresses may only be used by the kernel.

Virtual addresses can be assigned read, write, or execute permissions.

Question 14

How are virtual addresses created and initialized by exec()?

Answer 14

Exec() uses a blueprint from an ELF file to determine how the address space should look.

Virtual addresses (mainly) point to memory. Specifically:

Code is usually marked read-only

Data is marked read-write, but not executable

Heap is given an area used for dynamic allocations and marked read-write

Stack space is provided for the first thread

Question 15

What does fork() do to the address space of the calling process?

Answer 15

It creates a copy of it for the child. This means in physical memory, there are two copies of the objects from the original process.

The child has the same virtual addresses as the parent, but they point to different physical memory locations (their own corresponding copies of the data)

Question 16

What does the sbrk() system call do?

Answer 16

Handles dynamic memory allocation.

sbrk() asks the kernel to move the break point, or the point at which the process heap ends

Question 17

What does the mmap() system call do?

Answer 17

Creates virtual addresses that map to a portion of a file

Question 18

In a 32-bit MIPS architecture, what is the full range of available addresses?

Answer 18

0x0 to 0xFFFFFFFF

Question 19

What are the 4 address regions in our OS161 version of MIPS?

Answer 19

0x0 - 0x7FFFFFFF are the process virtual addresses (2GB)

0x80000000 - 0x9FFFFFFF are the kernel direct-mapped addresses that are translated by subtracting 0x80000000 (512 MB, cached)

0xA0000000 - 0xBFFFFFFF are the kernel direct-mapped addresses (512 MB, uncached)

0xC0000000 - 0xFFFFFFFF are kernel virtual addresses, which are translated by the kernel (1 GB)

Question 20

What does the hardware memory management unit (MMU) do during virtual address translation?

Answer 20

It speeds the process of translation once the kernel has told it how to translate an address or according to architectural conventions (MMU is the mechanism)

Question 21

What does the OS memory management subsystem do during virtual address translation?

Answer 21

It manages translation policies by telling the MMU what to do

Question 22

When considering virtual address translation mechanism vs its policy, what is the larger goal?

Answer 22

To have the system follow established OS policies while involving the OS directly in the translation as little as possible (because it’s time consuming to trap into the kernel)