Lecture 04

Question 1

How much memory address space is given on a 32-bit machine?

Answer 1

4GB (2^32)

Question 2

How much memory address space is given on a 16-bit machine?

Answer 2

64KB (2^16)

Question 3

How much memory is ‘Kernel Space’ on 32-bit Linux?

Answer 3

1GB

Question 4

How much memory is ‘User Space’ on 32-bit Linux?

Answer 4

3GB

Question 5

How much virtual address space does Intel 54 architecture support?

Answer 5

48-bit, or 256TB

Question 6

How much memory is ‘Kernel Space’ on 64-bit Linux?

Answer 6

128TB

Question 7

How much memory is ‘User Space’ on 64-bit Linux?

Answer 7

128TB

Question 8

On 64-bit Linux, addresses are 64-bits long. How many bits of this are used for addressing, and what is done with the other bits?

Answer 8

Only the lower order 48 bits are used for addressing.

Bits 63-48 must be all 1 or all 0.

Question 9

What are the five memory segments that a Linux process is divided into?

Answer 9

• Code Segment
• Data segment
• Block Started by Symbol (BSS)
• Stack Segment
• Heap

Question 10

What is the Code Segment of a Linux Process?

Answer 10

The program text

Question 11

What is the Data Segment of a Linux Process?

Answer 11

Initialised global/static variables and data structures.

Question 12

What is the Block Started by Symbol (BSS) Segment of a Linux Process?

Answer 12

The uninitialised global/static variables and data structures.

Question 13

What happens when one or more processes have their own, varying data at the same internal memory address?

Answer 13

Nothing.

Each process has its own independent address space so they cannot interfere with one another.

Question 14

What is a page table?

Answer 14

A table which maps an address in process memory to a physical memory address.

Question 15

Do processes have direct access to physical memory?

Answer 15

No, the CPU performs these accesses.

Question 16

How many ‘Protection Rings’ does a CPU have?

Answer 16

3.

Question 17

What is ‘Protection Ring 0’ on a CPU?

Answer 17

Full access. Can edit physical memory and edit the page table.

Question 18

What is ‘Protection Ring 3’ on a CPU?

Answer 18

Indirect memory access via the page table only.

Does not have the ability to edit the table and is blocked from some CPU instructions.

Question 19

What causes a ‘segmentation fault’?

Answer 19

When a process tries to access a memory location which is not currently mapped in the page table.

Question 20

What is the ‘exec’ system call used for?

Answer 20

Replacing the current program with a new program.

Question 21

When ‘exec’ is called on a process, what is NOT overwritten? (Pick as many as you think)

• Process ID
• Text
• Stack
• Heap
• Data
• Open file descriptors

Answer 21

Process ID and open file descriptors

Question 22

A pipe provides which level of communication?
Full-duplex
Half-duplex
Simplex

Answer 22

Half-duplex

Question 23

Do pipes allow one-way communication, or two way?

Answer 23

One way

Question 24

What is the function prototype for the ‘pipe’ function?

Answer 24

int pipe(int filedes[2]);