External Data & Text

Question 1

In C, where are local (automatic) variables allocated?

What is their scope?

What is their lifetime?

Answer 1

In the stack frame

The block of code where declared

The life of the block of code

Question 2

Where are static local variables stored?

What is the scope?

What is the lifetime?

Answer 2

Not in stack because it is designed to store things with a short lifetime. In a separate location of RAM

Block of code where declared

Life of program (persist from call to call of the function

Question 3

Where are global variables stored?

What is the scope?

What is the lifetime?

Answer 3

In a separate section of RAM

Global (even throughout files)

Life of program

Question 4

Where are static local variables stored?

What is the scope?

What is the lifetime?

Answer 4

In a separate location in RAM

Local to file from declaration point onwards

Lifetime: life of program

Question 5

What are the 3 sections of memory that programs may allocate?

Answer 5

.text
.data
.bss

Question 6

What does the text section contain?

Answer 6

Program text (machine code)

Read-only, programmer-initialized data

Question 7

What will happen if you try to write to text memory?

Answer 7

A segmentation fault will occur because text is read-only mem

Question 8

What does the data section contain?

Answer 8

Programmer-initialized data

Is read/write memory

Question 9

What does bss stand for and what does it have?

Answer 9

Block Starting Symbol

Reserved for zero-initialized data

Question 10

Where are the text, data and bss sections stored?

Answer 10

In low memory, just after the OS kernel section (see diagram)

Question 11

How do you indicate that what follows goes into a particular section?

Answer 11

Use pseudo-ops
.text (default section when assembling)
.data
.bss

Question 12

The assembler uses a _____ counter for each section (like text, data, bss)

Answer 12

Location counter

Question 13

What does the location counter do?

Answer 13

• starts at 0 and increases as instructions and data are processed
• final step of assembly gathers all code and data into appropriate sections

Question 14

What is the order of sections loaded when the OS loads a program into RAM?

Answer 14

1. text & data
2. bss section is zeroed

Question 15

External variables: low or high level concept?

Answer 15

low

Question 16

What are external variables and where are they allocated?

Answer 16

Non-local variables used to implement C language global & static variables. Allocated in the data or bss sections

Question 17

What pseudo-ops can be used to allocate & initialize external variables? What is the general form for allocating& initializing EVs?

Answer 17

.byte, .hword, .word, .dword

check notes

Question 18

What do the labels of external variables represent?

Answer 18

64-bit addresses

Question 19

External variables: how to put the address in a register and how to access one

Answer 19

Use adrp & add to put the address in a register

Use ldr or str to access the variable

Question 20

What is the .skip pseudo-op used for?

Answer 20

To allocate uninitialized space

Question 21

When is the .global pseudo-op used?

Answer 21

To make a variable available to other compilation units.

Question 22

What pseudo-op does the bss section usually only use?

Answer 22

.skip

Question 23

When is bss memory zeroed?

Answer 23

Before program execution

Question 24

Where are programmer-initialized constants put?

Answer 24

text section:
- is read only mem
- must be before or btw functions

Question 25

What type of memory is the bss section

Answer 25

R/W