External Data and Text

Question 1

In the C language, local (automatic) variables are always allocated in the stack frame of a function

What is its scope and lifetime?

Answer 1

Scope: local to block of code where declared

Lifetime: life of the block of code

Question 2

What is the scope and lifetime of static local variables?

Answer 2

Scope: block of code where declared

Lifetime: life of the program (persist from call to call of the function)

Question 3

Stack frames can’t be stored on the stack frame for the function. Why?

Answer 3

Stack memory may be used by other functions

Question 4

Where are static local variables stored?

Answer 4

In a separate section of RAM

Question 5

What does the ret instruction do?

Answer 5

It returns from a subroutine back to the calling code

It transfers control to the address stored in the link register (x30)

It jumps to the instruction immediately following the original bl in calling code

Question 6

What do the stp instructions do?

Answer 6

They create a frame record in each function’s stack frame

It safely stores the LR (x30), in case it is changed by a bl in the body of the function

It is restored by the ldp instruction, just before the ret

Question 7

The FP and the stored FP values in the frame records form a ___________

Answer 7

linked list

[diagram in notes]

Question 8

A called function must save/restore the state of the calling code

If it uses any of the registers __________ it must save their data to the stack at the beginning of the function

Are _________________

What must the function do in these registers just before it returns

Answer 8

x19-x28

callee-saved register

The function must restore the data in these registers just before it returns

Question 9

Which other registers can the callee use?

Answer 9

x9-x15

Question 10

By convention, these registers are not saved/restored by the called function

Thus .. ?

Answer 10

Are only safe to use in calling code between function calls

Question 11

The calling code can save these registers to the stack, if it is necessary to preserve their value over a function call

Are..

Answer 11

Caller-saved register

Question 12

How many arguments can be passed into a function using register x0 - x7?

Answer 12

8 or fewer
ints, short intsm and chars use w0 - w7

long ints use x0 - x7
[Example in notes]

Question 13

The subroutine is free to _______ registers x0 - x7 as it executes

Answer 13

overwrite

The register contents are not preserved over a function call

Question 14

Global Variables
Scope
Lifetime
Where they are stored

Answer 14

Scope: global (from declaration onwards)
Lifetime: life of program
Are stored in a separate section of RAM

[example in notes]

Question 15

Static Global Variables
Scope
Lifetime
Where they are stored

Answer 15

Scope: local to file (from declaration onwards)
Lifetime: life of a program
Are stored in a separate section of RAM

[example in notes]

Question 16

Program may allocate _ sections of memory:

What do they contain and which memory are they?

Answer 16

3
text
Contains:
Program text (machine code)
Read-only, programmer-initialized data
Is read-only memory
Attempts to write to this memory causes segmentation fault

data
Contains:
Programmer-initialized data
Is read/write memory

bss
Contains:
zero-initialized data
Is read/write memory

Question 17

These sections are located in _________, just after the section reserved for the OS kernel

Answer 17

low memory

[diagram in notes]

Question 18

Pseudo-ops are used to indicate …

Give examples

Answer 18

… what follows goes into a particular section

.text (is the default section when assembling)
.data
.bss

Question 19

The assembler uses a ____________ for each section

Answer 19

location counter

Starts at 0, and increases as instructions and data are processed
The final step of assembly gathers all code and data into the appropriate sections

Question 20

What happens when the OS loads the program into RAM?

Answer 20

The text and data sections are loaded first

The bss section is then zeroed

Question 21

What are external variables?

Answer 21

Are non-local variables, allocated in the data or bss sections

Are used to implement C language global and static local variables

Question 22

How can external variables be allocated and initialized?

Answer 22

Using the pseudo-ops:
.dword (8 bytes)
.word (4 bytes)
.hword (2 bytes)
.byte (1 byte)
General form:
label:   pseudo-op value1,[value 2, ...]

[Example in notes]

Question 23

What do the labels represent?

Answer 23

64-bit addresses

Use adrp and add to put the address into a register

They use ldr or str to access the variable

[Example in notes]

Question 24

How can uninitialized space be allocated?

Answer 24

With the .skip pseduo-op

Question 25

Which pseudo-op should be used to make a variable be made available to other compilation units?

Answer 25

.global

Question 26

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

Answer 26

.skip
All bss memory is zeroed before program execution
Initializing memory to non-zero values (with .word, .hword, etc.) doesn’t make sense

Question 27

Which section are programmer-initialized constants put into?

Answer 27

The text section

The must be before or between functions

Question 28

What does ASCII stand for and what does it do?

Answer 28

American Standard Code for Information Interchange

It encodes characters using 7 bits, stored in a byte

Question 29

How can character constants be encoded in assembly?

Answer 29

• The hex code
• The character in single quotes
  Note: may interfere with m4

Question 30

What is a string and how can it be initialized?

Answer 30

A string is an array of characters

Could be initialized in memory one byte at a time

But the .ascii pseudo-op is more convenient

Question 31

In C, strings are null terminated.. How is this done?

Answer 31

Could be done using two pseudo-ops

But more conveniently with .asciz or .string

Question 32

What is a string literal?

Answer 32

A string literal is a read-only array of characters, allocated in the text section

The literal usually has a label, which represents the address of the first character in the array

The address can be passed as a pointer argument into a function using an x register

Question 33

How are external Arrays of Pointers created?

Answer 33

Using a list of labels

[example in notes]

Question 34

What are command-line arguments used for?

Answer 34

They allow you to pass values from the shell into your program

In C: main (int argc, char argv[])

• argc is the number of arguments
• argv[] is an array of pointers to the arguments (represented as strings)

[example in notes]