Data and Program Representation

Question 1

How is memory seen by the program?

Answer 1

As an array of bytes.

Question 2

What is the address range of memory in bytes?

Answer 2

0 - (2^64 - 1). * this is for 64 bit architecture

Question 3

Is all of the address space of memory used?

Answer 3

No

Question 4

What are the sections in memory called?

Answer 4

memory mappings

Question 5

What are the memory mappings, in order form lowest to highest in address range.

Answer 5

Text, RoData, Data, BSS, Heap - Shared libs - Stack

Question 6

What does the text memory section hold?

Answer 6

Instructions that the program runs

Question 7

What does the data memory section hold?

Answer 7

Initialized global variables.

Question 8

What does the Bss memory section hold?

Answer 8

Uninitialized global variables. They are
initialized to zeroes.

Question 9

What does the Heap memory section hold?

Answer 9

Memory returned when calling malloc/new. It grows upwards.

Question 10

What does the Stack memory section hold?

Answer 10

It stores local variables and return
addresses. It grows downwards.

Question 11

What does the RoData memory section hold?

Answer 11

Read Only Data. String constants

Question 12

Which way does the Stack grow?

Answer 12

The stack grows downwards.

Question 13

Which was does the heap grow?

Answer 13

The heap grows upwards

Question 14

What are Dynamic Libraries?

Answer 14

They are libraries shared with other processes.

Question 15

What does each dynamic library have its own of?

Answer 15

text, data, and bss.

Question 16

True or False, Each program (process) has its own view of the memory that is independent of each other.

Answer 16

True

Question 17

True or False, If a process modifies a byte in its own address space, it will modify the address space of another process.

Answer 17

False

Question 18

Below, where is line x stored?

Program hello.c
int a = 5; // Line x
int b[20];
int main() {
int x;
int p;
p =(int) malloc(sizeof(int));
*p = 5;
printf(“Hello cs250\n”);
}

Answer 18

data section

Question 19

Below, where is line x stored?

Program hello.c
int a = 5;
int b[20]; // Line x
int main() {
int x;
int p;
p =(int) malloc(sizeof(int));
*p = 5;
printf(“Hello cs250\n”);
}

Answer 19

BSS

Question 20

Below, where is line x stored?

Program hello.c
int a = 5;
int b[20];
int main() { // Line x
int x;
int p;
p =(int) malloc(sizeof(int));
*p = 5;
printf(“Hello cs250\n”);
}

Answer 20

Text

Question 21

Below, where is line x stored?

Program hello.c
int a = 5;
int b[20];
int main() {
int x; //Line x
int p;
p =(int) malloc(sizeof(int));
*p = 5;
printf(“Hello cs250\n”);
}

Answer 21

Stack

Question 22

Below, where is line x stored?

Program hello.c
int a = 5;
int b[20];
int main() {
int x;
int p;
p =(int) malloc(sizeof(int)); //Line x
*p = 5;
printf(“Hello cs250\n”);
}

Answer 22

Heap

Question 23

What is a memory gap?

Answer 23

A space between memory sections where there is no memory mapping

Question 24

What happens if a program tries to access a memory gap?

Answer 24

The OS will send a SEGV signal that by default kills the program and dumps a core file.

Question 25

What does the “core file” contain?

Answer 25

The core file contains the value of the variables global and local at the time of the SEGV.

Question 26

What file can be used for “post mortem” debugging?

Answer 26

The core file.

Question 27

What is a program?

Answer 27

a file in a special format that contains
all the necessary information to load an application into memory and make it run

Question 28

What does a program file include?

Answer 28

● machine instructions
● initialized data
● List of library dependencies
● List of memory sections that the program will use
● List of undefined values in the executable that will be
known when the program is loaded into memory.

Question 29

True or false, there are different executable file formats?

Answer 29

True

Question 30

What is ELF – Executable Link File used in?

Answer 30

It is an executable file format used in most UNIX systems (Solaris, Linux).

Question 31

What is COFF – Common Object File Format used in?

Answer 31

It is used in Windows systems

Question 32

What is a.out used in?

Answer 32

Used in BSD (Berkeley Standard Distribution) and early UNIX
It was very restrictive. It is not used anymore.

Question 33

What are the steps to building a program? (extensive maybe split up flashcard)

Answer 33

● The programmer writes a program hello.c
● The preprocessor expands #define, #include,
#ifdef etc preprocessor statements and generates a
hello.i file.
● The compiler compiles hello.i, optimizes it and
generates an assembly instruction listing hello.s
● The assembler (as) assembles hello.s and
generates an object file hello.o
● The compiler (cc or gcc) by default hides all these
intermediate steps. You can use compiler options to
run each step independently.

Building a program

● The linker puts together all object files as well as
the object files in static libraries.
● The linker also takes the definitions in shared
libraries and verifies that the symbols (functions
and variables) needed by the program are
completely satisfied.
● If there is symbol that is not defined in either the
executable or shared libraries, the linker will give
an error.
● Static libraries (.a files) are added to the
executable. shared libraries (.so files) are not
added to the executable file.

Question 34

What line generates a hello executable?

Answer 34

“gcc –o hello hello.c”

Question 35

What are the steps to loading a program? (extensive maybe split up flashcard)

Answer 35

● The loader is a program that is used to run an
executable file in a process.
● Before the program starts running, the loader
allocates space for all the sections of the
executable file (text, data, bss etc)
● It loads into memory the executable and
shared libraries (if not loaded yet)

Loading a Program

● It also writes (resolves) any values in the executable
to point to the functions/variables in the shared
libraries.(E.g. calls to printf in hello.c)
● Once memory image is ready, the loader jumps to
the _start entry point that calls init() of all libraries
and initializes static constructors. Then it calls
main() and the program begins.
● _start also calls exit() when main() returns.
● The loader is also called “runtime linker” or “dynamic
linker”.

Question 36

What is the difference between static and shared libraries?

Answer 36

●Shared libraries are shared across different
processes.
● There is only one instance of each shared
library for the entire system.
● Static libraries are not shared.
● There is an instance of a static library for
each process.

Question 37

Do Static or Dynamic events happen during program building?

Answer 37

Static – Events that happen during
program building.

Example: Static linker,

Static type checking.

Question 38

Do Static or Dynamic events happen while the program is running?

Answer 38

Dynamic – Events that happen while
program is running.

Also called “Runtime”.
Example: Dynamic linker or
Runtime linker,
Dynamic Type checking