lecture 9

Question 1

given assembly program (pl.s), what will the assembler do?

Answer 1

assembler will create one relocatable object file
pl.s –> pl.o

Question 2

what format is the relocatable object file?

Answer 2

bytes are in Executable Linkable File (ELF) format

Question 3

what happens to each machine instruction in the assembly program when translated to ELF format?

Answer 3

each machine instruction generated by the assembler gets assigned to a specific ELF section with a temporary memory address

Question 4

ELF sections

Answer 4

.text
.rodata
.data
.symtab
.reltext
.reldata

Question 5

what gets placed in the .text section?

Answer 5

machine instructions (ie your program)

Question 6

what gets placed in the .rodata section?

Answer 6

read only data (ie constants)

Question 7

what gets placed in the .data section?

Answer 7

initialized global and static variables

Question 8

what gets placed in the .symtab section?

Answer 8

symbol table
- holds the name and address
locations of functions and
global/static variables

Question 9

what gets placed in the .reltext section?

Answer 9

relocation info for .text section
- linker will relocate unresolved
instructions and addresses (later
step) –> linker must resolve these
to create an executable

Question 10

what gets placed in the .reldata section?

Answer 10

relocation info for .data section
- linked will relocate unresolved
data and addresses (later step) –>
linker must resolve these to
create an executable

Question 11

what are the 3 types of ELF binary object files?

Answer 11

1. relocatable object file (only created by the assembler, .o)
2. executable object file (a.out)
3. shared object file (.so)

Question 12

what counts as symbols in the .symtab?

Answer 12

name of function
global variable
static variable

Question 13

what is included in a symbol table entry?

Answer 13

symbol name
section symbol (will go in either .text or .data)
32-bit address in memory

Question 14

pl.o: * assembly lang (in c for example)

int sum (int *a, int n);
int array[2] = {1, 2};
int main() {
int val = sum(array, 2);
return val;
}

.symtab section:

        .symtab (pl)  symbol | section | address  -------------------------------------------- main     | .text      |    ?  sum      | .text      |    ?   array    |  .data     |   ? 

what does the assembler do first?

Answer 14

assembler identifies all symbols in the assembly program and updates .symtab

Question 15

pl.o: * assembly lang (in c for example)

int sum (int *a, int n);
int array[2] = {1, 2};
int main() {
int val = sum(array, 2);
return val;
}

.symtab section:

        .symtab (pl)  symbol | section | address  -------------------------------------------- main     | .text      |    ?  sum      | .text      |    ?   array    |  .data     |   ? 

what does the ‘?’ mean under address?

Answer 15

for those symbols –> they have unknown address locations within the program

*must be resolved by either assembler (second pass) or linker (next step)

Question 16

what sections will the symbols be from in ELF?

Answer 16

.text or .data only

Question 17

pl.o: * assembly lang (in c for example)

int sum (int *a, int n);
int array[2] = {1, 2};
int main() {
int val = sum(array, 2);
return val;
}

.data section:

        .data (pl)  address| global variable (read/write)  --------------------------------------------
0        |   array = {1, 2} 

what does the assembler do second?

Answer 17

assembler then translates the assembly data to machine data
- updates the .data section with
each variable with an assigned
address within .data

Question 18

pl.o: * assembly lang (in c for example)

int sum (int *a, int n);
int array[2] = {1, 2};
int main() {
int val = sum(array, 2);
return val;
}

.data section:

        .data (pl)  address| global variable (read/write)  --------------------------------------------
0        |   array = {1, 2} 

how much memory is allocated within the .data section for the array variable?

Answer 18

8 bytes

array holds 2 integer variables –> 1 int = 4 bytes

4 + 4 = 8 bytes in total

*array address starts at 0 and would go to 8

Question 19

pl.o: * assembly lang (in c for example)

int sum (int *a, int n);
int array[2] = {1, 2};
int main() {
int val = sum(array, 2);
return val;
}

.data section:

        .data (pl)  address| global variable (read/write)  --------------------------------------------
0        |   array = {1, 2} 

are the total number of bytes for array fixed?

Answer 19

yes

Question 20

pl.o: * assembly lang (in c for example)

int sum (int *a, int n);
int array[2] = {1, 2};
int main() {
int val = sum(array, 2);
return val;
}

.data section:

        .data (pl)  address| global variable (read/write)  --------------------------------------------
0        |   array = {1, 2} 

how many bytes is the .data section after the assembler updated it with the global variable, array?

Answer 20

8 bytes in total

Question 21

pl.o: * assembly lang (in c for example)

int sum (int *a, int n);
int array[2] = {1, 2};
int main() {
int val = sum(array, 2);
return val;
}

.text section:

        .text (pl)  address| global variable (read/write)  --------------------------------------------
0        |   array = {1, 2} 

how many bytes is the .data section after the assembler updated it with the global variable, array?

Answer 21

8 bytes in total

Question 22

steps of assembler translating file to machine language:

Answer 22

1. recognizes all symbols in the program and adds them to .symtab (function names, globals, statics)
2. goes to .data –> translates variables to machine language, and adds to .data section with address location
3. goes to .text –> translates instructions to machine language, and adds to .text section with address location
4. second pass –> goes back to .symtab and updates all the addresses of the found symbols within the program
5. if any symbols that are variables are unknown –> assembler adds them to .reldata for linker to resolve
6. if any symbols that are instructions are unknown –> assembler adds them to .reltext for linker to resolve

Question 23

what does the linker create?

Answer 23

executable object file

Question 24

whats an executable object file

Answer 24

object file that combines relocatable object files and shared object files (libraries)
*includes all data and instructions that can be copied into memory and ran (executed)

Question 25

what are the 2 roles of the linker?

Answer 25

1. symbol resolution
2. relocation

Question 26

what is symbol resolution?

Answer 26

for each symbol in symbol table that has unknown address (?) after both passes of assembler –> linker attempts to locate the same symbol (name and section type) in other symbol tables

*if not found –> linking step fails and error occurs

Question 27

what is relocation?

Answer 27

*after symbol resolution

relocates (copies) the .text and .data sections from one or more relocatable object files to the .text and .data sections in the executable object file

Question 28

relocation copies _______ FROM their ___________ ___________ TO their ____________ ____________.

Answer 28

data and instructions

relative address locations in the relocatable object file

final absolute address locations in the executable object file

Question 29

what does “fully linked” mean?

Answer 29

all data and instructions are copied to the executable object file with a memory address

Question 30

after resolution is done, do the instructions get new address in executable object file?

Answer 30

yes

Question 31

in order to run program:

Answer 31

loader –> copies sections in executable to main memory
operating system –> then starts executing the instructions that are in .text section first

Question 32

before loader can copy sections, what must happen first?

Answer 32

OS must assign/create memory:
- stack
- heap
- text
- data
*all sections in main memory that are specific to your program

Question 33

loader copies what into the read only section of main memory

Answer 33

all the instructions in the text section and all the read-only data in the read-only data section in the fully linked executable (each instruction and data is given address) into READ ONLY segment in RAM

Question 34

loader copies what into the read/write section of main memory

Answer 34

all the read/write data in .data section in executable into read/write segment in RAM