C & Compilation

Question 1

What are the common languages of choice for low level programming

Answer 1

+ C due to the amount of control given to the programmer
+ Python because its easier for a programmer to understand and write code quickly
+ Rust because safety go burrr

Question 2

What do *.h files contain

Answer 2

The declarations for functions and variables which a given c file exposes

Question 3

What number can be special in some scenarios in c, what are some of these scenarios

Answer 3

0:
+ Any value which is not 0 is true
+ \0 is used to mark the end of strings
+ A pointer to address 0 is NULL and invalid

Question 4

When is cross compilation used

Answer 4

When compilation on the target device is impossible or impracticable, for example:
+ For the first compiler on new hardware or a new OS
+ For low capability targets such as the Pico’s

Question 5

What are the 4 stages of the compiler driver (to produce an object or ELF file from a C file)

Answer 5

Preprocessor
Compiler
Assembler
Linker

For cross-compilation we use:
Cross-Compiler
Cross-Assembler
Cross-Linker

Question 6

What is the job of the preprocess, what files does it consider

Answer 6

The preprocessor takes *.c files, expands macros and replaces include directives with the code blocks they reference. This produces a .i file

Question 7

What is the job of the compiler in the compile driver process

Answer 7

The compiler takes preprocessed code, parses it and optimises it.

Question 8

To what form does the compiler (as a step) parse the code it is given

Answer 8

The compiler parses code to an “Intermediate Representation”, this is a strongly typed, assembly like, RISC instruction set which is abstract from details of source code and the target machine

Question 9

What does the compiler (as a step) do once it has parsed code to an Intermediate Representation (IR)

Answer 9

It parses it to an optimiser such as the LLVM optimiser which apply’s a set of known optimisations (such as reversing the direction of for loops) to it

Question 10

Why is it useful to parse C code to an Intermediate Representation before optimising it

Answer 10

Because it allows the same optimiser to be used for multiple languages

Question 11

What does the assembler convert its input to

Answer 11

The assembler takes the intermediate representation of the program and converts it to a relocatable file such as .o. or .elf

Question 12

What are the 4 most important sections of a relocatable file (i.e. *.o or *.elf)

Answer 12

.text
.rodata
.data
.bss

Question 13

What does the .text section of a relocatable file contain, and where is this placed when the file is loaded

Answer 13

.text contains instructions and gets placed into flash memory when the program is loaded

Question 14

What does the .rodata section of a relocatable file contain and where is this placed when the file is loaded

Answer 14

.rodata contains read only data and is placed into flash memory when the program is loaded

Question 15

What does the .data section of a relocatable file conatain and where is this placed when the file is loaded

Answer 15

.data contains Initialised global variables which are placed into flash to be copied into RAM when the program is loaded

Question 16

What is the job of the linker

Answer 16

The linker takes Relocatable files and combines them to produce new Relocatable Files, merging the sections of the files

Question 17

What does the linker bottleneck refer to

Answer 17

The inker bottleneck refers to the fact compilation can be done in parallel, while linking cannot and MUST be done sequentially

Question 18

What operation does a loader perform

Answer 18

A loader accepts a relocatable file and places the sections into physical memory

Question 19

When a c program is loaded into memory, where are global and static variables placed (for one example memory location

Answer 19

Global and static variables are placed after the text segment and before the heap

They are split up into those which are initialized and those which are not

Question 20

What is the heap used for

Answer 20

The heap is the segment in memory which can be dynamically allocated, this can be accessed with commands such as malloc, realloc and free

Question 21

What is the stack used for

Answer 21

The stack is used to store stack frames, these are created whenever a function is called and removed when the function is left, they contain local variables

Question 22

How could the amount of memory available to a program be estimated

Answer 22

By creating a new item on the stack (new local var) s, and a new item on the heap (i.e. with malloc) h and taking s* - h*