Module 9: Intro to C

Question 1

Machine Code

Answer 1

binary files

Question 2

Assembly Code

Answer 2

text files (.asm)

Question 3

C code

Answer 3

text files (.c, .h)

Question 4

Compiler

Answer 4

converts a high level language (like C) to low level language (assembly) for example .c to .asm

Question 5

Assembler

Answer 5

converts assembly language to machine language object files (.asm to .obj)

Question 6

Linker

Answer 6

combines multiple object files (.obj) into a single executable object file; e.g. user_program + os.obj –> my_program.obj

Question 7

symbol table

Answer 7

created by a compiler to keep track of where data should be stored, the type of data stored, and the scope of the data being stored by the program it compiled

Question 8

local variables

Answer 8

the “scope” of these variables only exist within the {}. Variables must be declared at the top of the block!

Question 9

stack

Answer 9

in the context of the C language, the region in data memory where data required to support functions in the C language will be stored (functions such as arguments, local variables, and return values). it is organized like a physical stack: imagine piling one book on top of the next one in a stack of books. this is why it grows “backwards” in memory

Question 10

frame

Answer 10

logical grouping of the data on the stack that belongs to a single function. examples of data in the stack are arguments, local variables, and return values. as one function calls another each function will have its own frame.

Question 11

frame pointer

Answer 11

a register that holds a starting address for the frame or in the case of the LC4, a register holding the location where the calling function’s frame pointer was stored in the active function’s stack frame. sometimes called the base pointer.

Question 12

return address

Answer 12

address in program memory to return after function completes

Question 13

prologue

Answer 13

assembly code created by the compiler to construct the frame for any given function. it is the first code executed when a function is called. it is also responsible for “pushing” the stack frame for the active function onto the stack.

Question 14

epilogue

Answer 14

assembly code created by the compiler to “pop” the stack from the active function off of the stack. it copies the return value into the proper location on the stack so that the calling function has access to the returning data (if any) from the active function. it is executed every time a function exits.

Question 15

Big Ideas in C

Answer 15

a high level/low level language that binds the gap between a machine level language and high level languages

imperative

procedural

file-oriented (files: a simple abstraction for permanent storage and I/0)

portability: the same C code can be compiled for different ISAs

Question 16

prologue:

Answer 16

STR R7, R6, #-2
STR R5, R6, #-3
ADD R6, R6, #-3
ADD R5, R6, #0

arguments are pushed onto the stack from right to left
this lets the first argument from the left be the one closest to the callee
that is called “c convention”
it is needed for functions with variable argument counts e.g. printf
it can also lead to the order of evaluation being right to left (e.g. pow(a, ++a) may become pow(3, 3)

Question 17

pass by value vs. pass by reference

Answer 17

functions receive “copies” of local variables
recall that arguments to functions were copies of local variables
protects local variables from being modified accidentally

Question 18

Why are variables declared at the start of the function?

Answer 18

size of static/automatic variables are known at compile time

also, compiler may compiler line by line, hence right upfront!

Question 19

arrays’ order on the stack

Answer 19

the order of arrays on the stack is consistent with the memory address (i.e. in increasing order)
arr[0] - x7FF5
arr[1] - x7FF6
arr[2] - x7FF7