Welcome: 03 Assembly

C -> ASM

"Programming" means "writing C code".
On Linux-like (UNIX, *nix, POSIX) systems, the operating system API is primarily exposed to C programs through the system C library.
The hardware doesn't run C though - it runs amd64 machine code (on your laptop) or ARM machine code (on your phone) or maybe some other machine code.
Machine code is for machines, not humans, so it's hard to read.
Machine code is a series of instructions. If you write the instructions down as text, you get assembly language.
To run a C program, you need to translate to machine code (or "binary").
Conceptually, and historically, you first translate C -> ASM, then ASM -> binary.
You can still do this if you explicitly ask for it.

Note: For the first few homeworks you will be writing ASM programs. You should not have a compiler do this for you. Submitting compiler output for an assembly assignment is worth zero points.

Example:

// A C program is a collection of functions.
// Here's a minimal program with one function
int
main(int argc, char* argv[])
{
  printf("Hello C program\n");
  return 0;
}

# Direct C => binary
$ gcc -o hello hello.c
$ ./hello

# C => asm
$ gcc -S -o hello.s hello.c
# take a look at hello.s

# asm => binary
$ gcc -o hello hello.s
$ ./hello

Interesting stuff in hello.s:

The string is there, but no newline.
The main function exists
- Starts at label "main"
- Ends at "ret".
- Declared ".globl"
In the main function another function is called - not printf, but puts.
The optimizer got to us.

Let's tell it to be less clever:

# C => asm
$ gcc -fno-builtin -S -o hello.s hello.c
# take a look at hello.s

Now the string has a newline.
And the function called is "printf".

How about with two functions:

// add1.c
long
add1(long x)
{
    return x + 1;
}

int
main(int _ac, char* _av[])
  // initial _ marks args as not used
{
    long x = add1(5);
    printf("%ld\n", x);
    return 0;
}

# C => asm
$ gcc -S -o add1.s add1.c
# take a look at hello.s

Two functions: add1, main
- each starts at label, ends at "ret"
In main, the value 5 is moved to "%rdi"
- That must be where the function's first argument goes.
- No, that's "%edi"
- I said "%rdi", wait a second...
Then add1 is called
In add1, the value from %rdi goes to some places.
Eventually, "addq $1, ..." happens to it.
Back in main, %rax is moved to %rsi, and printf is called.

This almost makes sense, but it's a bit of a mess. Let's figure it out.

ntuck@neu

C -> ASM