The Language Hierarchy: electrons to web

CS 301: Assembly Language Programming Lecture, Dr. Lawlor

You can think of a computer as running very simple low-level operations like moving electrons, or as running very high-level operations like looking at a web page. The appropriate level depends on what you're trying to achieve, but it's useful to know at least a little about all the levels.

Web user interface, where you can click and type
JavaScript code, which mostly moves around JavaScript Object Notation (JSON) or Document Object Model (DOM) elements on a web page or across the network
JavaScript engine, like Google's v8, written in C++ to interpret JavaScript fast by converting it to machine code
C++, the main language used in our CS program
C, the simpler predecessor of C++
Assembly language, human-readable version of machine code
Machine code, raw binary CPU control, as discussed here
CPU transistors, as discussed in EE 341 (Digital Circuits), and EE 443 or CS 441 (Architecture)
Individual electrons, as discussed in Physics 212
Quantum wavefunctions, as directly manipulated by a quantum computer (currently either small, specialized, or theoretical!)

Field guide: C versus C++

Plain C is a simple, low-level language, and it's nearly a subset of C++, which was originally named "C with classes". If you look at the size of the author's book on each language, you can immediately see the big complexity difference!

C textbook Kernigan and Ritchie, a thin paperback, versus
Straustrup's Ocean Book on C++, a thick hardback.

Generally, these features are shared between C and C++:

Variables like "int" and "char"
Arithmetic operations like + - * /
Assignments like =, or += and such.
Functions, curly braces {}
if statements, switch statements
for loops, while loops, do loops
arrays
pointers

These features are only C++, not available in plain C:

class, constructor, destructor, inheritance, or virtual methods. Plain C uses functions or structs for similar operations.
new and delete. Plain C uses functions named malloc and free instead.
templates. Plain C uses macros for similar features.
std::string. Plain C uses arrays of characters for strings, which are more error-prone.
std::cout and std::cin. Plain C uses printf and scanf, which I actually prefer, but are harder to learn, and can't be extended.
std::vector. Plain C uses bare arrays, and a function named realloc.
std:: anything else!

In this class, we'll be covering both the implementation of C++ features in plain C and assembly, and plain C as a language on its own. Quite a few large projects are built in plain C, including the linux kernel, and many system libraries.

Field guide: assembly versus machine code

Machine code is the raw binary data that the CPU executes. The individual bits of machine code are the actual inputs to the transistors in the CPU's control system that make the CPU eventually (many stages later!) compute stuff.

Assembly language is a human-readable version of machine code.

If you compile a simple function that just does "return 7;", you can see assembly language instructions "mov" and "ret", and the binary machine code that the CPU actually runs. (Hit the "Disassemble" checkbox on NetRun to see this yourself; we'll be covering this starting next week.)

0000000000000000 <foo>:
   0:	b8 07 00 00 00       	mov    eax,0x7
   5:	c3                   	ret   
        Machine Code (in hex)   Assembly Language