Lesson 19 Notes

Readings

Assignment

C Header Template

Lesson Outline

Introduction

At this point in the course we have learned about the MSP430 and its instruction set. We've learned some assembly programming constructs and implemented a few programs. We've learned about some of the subsystems on our chip and used SPI.

But most engineers and developers don't program in assembly.

Compilers

Let's talk about compilers. Everyone do the reading? If you haven't, you should - it's pretty entertaining and informative.

What are some other programming languages you know? [List on board]

List of Programming Languages

The languages you listed are all what we call High Level Languages (HLL).

What are some benefits of using these over programming in assembly?

Some perspective: everything that happens on a computer is machine code. What do we use to generate machine code? The assembler. So every program you write ultimately becomes assembly code, then machine code.

Remember our workflow: assembly code --> assembler --> relocatable machine code --> linker --> executable

The compiler adds a layer on top of that. It converts code written in a higher language into assembly, which can then be fed into the rest of our process:

HLL --> compiler --> assembly code --> assembler --> relocatable machine code --> linker --> executable

or, since we'll be compiling and assembling code for a different architecture than the one our computer is running, we'll be using a cross-compiler / cross-assembler:

HLL --> cross-compiler --> assembly code --> cross-assembler --> relocatable machine code --> linker --> executable

To further our understanding, let's break it down a little further - into Compiled and Interpreted languages.

Compiled languages fit this model exactly.

Interpreted languages (scripting languages) run code on top of their own interpreter, which is written in a compiled language. [if there is interest, can talk about interpreter / bytecode / bytecode interpreter / JIT compilation / etc]

Some languages, like Java and C#, live somewhere in the middle. They're compiled into bytecode which can be run on a virtual machine.

REGARDLESS, all code run on a computer is assembly code, then machine code. Don't lose sight of that connection.

Are there any disadvantages to using an HLL over assembly?

Introduction to C

The HLL we'll use for the remainder of the course is C. C is one of the most widely used programming languages of all time - and is still used for a ton of huge / important projects:

C is a portable, higher-level assembly. That's the way you should think about it.

Great programmers use C very precisely to generate the exact assembly they want to perform a given task - so it's important to understand how the C constructs we'll learn about map to assembly. I'll save that for a later lesson.

The reason it's still used in many modern applications is that it gives you a lot of control over the generated assembly - making it FAST and MEMORY EFFICIENT.

Comments

// Single line comment

/* block comment that can span 
multiple lines */

int i = 0;  // a declaration

/*************************************
  ** The previous variable was
  ** declared just as an example.
*************************************/

Variables

Variable Types

Type Size Description
char 1 byte number or ASCII character
int 2 bytes larger number
float 2 bytes single-precision floating point number
double 4 bytes double-precision floating point number

Variable Modifiers

Modifier Description
short remains a 2-byte integer
long increases int size to 4 bytes
signed two's complement numbers (default)
unsigned allows unsigned arithmetic
static directly allocates memory to remember a value between function calls. Variable is allocated to "permanent" memory, not the stack.
extern atual storage and initial value of variable is defined elsewhere
const assigns a constant (read-only) value to a variable

Preferred Constant Declaration

The #define statment is a pre-processor directive. The pre-processor will go through and replace each instant of the variable with the value before compilation, similar to a .equ statement in assembly.

// #define MY_CONST some_value

#define SCREEN_WIDTH 640
#define SCREEN_HEIGHT 480

int numPixels = 0;
numPixels = SCREEN_WIDTH * SCREEN_HEIGHT;

Assignment, Arithmetic Operators

char myVar, a, b;  // variable declaration

myVar = a;  // assignment - note, all vars have undefined values at this point

myVar = a + b;  // addition
myVar = a - b;  // subtraction
myVar = a * b;  // multiplication
myVar = a / b;  // division

myVar = a % b;  // modulus (remainder)

myVar++;        // increment
myVar--;        // decrement

myVar += a;     // myVar = myVar + a
myVar -= a;     // myVar = myVar - a

Relational Operators

Operator Description Example
< less than
<= less than or equal to
> greater than
>= greater than or equal to
== equal to
!= not equal to
&& logical and
|| logical or

Example:

if ((a < 10) && (a > 5)) 
{
    // literally: if a is greater than 5 and less than 10, do whatever is in here
    // practically: if a is between 5 and 10, do whatever is in here
}

Bit-wise Operators

Operator Description Example
& AND
| OR
^ XOR
~ One's Complement
>> Bit-shift right
<< Bit-shift left
// Example with SPI

UCA0CTL1 &= ~UCSWRST;            // disable the subsystem (AND UCA0CTL1 with NOT UCSWRST)

// Do config stuff in here

UCA0CTL1 |= UCSWRST;             // enable the subsystem (OR UCA0CTL1 with UCSWRST)

if Statement

Conditional code execution based on a logical expression.

General case:

if (logical expression) 
{
    statements;
} else if (logical expression)
{
    statements;
}
...
else
{
    statements;
}

Example:

if (temp < MIN_TEMP) 
{
    flag = TOO_LOW;
} else if (temp > MAX_TEMP)
{
    flag = TOO_HIGH;
}
else
{
    flag = JUST_RIGHT;
}

switch Statement

Conditional code execution based on a value.

General case:

switch (value)
{
    case constant-expression1:
        statements;
        break;
    case constant-expression2:
        statements;
        break;
    default:
        // gets executed if no other case hits
        statements;
        break;
}

Example:

switch (GAME_STATE)
{
    case MENU:
        displayMenu();
        break;
    case PLAYING:
        updateState();
        break;
    case LOST:
        displayLost();
        break;
}

for Loop

Looping construct that tests logical expression and performs operation on an iterator variable.

General case:

for (initial; continue; increment)
{
    statements;
}

Example case:

for (i = 1; i <= 20; i++)
{
    sum += i;
}

while / do while Loop

Looping construct dependent on a logical expression.

General case:

while (condition) {
    statements;
}

do
{
    statements;
} while (condition);

do while is guaranteed to be executed once, while isn't.

Example:

int i = 5;

while (i < 10)
{
    i++;
}

do {
    i++
} while (i < 10);

// final value of i is 11

Basic C Program Structure

General case:

// #include statements
// #define statements

// global variables

void main(void)
{
    // Variable declarations
    // Useful code

    while (1) {}        // trap the CPU
}

Example:

#include "helper.h"
#define NUM_LOOPS 23

void main(void)
{
    unsigned char i;
    unsigned int summation = 0;

    for (i = 1; i <= NUM_LOOPS; i++)
    {
        summation += i;
    }

    while (1) {}        // trap the CPU

}

Header Example

Don't forget your code style!

/*--------------------------------------------------------------------
Name: <Your Name>
Date: <The date you stated working on the file>
Course: <The course's name>
File: <This file's name>
Event: <HW# and name>

Purp: A brief description of what this program does and 
    the general solution strategy. 

Doc:    <list the names of the people who you helped>
        <list the names of the people who assisted you>

Academic Integrity Statement: I certify that, while others may have 
assisted me in brain storming, debugging and validating this program, 
the program itself is my own work. I understand that submitting code 
which is the work of other individuals is a violation of the honor   
code.  I also understand that if I knowingly give my original work to 
another individual is also a violation of the honor code. 
-------------------------------------------------------------------------*/