Our first program in C is going to be a conversion of “PushButtonLEDTest.spin” from the Propeller Education Kit Labs: Fundamentals Chapter 3: Set Up and Testing Lab Page 35.
The schematic and hardware layout that go with this lab are:

Figure 3-7: Test Circuit Schematic Page 33
Figure 3-8: Test Circuit Wiring Diagram Page 34

The conversion is a direct replacement for the original spin program. Most of the content is dealt with in finer detail in later labs so don’t worry if there are gray areas all will be clear by the end of the labs. 

The aim was to introduce some C concepts and also show a small mix of the familiar instructions we saw in Spin. 
Before making hardware or software changes read the sections on page 37 of the Propeller Education Kit Labs: Fundamentals titled 

Before Changing or Adjusting Circuits
Propeller Supply Voltage Regulation – It’s Important!

One thing every C program contains is the “main” function, this is where the main body of our C program resides, contained within a set of curly brackets. Below is the main function written in one of its simplest forms, this will be fine for our conversion.

Int main()
{
return 0;
}


Here is the full listing of the conversion, there are some things that might have been done differently but this gives us the opportunity to examine some of the basic elements of programming the Propeller using GCC.

 
Line 1  #include
The include directive tells the compiler to add a header file or header files to our program. Header files contain information needed by certain functions and instructions contained in our program. In this conversion we have included the header file Propeller.h, all #include files have the .h extension. Which header files you use in the future depends which functions and instructions you intend to use and can be found by looking at the many example programs.


Line 2   #define PushButtonPin (x) (1<<x) 
The preprocessor directive #define is used here to create a function macro. The macro name is PushButtonPin () and it takes a parameter x. The last two brackets contain the definition. Whenever the preprocessor encounters PushButtonPin (x) in the code it will be replaced with the definition (1<<x). The two arrows represent the shift left bitwise operator and the definition is read as shift 1 left x number of places. For example PushButtonPin (4) would be replaced in our code with 1<<4 (shift 1 left 4 places) which equates to the binary value 10000 (the 1 starts at zero and moves 4 spaces left) or hex value 0x10. Look for its usage on line 13.


Line 4  int main()
This is the start of our main function, the curly brackets for this function are on lines 5 and 23.


Line 6  DIRA=0xFFFF;
DIRA is the instruction that sets the I/O direction of the Propeller pins. If a bit is set to a 1 that pin is an output, if a bit is set to a 0 that bit is an input. In a little more detail DIRA is a 32 bit register and we want to use P0 to P15 as outputs for the LED’s , if we look at 0xFFFF as a binary number it is easier to see what we need. (P0 is the right most digit and P31 is the left most digit)
DIRA=00000000000000001111111111111111
The first 16 bits are set to a 1 which are outputs, these are P0 to P15, the higher 16 bits are set to 0 and are set to inputs, these are P16 to P31. Convert the binary number above to a hexadecimal number and we get DIRA=0xFFFF. 

Look for the instructions that require a semi-colon, it is a common mistake to omit the semi-colon from the end of a line of code.

Line 8  While(1)
While is used to create a conditional loop. The way it is being used in our example, While(1), it creates a never ending loop constantly executing the code within its curly brackets (line 9 and line 20). It is good practice to indent each pairing of curly brackets from the previous pair, this makes it much easier to read your code.

Are you watching where semi-colons are not needed? 

Line 11  OUTA ^= DIRA;
^=  is the bitwise XOR operator, each pass through the While loop OUTA is XOR’d with DIRA , this has the effect of toggling the output pins on and off with each loop.

Line 13  if (pushbutton(18) & INA)
The if instruction looks at the condition within the brackets, if this condition is true then it executes the statement that follows the if instruction, if this condition is not true then the statement following the else instruction is executed. We are basically saying if the pushbutton is pressed do one thing else if not then do something else. 
So how do we derive the pin number the push button is on from (PushButtonPin (18) & INA)? First we have to go back to line 2 where we #defined pushbutton and the value we assigned (1<<pin). Our example is saying shift a bit value of 1 to the left 18 places 
PushButtonPin (18)
is replaced with
(1<<18)
which produces a 1 at bit 19 (P18)
00000000000001000000000000000000
If we do a bitwise AND with PushButtonPin(18) & INA we can detect when the push button  is pressed. The only time this condition will be true is when bit 19 of INA == bit 19 of PushButtonPin  in other words when the push button on P18 is pressed (P18 will be high or logic 1). If the push button is moved to a different pin then all we have to do is change the value in parenthesis and make sure the corresponding bit in DIRA is set to input. 
What we created above was a simple “mask”. A mask is used to filter a bit or several bits from another value.

Lines 15 & 18  waitcnt(CLKFREQ/4 + CNT); & waitcnt(CLKFREQ/20 + CNT);
These two instructions use the system clock to create a timed delay of ¼ of a second and 1/20 of a second respectively. Chapter 4: I/O And Timing Basics Lab discusses waitcnt in detail.

Finally download your program to the Propeller using Propgcc . The attached LED should start to blink and pressing the button will temporarily change the frequency.