Difference between Define and Variable

skybox63 · Joined: 30 Jun 2018 Posts: 17

Hello,

I have another question:

What is the difference between these two statements:

Jerson · Joined: 31 Jul 2009 Posts: 125 Location: Bombay, India

A #define is simply a text replacement command. So, whenever the compiler sees the #define keyword, it replaces the text following the #define keyword with the text after that. On the other hand, a variable is usually a RAM location that holds a value (which could also be something that you have #defined).

Ttelmah · Joined: 11 Mar 2010 Posts: 19548

Jerson has given the basic explanation.

#DEFINE replacement is done before the actual code compilation.
On most compilers there is a separate 'preprocessor' pass, and it is this that does the #define substitutions. In CCS it is done 'first', but only as an internal pass inside the compiler operation.

it is fractionally more powerful than basic text replacement, but only in very limited ways. So (for instance) you can concatenate text strings, and can use it with the preprocessor commands to do certain types of test.

Now, neither the lines you actually show would do anything worthwhile without other code. 'PIN_B0' in itself means nothing, unless you have already had a statement to make this a variable than can be read.

The statement pin=PIN_B0, means "at runtime, load the value of the variable PIN_B0, into the variable 'pin'". So what it does is dependant of what the variable 'PIN_B0' actually 'is'.

The #define just substitutes the text 'PIN_B0' whenever the text 'PinB0' is typed.

A variable uses RAM space.
A #define does not.
A typical use of #DEFINE it to improve readability. So:

temtronic · Posted: Sun Jul 08, 2018 5:17 am

couple of comments...

#DEFINE can also be used to create 'macros'. Small 'chunks of code' that
need get used throughout your program and it can save you a LOT of typing !

also

while your project is open, press F11, and the CCS manual 'magically' appears. You can easily search it 'tidbits' of information as well as lookup the correct syntax for functions and things. There are a few typos though generally it's accurate. As well, look at some of the examples CCS supplies in the 'examples' folder. Most are fairly well commented and cover most peripheral devices as well as internals like timers, UART, I2C.

Jay