Meaning of the statement in C++?

[areeb]

What does the following C++ code do??

Hi guys,

I am new to programming, what will be the output of the following code, please respond step by step how will this code work. Thanks!

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Code C - [expand]
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#define reloff PRT1DR &= 0x3a
 
char d=-20;
void main()
{
while(1)
  {
    d++;
    if(d==10)
    {
       d=290;
    }
    if(d>1000)
    {
       d=0;
    }
  }
}

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[andre_luis]

Re: What does the following C++ code do??

The code above seems more complying with C syntax than with C++.
There is no output declared there, so I'm assuming that variable d is what you are interested.

Moreover, there are some possible inconsistencies, such as declaring a variable as char type ( whose magnitude is 8 bits in most uCs ) but you're attempting to store values greater than 255.

 

[areeb]

Hi, can any please explain what does the following statement mean and what is the purpose? Thanks

#define reloff PRT1DR &= 0x7f

 

[K33rg4t3]

PRT1DR = PRT1DR AND 0x7f

it's a bitwise AND operation
(for each bit of PRT1DR and 0x7f)

and #define is making "reloff" work like compile-time function doing that operation

 

[areeb]

Ahan, and what is PRT1DR? And reloff?? I am not able to find any reference that explains these terms.

 

[K33rg4t3]

reloff in this case is user-defined name - you can use any other valid name you want (as long it's not used somewhere else)

PRT1DR - you need to tell me what is the source of this code and what machine, but it looks like a specific MCU register name...

 

[betwixt]

I would guess it's a reference to the port direction bits on a Cypress processor.

"PRT1DR &= 0x7f" means the contents of PRT1DR are ANDed with 0x7F and the result put back in PRT1DR, in other words, bit 7 is made zero with other bits unchanged.

"#define reloff PRT1DR &= 0x7f" just makes a new instruction in your program called 'reloff' which the compiler expands to 'PRT1DR &= 0x7' when it sees it.

So 'reloff' makes bit 7 of PRT1DR become zero.

Generally, we use #define like that to make the program more readable, for example you can use it so a port pin becomes synonymous with what is connected to it. So if you used "#define RelayOn PORTX,6=1" you could use the command "RelayOn" in your code rather than an instruction that didn't describe what it actually did. It also makes it a lot easier to modify the code afterwards. Example: if the relay was changed to bit 5 instead of 6 you only have to change the number in the definition and recompile. Without it you would have to go through the entire code and edit all the occurrences from 6 to 5.

Brian.