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is volatile array needed?

chao lin

There is a string buffer referenced by both an ISR and the main(). The ISR collects a sentence from UART and put it into the buffer. Once the received sentence is completed a flag is raised. The main() checks the flag and copies the sentence to another buffer. The code is like

unsigned char xdata RxBuffer[];
unsigned char xdata buffer[];
bit RxFlag = 0;

void uart( void )
{
  //receive data from sbuf
  RxBuffer[] = sbuf;
  //if done
    RxFlag = 1;
}

void main( void )
{
  while( 1 )
  {
    if( RxFlag )
      strcpy( buffer, RxBuffer );
  }
}
The question is "Should I declare RxBuffer as volatile?" RxFlag doesn't need to be volatile because it is an automatic variable. The assembly code is "JNB RxFlag, xxxx". By the way, strcpy() is a reentrant function and the UART is at 4800 bps. The worst case is while strcpy is coping data, RxBuffer starts storing another new sentence. The purpose is to keep the integrity of the received data very time. I know I can use ES = 0 and ES = 1 before and after strcpy().

Three more questions:
1. Do I misunderstand the usage of volatile on array?
2. Does the reentrancy play any significant role here?
3. What is the difference between memcpy() and strcpy()? What are the advantages and disadvatages on each?

Thank you for answering my questions.

chao.

Parents
  • 0

    "Should I declare RxBuffer as volatile?"

    Yes.

    "RxFlag doesn't need to be volatile"

    Yes it does, although it may be the case that bit variables are implicitly volatile in Keil. I'm not sure.

    To the compiler 'volatile' essentially means "don't optimise any usage of this variable".

    "Does the reentrancy play any significant role here"

    No, as you are not calling functions from your ISR.

    "What is the difference between memcpy() and strcpy()? What are the advantages and disadvatages on each?"

    memcpy() copies the specified number of bytes from source to destination, strcpy() copies bytes until the first zero byte in source. Check your C book.

    Stefan

Reply
  • 0

    "Should I declare RxBuffer as volatile?"

    Yes.

    "RxFlag doesn't need to be volatile"

    Yes it does, although it may be the case that bit variables are implicitly volatile in Keil. I'm not sure.

    To the compiler 'volatile' essentially means "don't optimise any usage of this variable".

    "Does the reentrancy play any significant role here"

    No, as you are not calling functions from your ISR.

    "What is the difference between memcpy() and strcpy()? What are the advantages and disadvatages on each?"

    memcpy() copies the specified number of bytes from source to destination, strcpy() copies bytes until the first zero byte in source. Check your C book.

    Stefan

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