Application of left and right shift of data in C language-EEWORLD

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I have been debugging a relatively complex program these two days. The source code is as follows:

unsigned long Peek(long address )

{

unsigned long value;

//if( address != PokePointer )

{SetAddress( address );}

//if( PeekPointer >= PeekLimit )

//{throw "Peek addressing error!";}

value += Read_Register( DATA_A ) << 24;

value += Read_Register( DATA_B ) << 16;

value += Read_Register( DATA_C ) << 8;

value += Read_Register( DATA_D );

//PeekPointer++; /* maintain local pointer */

return value;

}

The data returned by Read_Register( DATA_A ) are all byte type. I wonder if anyone can predict the result of the PPEK() function. Obviously, there is a low-level error in this function, because all bits of byte type data will become 0 after being shifted 8 bits to the left! Therefore, the PPEK function only returns Read_Register( DATA_D ); So what should the correct answer be? My correction is as follows:

value += Read_Register( DATA_A );

value =(value<<8) + Read_Register( DATA_B );

value =(value<<8) + Read_Register( DATA_C );

value =(value<<8) + Read_Register( DATA_D );

or:

value += （unsigned long） Read_Register( DATA_A ) << 24;

value += （unsigned long）Read_Register( DATA_B ) << 16;

value += （unsigned long）Read_Register( DATA_C ) << 8;

value += （unsigned long）Read_Register( DATA_D );

From the low-level mistakes I made above, I came to the conclusion that when writing code, when shifting left or right, you must pay attention to the type of the data being shifted, that is, the number of bits, to see if the operation will cause data overflow!

One more thing to add: the priority of left shift << and right shift >> is relatively low compared to mathematical operations. Let's take a look at the following expression:

c=a<<8+b; //Then this sentence means to shift a left by 8+b bits and assign it to c

Then if you want to shift a left by 8 bits and then add the value of b to assign it to c, you should write it as the following expression:

c=（a<<8）+b;

The above are all low-level mistakes I have made. I hope those who see this can learn from them! ! !

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