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Left Shift operator

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pilar

Joined: 30 Jan 2008
Posts: 197

Left Shift operator

Posted: Wed Aug 03, 2011 9:50 am

Hi, seem like a silly question but someone can tell me what this means?

Code:

data = 0xAB
data << 4;

Left Shift data 4 bits?

PCM programmer

Joined: 06 Sep 2003
Posts: 21708

Posted: Wed Aug 03, 2011 12:49 pm

Quote:

data = 0xAB;
data << 4;

This won't do anything because it doesn't assign the result to an 'lvalue'.
This means it doesn't put the result into a variable.

Compile a test program with that code and look at the .LST file.
Notice the compiler doesn't even generate any code for the left-shift line:

Code:

....................
.................... data = 0xAB;
0022: MOVLW AB
0024: MOVWF data
.................... data << 4;
....................

If you put the assignment symbol after the operand, then you will get
the left-shifted result in 'data':

Code:

data = 0xAB;
data <<= 4;

Now look at the .LST file. It's actually doing something this time.
It's generating the left-shift code and it puts the result into 'data':

Code:

.................... data = 0xAB;
0022: MOVLW AB
0024: MOVWF data
.................... data <<= 4;
0026: SWAPF data,F
0028: MOVLW F0
002A: ANDWF data,F
....................

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