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rikotech8
Joined: 10 Dec 2011
Posts: 376
Location: Sofiq,Bulgariq
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| #bit |
 Posted: Sat Dec 10, 2011 2:38 am |
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| Hello. I'm amateur in writing codes at CCS. Can anyone explain me in details, the function of: #bit. Excuse my poor english :D, thx! |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19537
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| Posted: Sat Dec 10, 2011 3:40 am |
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It all ties up with #byte....
OK. If you declare a variable 'fred' say, then this is a name for a piece of memory, which you can store number in, without worrying 'where' the numbers are actually stored.
#byte allows you to locate such a variable _at_ a specific location. So you can use:
| Code: |
int8 fred;
#byte fred = 0x60
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Which then locates 'fred' at memory location 0x60. You can do this with any normal variable (char, int, int16, float etc...).
However there is a shortcut, that if you just use:
#byte fred = 0x60
Then the compiler will automatically _create_ an int8 variable and put it at the specified location.
There is also a second similar shortcut, with:
#word tom = 0x60
Which will create an int16 variable 'tom' and put this at memory location 0x60.
So these functions allow you to put a variable that is one byte or larger in size at a specific location.
However what happens if you want to access just on bit at a specific location?. Now, hopefully you understand, that a 'bit', is a 'binary digit'. A single '1' or '0'. The smallest element of data in a computer.
You could (for instance) set the bottom bit of a variable with:
| Code: |
int8 fred;
fred=32;
//now set the bottom bit with:
fred |=1;
//fred now is 33
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This "OR's" the variable with 1, so in the binary representation:
00100000 - fred
00000001
00100001 - result of the two values ORed together.
The bit in the result is a '1' if it is '1' in fred _or_ the second value.
Similarly you can turn off a bit using 'AND', toggle a bit with 'XOR' etc..
However in each case, you have to read the whole byte, perform the arithmetic, and write the byte back. Similarly to test a single bit you would have to perfrom an 'AND', and then test the result. Lots of work.
Key though is that the PIC, has instructions to access the individual bits in a register. Offering the ability to perform all these operations in single instructions.
#bit allows you to use these, and treat a single bit in a memory location as if it was a register in it's own right.
So:
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int8 fred;
#bit fred_low_bit = fred.0 //The bottom bit of fred
fred=32;
//now set the bottom bit with:
fred_low_bit=1;
//fred now is 33
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So this creates a variable called 'fred_low_bit', which directly accesses _just_ the bottom bit of 'fred'. You can read it, write it, test it etc...
Biggest use, is where a variable has been moved with #byte to correspond to a physical register in the chip, where individual bits of registers give specific information, or control particular functions. Using #bit you can directly access these bits.
Best Wishes |
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rikotech8
Joined: 10 Dec 2011
Posts: 376
Location: Sofiq,Bulgariq
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| Posted: Sun Dec 11, 2011 2:08 am |
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| Thank you for exhaustive information! It was very usefull for me. |
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