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Dargar
Joined: 12 Dec 2003
Posts: 25
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| What does the " ^ " character signify in C?? |
 Posted: Wed Nov 02, 2005 10:32 am |
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I have far from seen it all, as this question will prove beyond a doubt. ;)
In the following code segment from http://www.ccsinfo.com/forum/viewtopic.php?p=53837#53837 (ckielstra's thread), the ^-sign is used to do "something" but I've never seen it before and I cant find it in the documentation or with google.. I guess that is more due to my searches beeing bad than anything else...
Anywas, if anyone could explain a couple of the lines using the ^operator I would be very happy:
| Code: |
int16 crc_1021(int16 old_crc, int8 data)
{
int16 crc;
int16 x;
x = make8(old_crc,1) ^ data; //x = ((old_crc>>8) ^ data) & 0xff;
x ^= x>>4;
crc = (old_crc << 8) ^ (x << 12) ^ (x <<5) ^ x;
crc &= 0xffff;
return crc;
}
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_________________
Owner of PCWH v. 3.224
Consider myself beginer\intermediate as far as CCS and Microchip PIC's goes.
Bachelor in Space Engineering and Undergraduate Masters in Space Engineering
soon to be undergrad Aerospace |
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Ttelmah
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| Posted: Wed Nov 02, 2005 10:40 am |
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Bitwise exclusive OR.
Best Wishes |
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Dargar
Joined: 12 Dec 2003
Posts: 25
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| Posted: Wed Nov 02, 2005 10:47 am |
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aha!
Good. Need to write an implementation for this in LabVIEW as well, for the ground control, and wasnt sure how to translate that! :D
Thanks!
_________________
Owner of PCWH v. 3.224
Consider myself beginer\intermediate as far as CCS and Microchip PIC's goes.
Bachelor in Space Engineering and Undergraduate Masters in Space Engineering
soon to be undergrad Aerospace |
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dyeatman
Joined: 06 Sep 2003
Posts: 1924
Location: Norman, OK
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| Posted: Wed Nov 02, 2005 2:12 pm |
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Dargar,
Just for reference it is documented in the October version of the CCS Manual on PDF page 82. |
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Dargar
Joined: 12 Dec 2003
Posts: 25
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| Posted: Wed Nov 02, 2005 2:42 pm |
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Thank you, always nice to know!
_________________
Owner of PCWH v. 3.224
Consider myself beginer\intermediate as far as CCS and Microchip PIC's goes.
Bachelor in Space Engineering and Undergraduate Masters in Space Engineering
soon to be undergrad Aerospace |
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Dargar
Joined: 12 Dec 2003
Posts: 25
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| Posted: Fri Nov 04, 2005 5:09 am |
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[edit]
after reading in the manual, I can not find any specific information for the << and >> operators, and they are not the same as the shift_left and shift_right are they? Sorry if I missed it, but I did search through the manual using pdf.
[/edit]
humm. would have been nice with some more explicit details on the ">>" and "<<" operators. do they shift in zeroes or wrap around or ones?
After answering the above, bear with me as I try to "compile" the c-code above into words to check that I understand whats going on:
(see comments for each line)
| Code: |
int16 crc_1021(int16 old_crc, int8 data)
{
int16 crc;
int16 x;
x = make8(old_crc,1) ^ data; //x = msb of old_crc
x ^= x>>4; //x is xor'ed with y, where y = x shifted 4 bits to the right
crc = (old_crc << 8) ^ (x << 12) ^ (x <<5) ^ x;
// "shift old_crc 8 steps to left" XOR'ed with "x shifted 12 steps to left"
// XOR'ed with "x shifted 5 steps to left" XOR'ed with x... does the shifts shift in 0's 1's or wraps?
crc &= 0xffff;
return crc;
}
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_________________
Owner of PCWH v. 3.224
Consider myself beginer\intermediate as far as CCS and Microchip PIC's goes.
Bachelor in Space Engineering and Undergraduate Masters in Space Engineering
soon to be undergrad Aerospace
Last edited by Dargar on Fri Nov 04, 2005 5:12 am; edited 1 time in total |
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Foppie
Joined: 16 Sep 2005
Posts: 138
Location: The Netherlands
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| Posted: Fri Nov 04, 2005 5:12 am |
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| they shift in zero's |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Posted: Fri Nov 04, 2005 6:06 am |
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| Standard C stuff. Rotate Left and Right. |
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Dargar
Joined: 12 Dec 2003
Posts: 25
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| Ok, got all of it. but still trying to understand it |
| Posted: Fri Nov 04, 2005 11:01 am |
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[edit]
for some stupid reason, I left all my CCS development kits in the states when I went to Norway for a 3 month stay, so I cant run the CCS code and print to a terminal like I would normally do!!
[/edit]
I got how it works and have reproduced it in LabVIEW, I think.
To validate, I took the PIC c-code and adapted it to run as a console application in Microsoft Visual Studio C++ (no flaming pls, I also own a Borland Builder C/C++ lisence! ;) ).
Anyway, my question has to do with what the CCS "Make8()" is equivalent to.
According to the CCS documentation:
| Quote: |
Extracts the byte at offset from var. Same as: i8 = (((var >> (offset*8)) & 0xff) except it is done with a single byte move.
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But... what is the purpose of the last "& 0xFF"? and'ing with 1's will leave 0's where there was 0's before, and 1's where there where 1's before, e.g. it wont change anything? (and the shift has already zeroed the upper parts of the 16 or 32 bit value.)
I tried to remove this in the c++ app, without any changes to the outputs!
(var's in c++ defined as such:
unsigned __int16 old_crc;
unsigned __int16 crc;
unsigned __int16 x;
unsigned __int8 data;
)
Also, is it recommended to start with "old_crc" as 0xFFFF or 0x0000 when running the first byte through?
[edit 2]
And it's the same with the very last instruction, crc &=0xFFFF; Why on earth waste cycles on this? Does it have to do with how CCS compiles or have I totally missed something here?
[/edit 2]
_________________
Owner of PCWH v. 3.224
Consider myself beginer\intermediate as far as CCS and Microchip PIC's goes.
Bachelor in Space Engineering and Undergraduate Masters in Space Engineering
soon to be undergrad Aerospace |
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Ttelmah
Guest
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| Posted: Fri Nov 04, 2005 11:12 am |
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The 'key' to this, lies in C.
A >> operator, _does not_ guarantee to fill with zeroes. It _will_, if the value is 'unsigned'. However for a 'signed' value, what happens is left to the processor. So in K&R, it says:
"Right shifting a signed quantity will fill with sign bits ("arithmetic shift") on some machines, and with 0 bits ("logical shift") on others".
Hence the generic method to obtain the top byte of a two byte number (which will work on either machine type), is to shift, then mask.
CCS, were posting the precise 'equivalent' function, that will work with processors that behave in either way.
Best Wishes |
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Dargar
Joined: 12 Dec 2003
Posts: 25
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| Posted: Fri Nov 04, 2005 11:25 am |
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aha!
That explains that bit.
I guess it actually explains why the
as well, only, in this case we actually have total control over the processors, so my question then is: For Microchip PIC targets compiled with the CCS compiler, is it strictly necessary or can this be dropped (foregoing "ANSI C") to speed up the code with a couple of cycles?
Actually, a better way to answer me on this one could be to explain the steps I can take in CCS to compare the code generated by the two different approaches and how the thought process goes when determining if it is necessary.
(From the previous post by Ttelmah, it should NOT be necessary since integers in CCS by default is unsigned?)
_________________
Owner of PCWH v. 3.224
Consider myself beginer\intermediate as far as CCS and Microchip PIC's goes.
Bachelor in Space Engineering and Undergraduate Masters in Space Engineering
soon to be undergrad Aerospace |
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Neutone
Joined: 08 Sep 2003
Posts: 839
Location: Houston
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| Posted: Fri Nov 04, 2005 11:46 am |
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| If crc is an int16 and you were to & with 0xFFFF you would see that no code is compiled for the operation. |
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Dargar
Joined: 12 Dec 2003
Posts: 25
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| Posted: Fri Nov 04, 2005 11:52 am |
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| Neutone wrote: | | If crc is an int16 and you were to & with 0xFFFF you would see that no code is compiled for the operation. |
to bad, I was starting to think I had actually managed to squeese a few more cycles out of it! :D
Thanks!
_________________
Owner of PCWH v. 3.224
Consider myself beginer\intermediate as far as CCS and Microchip PIC's goes.
Bachelor in Space Engineering and Undergraduate Masters in Space Engineering
soon to be undergrad Aerospace |
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