Using the SUBWF instruction

Hi
I am writing a simple code in ASM for PIC16F84A to compare two registers and depending on the result branch to locations... Its alredy clear for me that SUBLW substracts W from Literal and not the opposite as when using SUBWF. The only thing which is unclear for me is when is the Carry byt set or cleared. Because i have searched the web and found different explanation from which one must be the wrong.
So i write down the code here with explanation what is my desired result and i ask you please correct me if i am wrong.So:

Compare movf M1Time,W ;this is the first register to compare
subwf M3Time,W ;compare with the second register
BZ M3_Eq_M1 ;they are the same

movf M1Time,W
subwf M3Time,W
BNC M3_LT_M1 ;M3Time is less than M1Time

goto M3_GT_M1 ;M3Time is bigger than M1Time


M3_Eq_M1 movf .....

M3_LT_M1 movf .....

M3_GT_M1 movf ....


1, is this code correct?

2, if i substract a bigger value from a smaller value the Carry bit will be set? OR the carry bit will be cleared? its a little bit confusing me....this is about Carry and BORROW? C bit is set on overflow and is cleared on underflow?

3, please do not recommend me addlw and the twos complement solution )

Thank you very much

Hi,

Looking at my old instruction set notes I have this for the Status bit 0 Carry Flag.

If the result of a Sub is Pos or Zero the Carry is set to 1
If the result of a Sub is Negtive the Carry is set to 0

To confirm your code is correct, just run it in Mplabs Simualtor and single step those lines of code watching the Status bits on the bottom toolbar, Carry c =0 C=1

Some time ago I wrote a utility to help write assembler code for Pic16 series micros and posted it on eda board here.




No install is needed, just extract all the files to a directory. Call the directory PicHelp or whatever you like.
Generates code for common constructs in programming.

Please give it a go and see if it helps.