What is carry bit?

[Suraj143]

I have confused with carry bit in many cases, still I don’t know what the real meaning is.

Ex: before this subtraction occurred TEMP = 5 & COUNT = 1

	movf	TEMP,W
	subwf	COUNT,F
	btfss	STATUS,C

What is the result after the subtraction? What made to SET or CLEAR the carry bit?

Please explain this to me.

Thanks

 

[geko]

The subtract instruction sets or clears the carry flag.

If the flag is set after the subtract, there was no borrow

e.g 10-4 = 6 carry flag set
also 10-10 = 0 carry flag set

If the flag is clear after the subtract there was a borrow

e.g. 4-10 = -6 Carry flag cleared

In your example

Wreg = 5, Count = 1
1-5 = -4, Carry is clear

 

[Suraj143]

OK thanks Geko for the reply.

Can you tell me whats my result in my example? is it -4?

 

[geko]

Can you tell me whats my result in my example? is it -4?

Yes, with carry cleared.

 

[Suraj143]

Ok thanks geko now I got it.