What is carry bit? - Electronic Circuits Projects Diagrams Free

Suraj143 · 28th November 2007, 09:03 AM

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

Code:

	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 · 28th November 2007, 10:15 AM

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 · 28th November 2007, 10:23 AM

OK thanks Geko for the reply.

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

geko · 28th November 2007, 10:31 AM

Quote:

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

Yes, with carry cleared.

Suraj143 · 28th November 2007, 10:34 AM

Ok thanks geko now I got it.

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28th November 2007, 09:03 AM	(permalink)
Suraj143	What is carry bit? 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 Code: 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

28th November 2007, 10:15 AM	(permalink)
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 __________________ Pete picprojects.org.uk Last edited by geko; 28th November 2007 at 10:17 AM.

28th November 2007, 10:23 AM	(permalink)
Suraj143	OK thanks Geko for the reply. Can you tell me whats my result in my example? is it -4?

28th November 2007, 10:34 AM	(permalink)
Suraj143	Ok thanks geko now I got it.