> How does the carry bit on a 16c54 work with the DECF opcode?
>
> Using PSIM I saw that the carry bit is cleared when a subtraction results in
>a borrow.  If I decrement a variable though, and a borrow occurs, why does
>the carry bit not get affected?  The Data sheet on the chip says the DECF
>affects the carry bit.  When does it?
>
>Thanks
>Dave
>Applied Magnetics

Hi Dave.
The carry bit is not affected be INCF or DECF instructions. Only the zero
bit is affected. The instructions that affect the carry bit are:

 BSF    STATUS,C        ;C=1
 BCF    STATUS,C        ;C=0
 CLRF   STATUS          ;C=0
 ADDLW  DATA            ;depends of result, 1 if borrow
 ADDWF  REG(,W)         ;depends of result, 1 if borrow
 RLF    REG(,W)         ;C=MSB of REG
 RRF    REG(,W)         ;C=LSB of REG
 SUBLW  DATA            ;depends of result, 0 if borrow
 SUBWF  REG(,W)         ;depends of result, 0 if borrow

My data sheets told me this and my experience confirms it. I use the
1994 and 1995 data books. I hope this helps.

Fernando Soares