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