Post by thread:SUBWF v.s. DECF

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

> How does the carry bit on a 16c54 work with the DECF opcode? > It isn't changed. The DECF affects only the Z flag, setting it if the result is zero. If you don't even want to do that, use DECFSZ then NOP, this affects no flags at all. Only the add, subtract and rotate instructions affect the carry flag. Curiously, the RLF and RRF instructions don't affect Z, something else to keep in mind. >[...] > The Data sheet on the chip says the DECF >affects the carry bit. I haven't seen that. The table in my (1994) book and the quick reference card say that it doesn't. -Mike

> 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

On Wed, 30 Aug 1995 22:51:20 +0200, Fernando Soares <spam_OUTFernando.Manuel.Ramos.SoaresTakeThisOuTUNINOVA.PT> said: > 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: [useful list of instructions deleted] I suppose I am being nitpicky, but I think it should be pointed out that any instruction with STATUS as its destination can affect the carry. So while INCF and DECF usually don't affect the carry, INCF STATUS will. While I've never seen any reason to INCF or DECF STATUS, I have seen code that uses RRF STATUS and RLF STATUS. Cheers, Eric