Post by thread:Re: [PIC]: Does DECFSZ...?

Hello everybody! In the Easy PIC'n book, it is written that decfsz takes 1 clock cycle, and 2 clock cycle if result is 0. But in another book I have borrowed and read, it say that it only takes one clock cycle, eventhough the result is 0. Please help... ____________________________________________________________________ Get free email and a permanent address at http://www.netaddress.com/?N=1 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

>In the Easy PIC'n book, it is written that decfsz takes 1 clock cycle, and >2 >clock cycle if result is 0. >But in another book I have borrowed and read, it say that it only takes one >clock cycle, eventhough the result is 0. If you look in the MPLAB user's manual (or any datasheet that has an Instruction Set Summary page), you can see that it clearly describes it as requiring 1 cycle if nonzero and 2 cycles if zero. If it is nonzero, it really still takes 2 cycles to get to the same point in your code, except that it executes the next line instead of skipping it. When it "skips on zero", it really executes a "nop" instruction before it continues on. For example: decfsz FILE; 1 instruction cycle bsf FLAG, 1 ; executed normally if F != 0, or a NOP if F=0 movlw 0xFF ; this is instruction cycle 3, regardless of whether or not F=0 You can't "save" instruction cycles if it's zero or nonzero in going through your code. In the example above, it will always take 2 cycles to get from "decfsz" to "movlw". It doesn't matter if F=0 or F!=0. --Andrew _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

As a practical observation is showing the real difference is in two examples: ; decfsz some_reg non_goto_command_here ; will take 1(decfsz)+1(non_goto_command executed or skipped) clocks to get through for any given value of some_reg and ; decfsz some_reg goto_somewhere ;very particular case of it is call_somewhere ;) ; will take 1(decfsz)+1(skipped) to get through with skipping goto and 1(decfsz+2(goto) to get somewhere (in case of call it can take much longer ;) WBR Dmitry. Drew Vassallo wrote: {Quote hidden}> > >In the Easy PIC'n book, it is written that decfsz takes 1 clock cycle, and > >2 > >clock cycle if result is 0. > >But in another book I have borrowed and read, it say that it only takes one > >clock cycle, eventhough the result is 0. > > If you look in the MPLAB user's manual (or any datasheet that has an > Instruction Set Summary page), you can see that it clearly describes it as > requiring 1 cycle if nonzero and 2 cycles if zero. If it is nonzero, it > really still takes 2 cycles to get to the same point in your code, except > that it executes the next line instead of skipping it. When it "skips on > zero", it really executes a "nop" instruction before it continues on. For > example: > > decfsz FILE; 1 instruction cycle > bsf FLAG, 1 ; executed normally if F != 0, or a NOP if F=0 > movlw 0xFF ; this is instruction cycle 3, regardless of whether or not F=0 > > You can't "save" instruction cycles if it's zero or nonzero in going through > your code. In the example above, it will always take 2 cycles to get from > "decfsz" to "movlw". It doesn't matter if F=0 or F!=0. > > --Andrew > _________________________________________________________________ > Get your FREE download of MSN Explorer at http://explorer.msn.com > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Any instruction which can skip takes one cycle when it doesn't skip and two when it does. This is because of the way mchip implemented these skip instructions. PICs use a pipelined scheme to process instructions: as each instruction is being executed, the next instruction in the program is being fetched. All a PIC does in the case of a skip is ignore the fetched instruction and substitute a nop instead. So, instead of thinking of skipping instructions as taking two cycles when they skip, you can think of the skip taking one cycle and the following forced nop taking the second cycle. Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) {Original Message removed}

>> In the Easy PIC'n book, it is written that decfsz takes 1 clock cycle, and 2 clock cycle if result is 0. But in another book I have borrowed and read, it say that it only takes one clock cycle, eventhough the result is 0. << The only book that really matters is the Microchip data sheet. Go to the chapter "Instruction Set Summary" and look up the DECFSZ instruction. ******************************************************************** Olin Lathrop, embedded systems consultant in Littleton Massachusetts (978) 742-9014, spam_OUTolinTakeThisOuTembedinc.com, http://www.embedinc.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Thank you sir Bob! =) That made it so much clearer! Kindest regards... Bob Ammerman <.....RAMMERMANKILLspam@spam@PRODIGY.NET> wrote: Any instruction which can skip takes one cycle when it doesn't skip and two when it does. This is because of the way mchip implemented these skip instructions. PICs use a pipelined scheme to process instructions: as each instruction is being executed, the next instruction in the program is being fetched. All a PIC does in the case of a skip is ignore the fetched instruction and substitute a nop instead. So, instead of thinking of skipping instructions as taking two cycles when they skip, you can think of the skip taking one cycle and the following forced nop taking the second cycle. Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) {Original Message removed}