Post by thread:[PIC]: Syntax Confusion

Hi guys, Quick question on .asm syntax using decfsz. What's the difference between the two following examples please? assume 'count' is a variable loop code code decfsz count, F goto loop retlw loop code code decfsz count, 1 goto loop retlw I understand that decfsz is decrementing count and if count=0 after the decrement, the next 'goto loop' instruction is skipped. I see the two forms used all over the place and the second one makes more sense to me, it's the use of the 'F' that's puzzling me and I haven't found any reference (yet) to what it is. Ted Mawson spam_OUTTed.MawsonTakeThisOuTPortfolioPM.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

On 23-Jan-02 Ted Mawson wrote: > Hi guys, > > > > Quick question on .asm syntax using decfsz. What's the difference > between the two following examples please? > > decfsz count, F > > decfsz count, 1 decfsz is no different to all the other pic instructions that operate on a register file.... addwf variable,F ; variable = variable + W addwf variable,W ; W = variable + W so decfsz count, F ; count = count - 1 ; skip if count == 0 decfsz count, W ; W = count - 1 ; skip if W == 0 Note the Z flag is set on the result of the decrement so it doesn't matter where the result ends up (in count or W). The use of 0 or 1 in place of F or W is, while syntacticaly valid, just asking for confusion. I don't know if "0" means W or if "1" means W, but I don't need to know because I use "W" if I mean W and "F" if I mean F !! Peter. ---------------------------------- E-Mail: Peter Onion <.....ponionKILLspam@spam@srd.bt.co.uk> Date: 23-Jan-02 Time: 16:23:24 This message was sent by XFMail ---------------------------------- -- 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

On Wed, 23 Jan 2002 11:03:38 -0500, Ted Mawson wrote: {Quote hidden}>Hi guys, > > > >Quick question on .asm syntax using decfsz. What's the difference >between the two following examples please? > > > >assume 'count' is a variable > > > >loop code > > code > > decfsz count, F > > goto loop > > retlw > > > >loop code > > code > > decfsz count, 1 > > goto loop > > retlw > > > >I understand that decfsz is decrementing count and if count=0 after the >decrement, the next 'goto loop' instruction is skipped. I see the two >forms used all over the place and the second one makes more sense to me, >it's the use of the 'F' that's puzzling me and I haven't found any >reference (yet) to what it is. > > Functionally they're identical. The 'F' indicates that the result goes to the register and not to the W register. F and W are defined in the processor include file ( F equ 1, W equ 0). (Note that if you use "decfsz count, W", the register will not decrement and the code will loop forever if the count is non-zero.) Regards, Bob -- 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

>Quick question on .asm syntax using decfsz. What's the difference >between the two following examples please? > decfsz count, F > decfsz count, 1 Wow, I've seen some fairly confusing replies to this simple question. Hope we didn't confuse you, Ted. Simply: The variable or number after the "," is the destination for the result of the instruction. If you want the result to be stored in the original register ("count" in this example), then use "F" or "1". If you want the result to be transferred to the Working register, use "W" or "0". You can use "F" and "1" interchangeably. You can also use "W" and "0" interchangeably. This assumes you are using the appropriate .INC files for MPASM, because that's where "F" is defined as "1" and "W" is defined as "0". Pretty much every "standard" PIC ASM file that you write should have these .INC files included in the beginning of your code. Hope that clears things up. For more information, you can always check the MPLAB or MPASM help files, which should explain the destination options. --Andrew _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.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

> decfsz count, F > decfsz count, 1 Both have the same effect, because F is nothing more than a synonym for 1. The first is much more readable, though. Now guess what the effect of decfsz W, F will be? Wouter van Ooijen Van Ooijen Technische Informatica: http://www.voti.nl Jal compiler for PIC uC's: http://www.voti.nl/jal PICs kopen? http://www.voti.nl/shop -- 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

>Both have the same effect, because F is nothing more than a synonym for 1. >The first is much more readable, though. Now guess what the effect of >decfsz W, F will be? It won't be anything, because it won't compile. You can't use "W" as an actual register, as it's not directly accessible in this manner. You'd have to use: addlw -1 btfss STATUS, Z . . . --Andrew _________________________________________________________________ Chat with friends online, try MSN Messenger: http://messenger.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

On the 18C452 you have to use WREG for W as a register, but you can use W for the argument switch(F,W). Dave Mumert wrote: {Quote hidden}>Hi Drew > >You must have a different version of MPLAB. > >decfsz W,F works just fine. It even does what you would expect. > >The register in the FSR pointer gets decremented and the result saved back >to that register. > >It is the same as decfsz INDF,F because both INDF and W are defined as 0 > >Dave > >{Original Message removed}

Many thanks to all who offered advice on this one, you are all right but I found Drew's answer to be the clearest. Now I know where the F=1 and W=0 are defined :) Ted Mawson Ted.MawsonKILLspamPortfolioPM.com Drew Vassallo wrote... Sent: Wednesday, January 23, 2002 12:45 PM To: .....PICLISTKILLspam.....MITVMA.MIT.EDU Subject: Re: [PIC]: Syntax Confusion >Quick question on .asm syntax using decfsz. What's the difference >between the two following examples please? > decfsz count, F > decfsz count, 1 Wow, I've seen some fairly confusing replies to this simple question. Hope we didn't confuse you, Ted. Simply: The variable or number after the "," is the destination for the result of the instruction. If you want the result to be stored in the original register ("count" in this example), then use "F" or "1". If you want the result to be transferred to the Working register, use "W" or "0". You can use "F" and "1" interchangeably. You can also use "W" and "0" interchangeably. This assumes you are using the appropriate .INC files for MPASM, because that's where "F" is defined as "1" and "W" is defined as "0". Pretty much every "standard" PIC ASM file that you write should have these .INC files included in the beginning of your code. Hope that clears things up. For more information, you can always check the MPLAB or MPASM help files, which should explain the destination options. --Andrew _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.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 list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Ted Mawson <EraseMEPICLISTspam_OUTTakeThisOuTmitvma.mit.edu> wrote: > Many thanks to all who offered advice on this one, you are all right > but I found Drew's answer to be the clearest. Now I know where the F=1 > and W=0 are defined :) No, you only THINK you do. Drew's post to the contrary, W=0 and F=1 are defined internally within MPASM; "DECFSZ COUNT,W" and "DECFSZ COUNT,F" will work even if you don't include any .INC files. -Andy === Andrew Warren -- aiwspam_OUTcypress.com === Principal Design Engineer === Cypress Semiconductor Corporation === === Opinions expressed above do not === necessarily represent those of === Cypress Semiconductor Corporation -- 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

>decfsz W,F works just fine. It even does what you would expect. >The register in the FSR pointer gets decremented and the result saved back >to that register. Um, that's not what I would expect at all. If I were to take the instruction at face value (as written), then it should reduce the *W* register by 1 and place it back into the W register. What you claim happens is that it decrements the register pointed to by *FSR*, not W. How does that do what you expect?? >It is the same as decfsz INDF,F because both INDF and W are defined as 0 This may be true, but it is just coincidental. Of course it would work in this manner, as they are both defined as 0 like you said. But that really has nothing to do with the Working register being affected by the instruction "decfsz," as was the original question. It's just not usable in that way. --Andrew _________________________________________________________________ Join the world s largest e-mail service with MSN Hotmail. http://www.hotmail.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

> Drew's post to the contrary, W=0 and F=1 are defined internally > within MPASM; "DECFSZ COUNT,W" and "DECFSZ COUNT,F" will work >even if > you don't include any .INC files. Interesting. I guess I never bothered to check, except that I wonder why they would define them in their .INC files in that case. I suppose you could use another compiler, and in that event they likely would not be internally defined, so you'd need to know where they were :) --Andrew _________________________________________________________________ Join the world s largest e-mail service with MSN Hotmail. http://www.hotmail.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

> >Both have the same effect, because F is nothing more than a synonym for 1. > >The first is much more readable, though. Now guess what the effect of > >decfsz W, F will be? > > It won't be anything, because it won't compile. You can't use "W" as an > actual register, as it's not directly accessible in this manner. Indeed it won't do what an innocent reader might expect, but this will not prevent MPASM from accepting it! 0139 0B80 decfsz W,F 013A 0B80 decfsz 0,1 Wouter van Ooijen Van Ooijen Technische Informatica: http://www.voti.nl Jal compiler for PIC uC's: http://www.voti.nl/jal PICs kopen? http://www.voti.nl/shop -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.