On Sat, Mar 27, 2010 at 08:46:35PM -0400, Bo Dirigo wrote:
> While studying various PIC instructions' formats, I noticed a number of 
> instructions that had a 'd' (destination) in the Mnemonic; e.g., MOVF 
> f,d; ADDWF f,d; INCF f,d, et al.
> 
> If d=0, the results are stored in the W register - ADDWF  FSR, 0
> If d=1, the results are stored in the f register - ADDWF  FSR, 1

Correct. Every instruction that has W and a file register as an operand may
specify either as the target of the result.

> 
> My question is "Do you need to specify the "0" value if one knows they 
> want the resultant value to be stored in the W register?"

No. There is a default. But honestly I can never remember which one it is.

>  Obviously, if 
> one wants the result to be stored in the f register, they must specify 
> the "1" value.  I'm probably all wet here, but was thinking perhaps the 
> assembler would automatically default to the "0" value if nothing 
> specified.

Again, I can never remember. So I always specify the target. Fortunately
both MPASM and gpasm will generate a warning if you use the default.

>  Before you shoot me, I am in agreement it would be better to 
> specify either a "0" or "1" depending on what it is one wants done.  I'm 
> just curious.

Finally in the include files the names W and F are defined to set that
destination. So no one ever uses the 0 or 1 value to specify the target.
This is done precisely because the numbers convey no semantic information.
An instruction like:

	addwf	PCL,F	

In much clearer than:

	addwf	PCL,1	

I cannot remember the last time I've seen the latter notation in a piece of
code. And no matter what the default is:

	addwf	PCL

Is just a train wreck waiting to happen. But just for the record this piece
of code:

	LIST    P=16F88
#include "p16f88.inc"  

        ORG     0
        addwf   PCL,W
        addwf   PCL,F
        addwf   PCL  
        END          

Generates the following listing:

              00001                 
               00002                 
               00003                 
               00004         LIST    P=16F88
               00005 #include "p16f88.inc"  
               00001         LIST           
               00002 ; P16F88.INC  Standard Header File, Version 1.00
Microchip Technology, Inc.
               00401         LIST                                                                  
               00006                                                                               
0000           00007         ORG     0                                                             
0000 0702      00008         addwf   PCL,W                                                         
0001 0782      00009         addwf   PCL,F                                                         
Message [305] : Using default destination of 1 (file).                                             
0002 0782      00010         addwf   PCL                                       

You can see by the warning message that the default destination is the
file, not the W register.

Hope this helps,

BAJ
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist