At 12:13 1/11/99 +1100, you wrote:
>John Considine wrote:
>>
>> I am trying to have a variable be decremented and that result be used as
>> the BTFSC test bit
>>
>> For example
>>
>> movlw    H'07'
>> movwf    TestBit
>> decf        TestBit, 1
>> btfsc       PORTB, TestBit
>>
>> Is this possible,  I tried using indirect referencing like
>>
>> movlw    TestBit
>> movwf    FSR
>> btfsc       PORTB, INDF
>>
>> but that will not work. Any suggestions.
>> Thanks
>
>You cannot use BTFSC like that. (?)
>
>You may need to try something like...
>
>        movlw b'10000000'
>        movwf TestBit
>
>        - - - -
>
>        bcf status,carry        ; shift bit one position to right
>        rrf TestBit
>        btfsc status,carry      ; if carry = 1 then reset bit 7 = 1
>        rrf TextBit
>
>        - - - -
>
>        movf PORTB,w            ; do bit comparison on port
>        andwf TestBit,w
>        btfsc status,Z
>        goto BitIsOne
>
>--
>Best regards
>
>Tony
>
>http://www.picnpoke.com
>Email sales@picnpoke.com
>
>

I realy can't see what is going on here,
If you are decementing to see what the results is you can have either a 1
or a 0, no matter what.
Or is it that you are trying to use this as a mask for testing of each
potential bit location.
Ah yes the latter
If the latter is true, then there is the problem in that the compiler +
opcodes expect this to be a litteral value, hence the method used by Tony.

E.G BTFSC myval, 0-7, where the 0 to 7 are fixed.

If this is the case then perhaps you could waste a bit of space
and use a table type function

                                        ;Test byte is the location to be compare
d with the counter
                                        ;value. This makes the code resuable for
 testing of bit
                                        ;compares for any direct RAM location
                movf    PORTB
                movwf   test_byte,w     ;do this bit on pre entry

                counter         ;the current bit location to look at
                                        ;the counter is 0-7 and indicates the bi
t that we want to look
                                        ;at. Note that this vale must be preload
ed and maintained by
                                        ;the calling code

                                        ;function body


;***********
bit compare:
;***********
;                                       Perequsites
;                                       ===========
;                                       Counter must be set with the bit to test
;                                       w must contain the table offset
;                                       test_byte must be the byte to test to
;                                       LOCAL variable  :- test_byte -> effectiv
e only, not to be preserved

                movlw   3               ;so that we get to the correct location
in the table
                addw    counter
                add     pcl,w           ;point to the location to jump to
TABLE
                nop
                nop
                nop
                btfsc   test_byte,0
                goto    return_true
                goto    return_false
                btfsc   portb,1
                ...

return_true
                do stuff

return_false
                do other stuff

I know that there is stuff missing in the table but you can get the general
idea. Tonys stuff also works, but
resides on the assumption that the value (As you requested) is dectmented
by one each time, where the table value lets you attack any bit at any time.

On further thinking Tonys way on bit test will work at all times, so ignore
the rubbish I just came up with

Dennis