Lasse Madsen wrote:
>
>If forinstance i have a counter which counts from 0 towards 8 and at the
>present moment it has reached 5 how would i drag that 5 out from memory and
>compare it with numbers from 0 to 8 and if a match is found proceed with
>another function ?
>
>If the counter counted maybe i could say:
>
>N    equ    0x1a
>
>       movlw    8
>go  sublw    N
>       btfss    status,5
>       goto    go
>       goto    do_something
>
>Right ?
>


Since I haven't seen any other responses to this message, I'll take a shot
at it:

I'm not quite sure exactly what you're trying to accomplish here. It appears
that you have confused the SUBLW (Subtract W from Literal) instruction with
SUBWF (Subtract W from f).

Your code will repeatedly subtract the address of your N variable (not its
value) from W until it reaches zero. Since W is initially set to 8 and the
address of N is specified as 0x1a, the number of loops is constant and not
dependent on the value of the N variable. Subtract 26 from W (putting the
result in W) repeatedly using 8-bit math if you really want to find out how
many. It should eventually wrap to zero. If it doesn't, it will loop
forever.

Well, DUH, the paragraph I just wrote turns out to be total rubbish. I just
looked up the bits in the STATUS register. The Z flag is bit 2 rather than
bit 5. I ASSumed that you were testing for the zero condition. This code
will actually loop until bit RP0 in STATUS gets set. Since there is nothing
here to set RP0, it should loop forever.

<soapbox>

Some suggestions:

1) Use the CBLOCK...ENDC directive pair to assign addresses for your
variables rather than hard-coding absolute addresses for them.

2) Use *descriptive* variable names. (Single-letter names are fine for
algebra but they really suck for programming.) Avoid 'magic numbers' by
equating constants to meaningful names.

3) Use the standard Microchip include files (e.g. p16f84.inc) to provide
definitions for processor registers and bit positions. A line that says
"btfss  STATUS, Z" tells you what's actually happening - skip if the Z bit
in STATUS is set. (Some people prefer to use skpz and skpnz macros, YMMV.)

4) Use the MPLAB simulator to learn what's really happening as your code
executes. In assembly language, there are far more details to concern
yourself with than in higher level languages.

5) Study other people's code for further ideas. Many PIClisters consistently
use excellent programming practices.

</soapbox>

    To check for a variable matching a specific value, something along these
lines should do the trick: (Please ignore the dots, my mailer doesn't do
multiple spaces well.)

check_value..equ..5
.
.
...movlw...check_value......(the constant you are comparing to)
...subwf...your_variable,w..(result to W, don't trash your variable)
...btfsc...STATUS,Z.........(if Z is clear, skip for no match)
...goto....matched..........(if Z is set, result = 0, value matched)
.
..........(continue with non-match code here)
.
match:....(here if the variable matched check_value)
.
.


Sorry for such a lengthy reply. I hope this helps.

Regards, Bob


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