limiting to "no less than" zero

[Byte_Butcher]

Wow... thanks Darrel. THAT is an interesting item to know about.
I'm sure I can make good use of that!

Ioannis, I'm using a 16F887. And thanks for helping explain the STATUS thing!


steve

[Heckler]

Darrel = After a subtraction, if the result is less than 0 (BORROW), the carry bit will be cleared (0).
Ioannis = If the previous subtraction produced negative results, bit 0 of STATUS is set (to 1)

OK guys(gals?)
if I read those two statements correctly... they are contradictory. You can't have it both ways... which is it??

[Darrel Taylor]

Don't believe anything just because somebody said so.
Especially when there's conflicting information.

Check the datasheet, and test it for yourself. Then you will know the truth.

This program will help with the testing part.

Volume   VAR BYTE
Vstep    VAR BYTE
Result   VAR BYTE
Carry    VAR BIT
HIGH PORTD.0     ; initialize serial line
PAUSE 100
FOR Volume = 5 TO 0 STEP -1
    FOR Vstep = 5 TO 0 STEP - 1
        Result = Volume - Vstep
        Carry = STATUS.0
        SEROUT2 PORTD.0,84,[DEC Volume," - ",DEC Vstep," = ",DEC Result,"   C = ",DEC Carry,13,10]
    NEXT Vstep
NEXT Volume
STOP

[Byte_Butcher]

IF !STATUS.0 THEN Volume = 0

Darrels way works with a 16F887.
Can't vouch for any other chips...

[aratti]

Darrel, what is the value of STATUS.0 at start up? I assume it should be zero (no borrow no carry) , are you going to set it at start up, in order to have your snippet @ post #12 working?
Cheers

Al.

[cncmachineguy]

Not that Darrell needs it, but I feel compelled to jump in here and row his boat. To my knowledge, borrow/carry flag in the status reg is the same across ALL PIC's. It is not a compilier thing. It is a basic hardware reg no different then any other. It exists the same as W. That said, I believe it is cleared at POR, but it doesn't matter. it only has value directly after an addition or subtraction. Well it is also used on rotate R/L through carry, but PBP doesn't support that. Some devices even have 1 for borrow or carry between nibbles, but again only used during math.

So startup value makes no difference for the snippet in post 12 as it follows a math operation.

[Heckler]

Here is my test code and the debug results below...

 DEFINE DEBUG_REGG PORTA        'set debug port to porta
 DEFINE DEBUG_BIT 0             'use pin a0 of porta for debug
 DEFINE DEBUG_BAUD 2400         'set baud rate to 2400
 DEFINE DEBUG_MODE 0            'communicate in true mode
valA var byte
valB var byte
resultA var byte
resultB var byte
resultC var byte
resultD var byte
   Carry var bit
   vala=51
   valb=50

resulta=vala-valb
Carry = STATUS.0
debug "resultA : 51-50=",dec resulta," Carry=",dec carry,13,10

resultb=valb-vala
Carry = STATUS.0
debug "resultB : 50-51=",dec resultb," Carry=",dec carry,13,10

resultC=vala+valb
Carry = STATUS.0
debug "resultC : 51+50=",dec resultc," Carry=",dec carry,13,10

vala=55
valb=201

resultD=vala+valb
Carry = STATUS.0
debug "resultD : 55+201=",dec resultd," Carry=",dec carry,13,10
end

and the Debug results...


resultA : 51-50=1 Carry=1
resultB : 50-51=255 Carry=0
resultC : 51+50=101 Carry=0
resultD : 55+201=0 Carry=1

[Heckler]

DANG EDIT TIMER...

Therefore
If you subtract two BYTE variables and the result is NEGATIVE, the Carry Bit will be Cleared (0)
If you add two Byte variables and the result is >255 then the Carry Bit will be SET (1)

HEY!! I think that's what Darrel said earlier.

[Darrel Taylor]

Sweet!
And you made your own test program ... Perfect!

I'm sure it all seems much clearer now.

[Ioannis]

Oops, sorry. I should have said borrow...

Darrel was too polite! Thanks.

@ Steve: I meant which audio chip are you controlling with the PIC©

Ioannis