limiting to "no less than" zero

[Byte_Butcher]

Thanks guys!

Well... I guess the answer to my question is that there isn't really a way that's as easy as using "min" to limit the upper end.

The method that Charles posted is what I've been using. But I was hoping for something that was... simpler.
I guess I'd hoped there was some way as clean and easy as "MAX" but that worked all the way down to zero.

Darrels method using volume.15 is great, but it "wastes" half of a variable.
Most of the variables that I need to limit are declared as bytes, and I need the full 255 step range for some of them.
I *can* use it for my temperature setpoint because that needs a word anyway and I can afford to "lose" part of it.

I'm starting to run low on code space on this project so hoping to find more efficient ways to do a few things...

Thanks for the input!

Steve

[Darrel Taylor]

Darrels method using volume.15 is great, but it "wastes" half of a variable.
Most of the variables that I need to limit are declared as bytes, and I need the full 255 step range for some of them.

With 8-bit values, the CARRY flag is valid after addition or subtraction.

So it can be ...

if buttonB then
    Volume = Volume - Vstep
    IF !STATUS.0 THEN Volume = 0   ; Zero if BORROWED
endif

[Byte_Butcher]

STATUS ?
STATUS.0 ?

Ahhh, Darrel, you confuseth me. From whence comes... STATUS ?

steve

[Darrel Taylor]

STATUS is a register in the PIC that contains the results (status) of arithmetic operations.
Depening on the chip family, it may also have Watchdog Timer status and FSR/page bits.

STATUS.0 is the CARRY flag.
After an addition, if the result is larger than 255 the carry bit will be set (1).
After a subtraction, if the result is less than 0 (BORROW), the carry bit will be cleared (0).

The carry bit is only valid after addition/subtraction with 8-bit values.
After math with WORD values, the carry bit cannot be used because not all of the math is done by the hardware.

[Byte_Butcher]

The carry bit is only valid after addition/subtraction with 8-bit values.
After math with WORD values, the carry bit cannot be used because not all of the math is done by the hardware.

OK, curiosity got me so I tried it with WORD values and it still seems to work...


x = 32768
Y = 32769
x = x + y
Result: X = 1, STATUS.0 = 1
'--------------------------------

x = 32768
Y = 32765
x = x + y
Result: X = 65533, STATUS.0 = 0
'-------------------------------------

x = 32768
Y = 32765
x = x - y
Result: X = 3, STATUS.0 = 1
'--------------------------------------

x = 32765
Y = 32771
x = x - y
Result: X = 65530, STATUS.0 = 0
'------------------------------------

I thought it was only 'sposed to work with 8 bit numbers?


Steve

[Darrel Taylor]

Try it with ...

X = 32768
Y = 65523

[Byte_Butcher]

Try it with ...

X = 32768
Y = 65523

Nope, that didn't work.
OK, 8 bit math then....

Thanks!

[Ioannis]

If the previous subtraction produced negative results, bit 0 of STATUS is set (to 1). Then you check this condition and if true, set Volume to 0.

By the way which chip are you controlling?

Ioannis

[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