How to prevent variable going below zero?

[Ioannis]

reader:
adcin 0,Z
pause 100
IF Z>100 AND Z<130 THEN I=I+5
IF Z<100 THEN I=I-5
if i>255 then i=255
if i<5 then i=0
HPWM 2,I,10000
XY=I
gosub decoder
goto reader

1. tumbleweed caught one problem in your code.

2. The variable I is altered some where? What check are you making ensuring it will not get a value to lead in negative results doing a -5 subtraction inside the reader routine?

Ioannis

[peterdeco1]

I like Tumbleweed's code. Shorter and more to the point.

[richard]

With a few proviso's mikes way beats it hands down

IF Z<100 THEN I=I-5
asm
     BTFSS STATUS,C
     CLRF   _I
 ENDASM

if i attempts to underflow its set to 0

[Ioannis]

I like it how a simple logic problem CuriousOne had, lead to another topic with negative numbers and STATUS register!

Quite interesting!

Ioannis

[richard]

anyone familiar with assembly programming will be well aware of the status reg and its use. what surprised me is that
the value of the status reg was/is preserved even after the completion of the transaction. because the result is transferred from the
working regs back into the result variable during the process i expected the status to be no longer representative of the calculation.
surprised yet again !

[Ioannis]

You are absolutely right! I do not trust that this may be the case for any PIC though.

Ioannis

[richard]

I do not trust that this may be the case for any PIC though.

its a quick easy test to check, it works on all chips that i tried just make sure the compiler dosn't compress the subtract into a decrement
i var can be byte or word

j=8 
i=6 
while j 
status=1
 i=i-2  ;subtractant cannot be 1 
s=status 
debug 13,10,"status ",bin8 s ,"  i ",    dec i 
j=j-1 
wend