How to prevent variable going below zero?

[mpgmike]

mike would you like to provide some pbp code that demonstrates use of status register following pbp addition or subtraction


the status reg here is not in any way influenced by the subtraction result

i = byte

j=8
i=6
while j
status=1
i=i-1
s=status
debug 13,10,"status ",bin8 s ,"  i ",    dec i
j=j-1
wend

result

status 00011001 i 5
status 00011001 i 4
status 00011001 i 3
status 00011001 i 2
status 00011001 i 1
status 00011101 i 0
status 00011001 i 255
status 00011001 i 254

You don't state what PIC this is for, so I randomly grabbed the data sheet for a PIC16F1824; STATUS Register 3-1, page 24.

STATUS <7:5> = Unimplemented, read as 0
STATUS.4 = TO: Time-Out bit
STATUS.3 = PD: Power-Down bit
STATUS.2 = Z: Zero bit
STATUS.1 = DC: Digit Carry
STATUS.0 = C: Carry

You can see when you reached zero the STATUS.Z reflected that. However, you alter the STATUS Register in your code. About the only thing you can change is the C bit. Delete the "status=1" line and try again.

[richard]

You can see when you reached zero the STATUS.Z reflected that. However, you alter the STATUS Register in your code. About the only thing you can change is the C bit. Delete the "status=1" line and try again.

your assertion is

If subtracting and want to see if it rolled from 0 to 255, "STATUS.C = 0". Look in your data sheet for the STATUS Register and how it works.

You don't state what PIC this is for, so I randomly grabbed the data sheet for a PIC16F1824; STATUS Register 3-1, page 24.

if your assertion was correct it would not matter but its a12f1840

i repeat
mike would you like to provide some pbp code that demonstrates use of status register following pbp addition or subtraction
for any chip

[richard]

a look at the lst file yields the reasons .
the status reg is not useful when you add or subtract a const 1
if you use a number greater than 1 or another var then the code does a proper addition/subtraction rather than inc/dec and sets status reg correctly

caveat emptor