PASM adds and subs

Duane Degn

I'm not sure if I understand signed add and subtract.

I want to adjust privateTarget0 up or down based on the change in targetSpeed0.

targetSpeed0 can be positive or negative (so can privateTarget0).

In this portion of the program, I save the earlier targetSpeed0 to previousSpeed0. I use the temporary variable "difference" to calculate, what I think should be, the change in targetSpeed0.

I plan to modify privateTarget0 in a another part of the program so it wont stay equal to targetSpeed0. But for now, the two values should match (or so I think). Both values start at zero. This is the only place targetSpeed0 is changed (by reading value from hub).

updateTarget            mov     previousTargetSpeed0, targetSpeed0
                        rdlong  targetSpeed0,targetAddress0
                        mov     difference, targetSpeed0
                        subs    difference, previousTargetSpeed0   ' has target speed changed?                
                        adds    privateTarget0, difference         ' adjust privateTarget speed to reflect change
 
                        wrlong  privateTarget0, privateTargetAddress0

For now, I'm writing privateTarget0 to hub RAM for debugging. I don't think I'll need access to it in the final version of the program.

When I watch the variables from a different cog I see privateTarget0 increase when targetSpeed0 is increased but privateTarget0 doesn't decrease when targetSpeed0 is decreased.

Is there an error in my understanding of subs and/or adds or is my problem hiding somewhere else?

Thanks,

Duane

Dave Hein

The code looks OK to me, so the error must be in some other part of the code. BTW, adds/subs is the same as add/sub, except for how they set the carry flag.

Duane Degn

Thank you Dave. I must have and error where the code checks to see if that subroutine is executed.

This should help me find the problem.

Dave Hein wrote: »

BTW, adds/subs is the same as add/sub, except for how they set the carry flag.

Thank you for this additional information. I didn't understand the difference between signed and unsigned add and subtract.

I'm trying to control four motors with encoders from within one cog.

Duane