inc and dec limitations ?

[Grevle]

I had some problems with the inc and dec instrucions.

 

when i had one instance of incrementing a variable (PX)

 

inc px

 

the loop worked

 

and then was trying to use one more variable (P2X)

 

inc p2x.

 

this was in the same loop but that didnt work, i tried many different ways of using both variables but ,still it would now work, So i had to use some other metode that didnt use two insances of the inc command , and that worked right away so this lead me to think that the inc and dec instructions have some limitations, does it have something to do with the accumulator ?

[flashjazzcat]

Both instructions affect the N and Z flags only, so likely they upset some other logic in your loop. It would help to see the code in question, of course.

Edited January 20, 2016 by flashjazzcat

[GroovyBee]

Can you post your code that didn't work?

[Grevle]

well i had so many tries and diffent versions and test runs so the code all a mess, it also involves returns to the vbi since its a player movement routine. but i i fixed my program using a different metode by using some instances of the dex command.

[snicklin]

I had some problems with the inc and dec instrucions.

 

Intermittently so have I.

 

I don't have any code to show at the moment, but if it comes up again, I'll post it.

[Rybags]

There can be traps - often it's nice to use BPL in e.g. a loop that clears hardware registers. You might start at $1F counting back to zero.

BPL for such loops falls over if your starting value is over $80 to begin with.

[popmilo]

Inc and dec don't change C and V flag, so if you use bcc,bcs,bvc and bvs after inc or dec they jump based on some previous operation.

Other than that it should be pretty simple. Values go from 0-255, and new value influences Z and N flag.

 

Your error was probably somewhere else and not because of dec or inc. Could it be something with "p2x" variable name ? I'm not sure if such variable name with 'number between letters' is allowed in assembler you're using ?

[Grevle]

im using Atasm, hmm i think i used number between letter some other places, like the player equates p0x, p1x, p2x, p3x.. player X postionregisters and it seem to work normal.

 

The error probably is somwhere else. the code is getting quite complicated, for me anyway.

[popmilo]

No worries, if you get stuck on another bug, just dump your code here somewhere and you'll get it fixed in no time. Plenty of bored coders around here

[Heaven/TQA]

if I remember my mistakes with inc/dec it was always the BPL/BMI thing most of the time...