[sdiy] DSPIC computed branch and include help

[Richie Burnett]

That's it! Thanks Scott. The GOTO instruction takes up two instruction locations in program memory! I knew it took 2 instruction cycles of time to execute a GOTO operation because of the instruction pipeline, but I had always thought all Dspic instructions were the same size in program memory. It's right there in the programmers reference manual though that it takes up 2 words.

The BRA Wx instruction works correctly when it's followed by single word branch instructions instead of dual-word GOTOs.  I'm just surprised it didn't generate an exception or something branching into the middle of a two-word instruction! At least then I would have realised what was wrong.

Thanks for the help, now I just need to figure out why MPLAB debug and simulation won't follow program execution into multiple included source files. Had I actually been able to single-step the program execution into included sources I would have solved the computed branch bug much quicker.

-Richie,



Sent from my Xperia SP on O2

---- Scott Gravenhorst wrote ----

>"Richie Burnett" <rburnett at richieburnett.co.uk> wrote:
> >Your example is exactly what I'm trying to do.  But I have used the 
> >"goto" instruction instead of bra for some reason so it looks like:
> >
> >bra w0
> >goto light_red_led
> >goto light_amber_led
> >goto light_green_led
> >goto light_blue_led
> >
> >etc...
> >
> >I will change the goto instructions to bra and see if it works.
>
>The GOTO Expr instruction consumes 2 words of program memory while the 
>BRA Expr instruction uses one word.  This means that a GOTO table as 
>shown above cannot work without some additional arithmetic on register 
>W0.  If the relative branch range of the BRA Expr instruction is 
>sufficient for your needs, it should be used instead of GOTO.
>
>-- ScottG
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