Re: [hatari-devel] DSP bug found (and maybe solved)

[Andreas Grabher <andreas_g86@xxxxxxxxxx>]

> Am 14.04.2024 um 23:48 schrieb Laurent Sallafranque <laurent.sallafranque@xxxxxxx>:
> 
> Hi all,
> 
> 
> I've spent the last 2 weeks tracing Audio Fun Machine to find the bug when the equalizer is ON (sound becomes noisy).
> 
> 
> I've isolated the bug : it's about Rn + Nn + Mn updating.
> An example :
>             move    #>0,r0                     ; R0 = 0
>             movec    #$01,m0                ; modulo 2
>             move    #$02,n0                   ; incrementor = 2
> 
>             move    #>1,a
>             move    #>1,b
>             move    a,x:(r0)+                 ; r0 = 1
>             nop
>             nop
>             move    b,x:(r0)+                 ; r0 = 0  (because of the modulo)
>             nop
>             nop
>             move    (r0)+n0                   ; R0 = 0 (should be 2)
>             nop
>             nop
>             move    a,x:(r0)+                 ; R0 = 1 (should be 3)
> 
> 
> I suggest the following fix in the function :
> static void dsp_update_rn_modulo(uint32_t numreg, int16_t modifier)
> 
> Line 1694 :
> replace
>     if (abs_modifier>modulo) {
> by
>     if (abs_modifier>modulo-1) {
> 
> 
> In line 1673, modulo is added +1 for buffer boundaries computing.
> But I think modulo should be -1 when tested with modifier.
> 
> At least, Audio Fun Machine sound becomes good with this patch, but DSP programs should be tested for non regression.
> I've tested 2 or 3 of them, but not all of them.
> 
> @eero, could you test my patch and tell me if AFM works well for you ?
> 
> 
> Regards
> 
> Laurent
> 
Great discovery! Obviously the special case where abs_modifier equals modulo was not handled. I think the more logical solution would be

if (abs_modifier>=modulo)