Am 15.04.2024 um 11:50 schrieb 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)
I was wrong with my first patch and I think the solution from Laurent is also wrong, because it does not correctly handle the case where M equals |Nn| but is not equal to 2^k.
I created a patch (appended). I think this is the cleanest way: It first checks for the case where |Nn| is not a multiple of 2^k. It then checks if Nn is greater than M which means unpredictable result else it does normal modulo operation. In the special case where |Nn| is a multiple of 2^k it jumps to the next buffer (this includes the case where P is 0 and makes this case a bit faster).
Special case is described in the data sheet of the 56001 DSP:
"If an offset, Nn, is used in the address calculations, the 16-bit absolute value, INnl, must be less than or equal to M for proper modulo addressing. If Nn>M, the result is data dependent and unpredictable, except for the special case where Nn = P x 2^k, a multiple of the block size where P is a positive integer. For this special case, when using the (Rn) + Nn addressing mode, the pointer, Rn, will jump linearly to the same relative address in a new buffer, which is P blocks forward in memory (see Figure 5-12). Similarly, for (Rn) - Nn, the pointer will jump P blocks backward in memory.“
I think the data sheet is not very clear because if |Nn| is equal to M there are two possible cases but only one is described.
Please verify my thoughts. I hope there is no mistake.