Talk:APU DMC: Difference between revisions
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:The generated addresses are linear, and I don't think a shift register can do that. So I'm conjecturing an up-counter here. --[[User:Tepples|Tepples]] 02:30, 6 June 2011 (UTC) | :The generated addresses are linear, and I don't think a shift register can do that. So I'm conjecturing an up-counter here. --[[User:Tepples|Tepples]] 02:30, 6 June 2011 (UTC) | ||
::The DPCM address register is only 15 bits wide (with A15 being connected to the VCC rail @ 2850,5450), and the upper bit (A14) gets initialized to 1 (@ 2565,5365) instead of the last value written to $4012 (and the bottom 6 bits get initialized to 0, but we already know this). --[[User:Quietust|Quietust]] 03:03, 6 June 2011 (UTC) | ::The DPCM address register is only 15 bits wide (with A15 being connected to the VCC rail @ 2850,5450), and the upper bit (A14) gets initialized to 1 (@ 2565,5365) instead of the last value written to $4012 (and the bottom 6 bits get initialized to 0, but we already know this). --[[User:Quietust|Quietust]] 03:03, 6 June 2011 (UTC) | ||
== DMC find == | |||
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Looks like the DMC silent flag makes difference for $4011 raw output. If this flag is non-zero, the $4011 value written is ignored by the raw output ''only''. I could test a few NSFs and it works flawlessly. --[[User:Zepper|Zepper]] 14:53, 14 November 2011 (UTC) | |||
Revision as of 14:53, 14 November 2011
Similarly to the noise channel, the DPCM channel's frequency counter on the die is a 9-bit linear feedback shift register (with taps at the 5th and 9th bits); when I take the counter values from the on-die ROM and run the LFSR until the result is '100000000', the cycle counts (for NTSC) match once multiplied by 2. --Quietust 05:00, 23 January 2011 (UTC)
I suspect there's no "address increment", but another shift register. Just pay attention to the address wrap - if we had a counter, it would wrap to zero, but instead, it wraps to $8000, like ($10000 >> 1). --Zepper 01:49, 6 June 2011 (UTC)
- The generated addresses are linear, and I don't think a shift register can do that. So I'm conjecturing an up-counter here. --Tepples 02:30, 6 June 2011 (UTC)
- The DPCM address register is only 15 bits wide (with A15 being connected to the VCC rail @ 2850,5450), and the upper bit (A14) gets initialized to 1 (@ 2565,5365) instead of the last value written to $4012 (and the bottom 6 bits get initialized to 0, but we already know this). --Quietust 03:03, 6 June 2011 (UTC)
DMC find
Looks like the DMC silent flag makes difference for $4011 raw output. If this flag is non-zero, the $4011 value written is ignored by the raw output only. I could test a few NSFs and it works flawlessly. --Zepper 14:53, 14 November 2011 (UTC)
