APU DMC

The NES APU's delta modulation channel (DMC) can output 1-bit delta-encoded samples or can have its 7-bit counter directly loaded, allowing flexible manual sample playback.

The DMC channel contains the following: memory reader, interrupt flag, sample buffer, timer, output unit, 7-bit counter.

                        Timer
                          |
                          v
Reader ---> Buffer ---> Output ---> Counter ---> (to mixer)

$4010	IL--.FFFF	Flags and frequency (write)
bit 7	I--- ----	IRQ enabled flag. If clear, the interrupt flag is cleared.
bit 6	-L-- ----	Loop flag
bits 3-0	---- RRRR	Rate index Rate $0 $1 $2 $3 $4 $5 $6 $7 $8 $9 $A $B $C $D $E $F ------------------------------------------------------------------------------ NTSC 428, 380, 340, 320, 286, 254, 226, 214, 190, 160, 142, 128, 106, 84, 72, 54 PAL 398, 354, 316, 298, 276, 236, 210, 198, 176, 148, 132, 118, 98, 78, 66, 50
$4011	-DDD.DDDD	Direct load (write)
bits 6-0	-DDD DDDD	The counter is loaded with D. If a sample is currently playing, the counter is occasionally not changed properly.
$4012	AAAA.AAAA	Sample address (write)
bits 7-0	AAAA AAAA	Sample address = %11AAAAAA.AA000000
$4013	LLLL.LLLL	Sample length (write)
bits 7-0	LLLL LLLL	Sample length = %LLLL.LLLL0001

The counter's value is sent to the mixer. It is loaded with 0 on power-up.

Automatic 1-bit delta-encoded sample playback is carried out by a combination of three units. The memory reader fills the 8-bit sample buffer whenever it is emptied by the sample output unit. The status register is used to start and stop automatic sample playback.

The sample buffer either holds a single 8-bit sample byte or is empty. It is filled by the reader and can only be emptied by the output unit; once loaded with a sample byte it will be played back.

Memory reader

When the sample buffer is emptied, the memory reader fills the sample buffer with the next byte from the currently playing sample. It has an address counter and a bytes remaining counter.

When a sample is (re)started, the current address is set to the sample address, and bytes remaining is set to the sample length.

Any time the sample buffer is in an empty state and bytes remaining is not zero, the following occur:

• The sample buffer is filled with the next sample byte read from the current address, subject to whatever mapping hardware is present. The CPU is suspend for four clock cycles.
• The address is incremented; if it exceeds $FFFF, it is wrapped around to $8000.
• The bytes remaining counter is decremented; if it becomes zero and the loop flag is set, the sample is restarted (see above), otherwise if the bytes remaining counter becomes zero and the IRQ enabled flag is set, the interrupt flag is set.

At any time, if the interrupt flag is set, the CPU's IRQ line is continuously asserted until the interrupt flag is cleared.

Output unit

The output unit continuously outputs a 7-bit value to the mixer. It contains an 8-bit right shift register, a bits-remaining counter, a 7-bit delta-counter, and a silence flag.

When an output cycle ends, a new cycle is started as follows:

• The bits-remaining counter is loaded with 8
• If the sample buffer is empty, the silence flag is set, otherwise the silence flag is cleared and the sample buffer is emptied into the shift register.

When the timer outputs a clock, the following actions occur in order:

1. If the silence flag is clear, bit 0 of the shift register is applied to the counter as follows: If bit 0 is clear and the delta-counter is greater than 1, the counter is decremented by 2, otherwise if bit 0 is set and the delta-counter is less than 126, the counter is incremented by 2.
2. The right shift register is clocked.
3. The bits-remaining counter is decremented. If it becomes zero, a new cycle is started.

Nothing can interrupt a cycle; every cycle runs to completion before a new cycle is started.

Likely internal implementation of the read

The following is speculation, and thus not necessarily 100% accurate. It does accurately predict observed behavior.

The 6502 cannot be pulled off of the bus normally. The 2A03 DMC gets around this by pulling RDY low internally. This causes the CPU to pause during the next read cycle, until RDY goes high again. The DMC unit holds RDY low for 4 cycles. The first three cycles it idles, as the CPU could have just started an interrupt cycle, and thus be writing for 3 consecutive cycles (and thus ignoring RDY). On the fourth cycle, the DMC unit drives the next sample address onto the address lines, and reads that byte from memory. It then drives RDY high again, and the CPU picks up where it left off.

This matters, because it can interfere with the expected operation of the controller registers, reads of the PPU status register, and CPU VRAM or SPR reads if they happen to occur in the same cycle that the DMC unit pulls RDY low.

For the controller registers, this can cause an extra rising clock edge to occur, and thus shift an extra bit out. For the others, the PPU will see multiple reads, which will cause extra increments of the address latches, or clear the vblank flag.