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dma.md

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DMA protection

Using the Z80 and DMA at the same time may cause issues on a real Mega Drive system. Therefore, some precautions need to be taken to ensure that your program will run properly when using the DMA.

Background

A fairly well known issue in Mega Drive programming is the conflict that occurs when the Z80 tries to access the 68000 bus just when the VDP is starting a DMA transfer.

Normally, if a DMA is ongoing and the Z80 tries to access the 68000 bus, the bus arbiter will simply wait for the VDP to finish the DMA before letting the Z80 know it is ready to read (or write) memory from the bus.

However, what may happen when a DMA is just starting is that a bus contention conflict occurs between the VDP and the bus arbiter, causing glitches to appear on the address lines.

In the best case, the the data transferred by the DMA to VRAM will be corrupted. In the worst case, the glitch occurs while the RAM (or a flashcart) is refreshing its internal memory, the memory enters a metastable state and you may get corruption on 68K RAM or flashcart ROM.

The usual method to solve this is to halt the Z80 before starting a DMA. However, since DMAs might last for quite a while, stopping the Z80 for all this time will cause the PCM sample quality to be degraded. The fix is to have the Z80 buffer samples during active scan and make sure it only reads from the buffer while a DMA is ongoing. MDSDRV contains a buffer, however the Z80 cannot by itself determine if a DMA is (or will be) ongoing.

Using DMA protection

After initializing MDSDRV using mds_init, the DMA protection is disabled. This allows for low latency PCM and allows the Z80 to run without needing an acknowledgement from the 68k. The 68k will simply send a bus request whenever it does a DMA, as with other sound drivers.

Enabling DMA protection

To enable the DMA protection, call mds_command with command number $11 (set_pcmmode) in d0 and set d2 to the requested buffer size. The byte at d1 must contain a valid PCM mixing mode or can be set to 0 to use the previous PCM mixing mode.

Example:

	lea		mds_work,a0
	moveq	#$11,d0			; set_pcmmode
	moveq	#$2,d1			; PCM mode 2
	move.w	#100,d2			; DMA protection on, buffer 100 bytes
	jsr		mds_command
	;		( or mdsdrv+$0c )

The requested buffer size must be larger than 40 bytes and smaller than 220 bytes.

A buffer of 100 bytes works well for the 2 channel mixing mode. If you notice that the sound becomes crackly, increase the buffer size by changing the value of d2. Notice that a larger buffer will introduce latency.

The minimum buffer size threshold is 40 bytes. If the buffer falls below this limit in the vertical interrupt routine, it will temporarily stop playing back samples, which will affect the sound quality.

Acknowledging interrupts

When DMA protection is enabled, in each vertical interrupt the 68k must acknowledge to the Z80 that it has finished any DMAs and that the Z80 may start filling the buffer from ROM again. This is done by writing a non-zero value to z_vbl_ack (at $A00E06). Failure to acknowledge while DMA protection is enabled will result in stopped PCM playback.

Example:

; Run this code in the vertical interrupts after finishing all DMAs
	move.w	#$100,$a11100			; Send a bus request
@wait
	btst	#0,$a11100
	bne.s	@wait
	st.b	$a00e06					; Set z_vbl_ack to $ff
	move.w	#$000,$a11100			; Clear bus request

Disabling DMA protection

While DMA protection is active, if the 68k needs to disable interrupts for a long time, therefore becoming unable to acknowledge the Z80, it is possible to disable the DMA protection by issuing set_pcmmode with the bytes in d1 and d2 being set to zero.

Example:

	lea		mds_work,a0
	moveq	#$11,d0			; set_pcmmode
	moveq	#0,d1			; Keep mixing mode
	moveq	#0,d2			; Disable DMA protection
	jsr		mds_command
	;		( or mdsdrv+$0c )

Note that when the DMA protection is re-enabled without resetting the PCM mode, it will take effect after a short delay (just before the next sample batch).

The Z80 PCM driver

This section contains some information on the Z80 driver that can be useful when working with the DMA protection.

The Z80 driver reads samples in "batches". Depending on the mixing mode, the batch is either 32 or 30 bytes long. The size of the batch has been selected to allow mixing, volume control and bank switching of multiple channels without causing playback interruptions (which reduces sound quality) or high latency.

The PCM buffer is organized as a ring buffer with a maximum width of 256 bytes.

z_load ($A00E01)

This variable contains the current size of the sample buffer. It is updated by the Z80 after a batch has been written to the buffer, or when a sample has been read from the buffer while "idle" (that is, a batch is not currently being written to the buffer).

If this is often being set to 40 (hex $28) in the vertical interrupt routine, that is a sign that the buffer needs to be increased (or simply that the Z80 was not able to buffer enough samples in time).

z_min_buffer ($A00E04)

This variable specifies the minimum size of the buffer. If the current size of the buffer is less than the value of this variable, the Z80 will start reading a new batch.

When DMA protection has been enabled, writing to this variable has the same effect as adjusting the buffer size with set_pcmmode.

z_vbl_ack ($A00E06)

The vertical interrupt acknowledge flag. At the beginning of the vertical interrupt, the Z80 sets this to 0. It will then wait for the 68k to set this to a non-zero value, while depleting the sample buffer.

Z80 vertical interrupt entry point ($A00038)

The DMA protection can be enabled or disabled by replacing the first instruction of the interrupt routine. For more information, read the source code of mdssub.z80.