EarthBound/VRAM DMA Circular Queue

This is a sub-page of EarthBound.

VRAM DMA Circular Queue
Game	EarthBound
Start Address	0x7E0400
End Address	0x7E04FF
# of Entries	32
Entry Length	8 bytes (0x08)
Total Length	256 bytes (0x100)
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A circular queue storing queued-up VRAM DMA transfers to be made at the next VBLANK period.

Format

Transfer Style

• Length: 1 byte (0x1)
• Offset within entry: 0x00
• Range of values: 0x00-0x33

Offset into the VRAM DMA Transfer Style Table to get miscellaneous parameters for a DMA transfer.

Size

• Length: 2 bytes (0x2)
• Offset within entry: 0x01
• Range of values: 0x0000-0xFFFF

Transfer size in bytes.

Memory Address

• Length: 3 bytes (0x3)
• Offset within entry: 0x03
• Range of values: 0x000000-0xFFFFFF

CPU memory address (long pointer) for the "A" side of the DMA transfer.

VRAM Address

• Length: 2 bytes (0x2)
• Offset within entry: 0x06
• Range of values: 0x0000-0x7FFF

VRAM word address for the "B" side of the DMA transfer.

Auxiliary Data

• $7E0000 (8-bit): Entries end offset. Next queue position after the last entry. Also represents the position to write to on adding new entries.
• $7E0001 (8-bit): Entries start offset. Position of the start of the first entry.

Usage

Entries are added to the queue by the VRAM DMA Transfer Routine (unless that routine initiates a DMA transfer directly). If the routine finds that the queue is full, it simply loops until it is no longer full.

Entries are removed by the VBLANK Interrupt Routine where the encoded DMA transfers are initiated.