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Blitter

The Blitter is an arrangment of logic gates, registers, and counters that may be used to copy rectangular areas up to 127×127 pixels from Sprite RAM to the framebuffer. This copy runs in parallel to CPU execution and copies at a rate of 1 pixel per CPU clock cycle. If the DMA_IRQ flag is set, the Blitter will assert an IRQ signal when a copy operation completes.

Blitter Registers

There are seven registers used for preparing a copy operation and an additional address used for initiating the copy operation. In order to access these registers, DMA_ENABLE must be set. Modifying these registers during a copy operation is possible, but not presently modeled in the GameTank Emulator.

Addr	Name	Use
$4000	VX	X coordinate in framebuffer of left column of drawing
$4001	VY	Y coordinate in framebuffer of top row of drawing
$4002	GX	X coordinate in Sprite RAM for first column of source data
$4003	GY	Y coordinate in Sprite RAM for first row of source data
$4004	WIDTH	Width of drawing operation. Bit 7 controls horizontal flipping.
$4005	HEIGHT	Height of drawing operation. Bit 7 controls vertical flipping.
$4006	START	Write 1 to clear IRQ and begin a blit operation. Write 0 to clear IRQ without starting a blit.
$4007	COLOR	Value to use for Color Fill Mode. Only used when DMA_COLORFILL_ENABLE is set.