3.2 KiB
Registers
0xf9000 OBJ Pointer
LSB byte sets the starting position of the OBJ copy pointer. The pointer is decremented each time an object is copied. Once the pointer goes below zero, no more objects will be copied. The starting value of the pointer will be ~LSB.
The pointer is counts backwards during copies and forwards during raster. At most, 210 objects can be processed in a single line, so the maximum value for the pointer register is 0x2e.
0xf9002 Direct Memory access
When set to 0x0001, reads and writes to 0xf8000-0xf8fff go directly to sprite RAM.
When set to 0x0002, reads and writes to 0xf8000-0xf8fff go directly to color RAM.
0xf9004 Copy Mode
0x0000 copy all objects except for those with their layer set to 7 (0b111).
0x0001 copy all objects in layer order.
0x0002 Split the objects up based on Y position. Objects < 0xff (lower half of the screen) get copied to sprite RAM 0x000-0x3ff. Objects > 0xff get copied to sprite RAM 0x400-0x7ff. Sprite RAM 0x400-0x7ff is what is rasterized.
Maybe used for split screen or something?
Can the Y position be changed?
0x0008 copy all objects regardless of layer value
0x0100 copy to upper 1024 colors
0x0200 copy to VID_CTRL14 bank, obj prio
0x0400 copy to VID_CTRL15 bank
0xf9008 Initiate transfer
Clears sprite RAM. Copies colors to color RAM. Performs the obj transfer. DMA_BUSY signal will be asserted until transfer is complete.
Timing Notes
GA22 sends hcount signal to GA21 which is used to generate the obj ram address. It counts at 13.333Mhz, 2x the pixel clock. It increments for 848 cycles and then resets to 0. 846 is the highest value it reaches, then it is 0 for two systems. 848 @ 13.333Mhz is 63.6us, the line length. Since the maximum value (846) is midway through the 212th sprite, 211 is likely the maximum number of sprites.
X=44 is the start of the line buffer First 8 pixels are not draw, so a sprite will no appear in the line buffer until X=48 X=467 is the last position that will draw into the line buffer
X=96 is the first visible pixel X=415 is the last visible pixel
111088 clks per frame 2160 clks vsync 1928 clks vsync start to v_pulse 424 clks v_pulse 104 clks hblank 31,40,33 hfp, hsync, hbp hpulse on 20th hsync clk, every 4 clocks: read from 0xf000, row scroll 1-3. the sync ends
Colors
System supports a basic palette of 2048 colors. 4 bits from obj or tile data, and 7 bits set on a per-tile or per-obj basis. Colors are copied from the buffer to palette memory as part of the GA21 transfer process. Only 1024 colors are transferred each time, the lower or upper 1024 destination colors are selected using the 0x0100 bit of the 0xf9004 register.
An additional 2048 colors are available for use in two ways. If object priority is disabled by clearing VID_CTRL13 then the object priority bit is used to select this set of 2048 colors. Which set the playfield uses depends on the value of VID_CTRL14. GA21 transfers colors to this region when the 0x0200 bit of the 0xf9004 register is set.
Finally a separate bank of 4096 colors can be selected using VID_CTRL15. GA21 transfers colors to this region when the 0x0400 bit of the 0xf9004 register is set.