Nintendo NES-001 User Guide - Page 34

Appendix B NES I/O Registers The following information is based on [7]: Address Access Level Description $2000 Write PPU Control Register 1: $2001 Write • Bits 0-1 - Name table address, changes between the four name tables at $2000 (0), $2400 (1), $2800 (2) and $2C00 (3). • Bit 2 - Specifies amount to increment address by, either 1 if this is 0 or 32 if this is 1. • Bit 3 - Identifies which pattern table sprites are stored in, either $0000 (0) or $1000 (1). • Bit 4 - Identifies which pattern table the background is stored in, either $0000 (0) or $1000 (1). • Bit 5 - Specifies the size of sprites in pixels, 8x8 if this is 0, otherwise 8x16. • Bit 6 - Changes PPU between master and slave modes. This is not used by the NES. • Bit 7 - Indicates whether a NMI should occur upon V-Blank. PPU Control Register 2: $2002 Read • Bit 0 - Indicates whether the system is in colour (0) or monochrome mode (1), • Bit 1 - Specifies whether to clip the background, that is whether to hide the background in the left 8 pixels on screen (0) or to show them (1). • Bit 2 - Specifies whether to clip the sprites, that is whether to hide sprites in the left 8 pixels on screen (0) or to show them (1). • Bit 3 - If this is 0, the background should not be displayed. • Bit 4 - If this is 0, sprites should not be displayed. • Bits 5-7 - Indicates background colour in monochrome mode or colour intensity in colour mode. PPU Status Register: $2003 Write • Bit 4 - If set, indicates that writes to VRAM should be ignored. • Bit 5 - Scanline sprite count, if set, indicates more than 8 sprites on the current scanline. • Bit 6 - Sprite 0 hit flag, set when a non-transparent pixel of sprite 0 overlaps a non-transparent background pixel. • Bit 7 - Indicates whether V-Blank is occurring. SPR-RAM Address Register: $2004 Write Holds the address in SPR-RAM to access on the next write to $2004. SPR-RAM I/O Register: $2005 Write Writes a byte to SPR-RAM at the address indicated by $2003. VRAM Address Register 1. 34