User:Ddribin/PPU Sandbox

PPU Register Overview

The PPU exposes only eight memory-mapped registers to the CPU. These nominally sit at $2000 through $2007 in the CPU's address space, but because they're incompletely decoded, they're mirrored in every 8 bytes from $2008 through $3FFF, so a write to $3456 is the same as a write to $2006.

PPU Registers

Address	Name	(Alt. Names?)	Function
$2000	PPUCTRL	PPUCR PCR	PPU Control Register
$2001	PPUMASK	PPUMR PMR	PPU Mask Register
$2002	PPUSTATUS	PPUSR PSR	PPU Status Register
$2003	OAMADDR	OAMAR OAR	OAM Address Register
$2004	OAMDATA	OAMDR ODR	OAM Data Register
$2005	PPUSCROLL	PPUSCR PSCR	PPU Scroll Register
$2006	PPUADDR	PPUAR PAR	PPU Address Register
$2007	PPUDATA	PPUDR PDR	PPU Data Register
$4014	OAMDMA	ODMA	OAM DMA

PPUCTRL: PPU Control Register

Bit	7	6	5	4	3	2	1	0
$2000	NMIE	MSE	SSZ	BPT	SPT	VDN	NTA1	NTA0
Read/Write	W	W	W	W	W	W	W	W
Initial Value	0	X	0	0	0	0	0	0

Bit 7 - NMIE: NMI Enable

Setting NMIE to one causes an NMI to be generated at the start of the vertical blanking interval.

Bit 6 - MSE: Master/Slave Enable

Has no effect on the NES.

Bit 5 - SSZ: Sprite Size

0: 8x8; 1: 8x16

Bit 4 - BPT: Background Pattern Table

Background pattern table address (0: $0000; 1: $1000)

Bit 3 - SPT: Sprite Pattern Table

Sprite pattern table address for 8x8 sprites (0: $0000; 1: $1000)

Bit 2 - VDN: VRAM Increment Down

VRAM address increment per CPU read/write of PPUDATA (0: increment by 1, going across; 1: increment by 32, going down)

Bits 1, 0 - NTA1 and NTA0: Base Nametable Address 1 and 0

NTA1	NTA0	Base VRAM Address
0	0	$2000 (Nametable 0)
0	1	$2400 (Nametable 1)
1	0	$2800 (Nametable 2)
1	1	$2C00 (Nametable 3)

PPUMASK: PPU Mask Register

Bit	7	6	5	4	3	2	1	0
$2001	INB	ING	INR	SRE	BRE	SCD	BCD	GSE
Read/Write	W	W	W	W	W	W	W	W
Initial Value	0	0	0	0	0	X	X	0

Bit 7 - INB: Intensify Blues

Intensify blues and darken other colors.

Bit 6 - ING: Intensify Greens

Intensify greens and darken other colors.

Bit 5 - INR: Intensify Reds

Intensify reds and darken other colors.

Bit 4 - SRE: Sprite Render Enable

Enable sprite rendering.

Bit 3 - BRE: Background Render Enable

Enable background rendering.

Bit 2 - SCD: Sprite Clip Disable

Control sprite clipping in leftmost 8 pixels of screen (0: clip; 1: display).

Bit 1 - BCD: Background Clip Disable

Control background clipping in leftmost 8 pixels of screen (0: clip; 1: display).

Bit 0 - GSE: Grayscale Enable

0: normal color; 1: AND all palette entries with 0x30, effectively producing a monochrome display; note that colour emphasis STILL works when this is on!

PPUSTATUS: PPU Status Register

Bit	7	6	5	4	3	2	1	0
$2002	VBL	S0H	SOV	---	---	---	---	---
Read/Write	R	R	R	R	R	R	R	R
Initial Value	X	0	X	X	X	X	X	X

CA65 Definitions

.define bit2mask(bitnum) (1 << bitnum)
.define bits2mask(bits, bitnum) (bits << bitnum)

        ;; PPU Registers
        ppuctrl := $2000
        ppumask := $2001
        ppustatus := $2002
        oamaddr := $2003
        oamdata := $2004
        ppuscroll := $2005
        ppuaddr := $2006
        ppuaddr := $2007
        oamdma := $4014


        ppuctrl_nmie = bit2mask(7)
        ppuctrl_mse = bit2mask(6)
        ppuctrl_ssz = bit2mask(5)
        ppuctrl_bpt = bit2mask(4)
        ppuctrl_spt = bit2mask(3)
        ppuctrl_vdn = bit2mask(2)
        ppuctrl_nta1 = bit2mask(1)
        ppuctrl_nta0 = bit2mask(0)

        ppuctrl_nta_2000 = bits2mask(%00, ppuctrl_nta0)
        ppuctrl_nta_2400 = bits2mask(%01, ppuctrl_nta0)
        ppuctrl_nta_2800 = bits2mask(%10, ppuctrl_nta0)
        ppuctrl_nta_2c00 = bits2mask(%11, ppuctrl_nta0)
        
        ppumask_inb = bit2mask(7)
        ppumask_ing = bit2mask(6)
        ppumask_inr = bit2mask(5)
        ppumask_sre = bit2mask(4)
        ppumask_bre = bit2mask(3)
        ppumask_scd = bit2mask(2)
        ppumask_bcd = bit2mask(1)
        ppumask_gse= bit2mask(0)

        ppustatus_vbl = bit2mask(7)
        ppustatus_s0h = bit2mask(6)
        ppustatus_sov = bit2mask(5)